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8 1

SECOND AND THIRD

TRANSITION SERIESIn general, the elements of second and third transition series of a given group have

similar chem ical pr opert ies but both show pronounced differences from t heir light congener s.

For example, a number of cobalt (+2) complexes having tetrahedral and octahedral geometry

ar e known and also a char acterist ic sta te in ordin ar y aqueous solution. But few complexes

of Rh(+2) is known and Ir(+2) is unknown. Similarly Mn(+2) ion is very stable whereas few

compoun ds of Tc and Re in +2 oxidat ion st at e ar e kn own. Cr(+3) is th e most st able oxidation

states and forms number of complexes. Cr(+6) is quite unstable Mo(III) and W(III) compounds

are not stable as Cr(+3) compounds. Mo(VI) and W(+6) are quite stable oxidation states.

This is not to say that there is no valid analogy between the chemistry of three series

of transition elements. For example, the chemistry of Rh (III) complexes is general similar

to that of Co (III) complexes, and here as elsewhere, the ligand field bonds in the spectra

of the complexes in corresponding to oxidation states are similar. However, there are some

differences be tween the 1s t row, second and th i rd row t rans ion e lements . Genera l

characteristic of 4d and 5d elements are discussed below.

ELECTRONIC CONFIGURATIONThe elements Y39 to Cd48 (10 elements) constitute the second transition series whereas

La 157 ,Hf72Hg constitu te t hir d tr an sition series. The filling of 4d and 5d orbitals continues

as we move from left to right in the periods. The electronic configurations of 4d and 5d

transition series are given in Table 2.1. The electronic configuration of first raw transition

element s is regula r wit h only exception of Cr (3d54s1) and Cu(3d104s1) electr onic configur at ion.

However, for second and third row transition elements the electronic configuration is not

regular. There are evidently some pronounced irregularities in the configuration of these

element s. Like Cr an d Cu th e electr onic configurat ion of Mo(4d55s1), Ag(4d105s1) Re (5d56s2)

an d Au (5d106s1) can be easily un derst ood on t he ba sis of higher sta bility of exactly ha lf filled

and fully filled d orbital. However, this concept cannot explain the anomalous configuration

of Nb, Ru, Rh, Pd, W, Pt. As a matter of fact, no simple explanation for such anomalies can

be offered. There are two factors which play a significant role in determining theseconfigurations.

(1) Electr on-Nuclear att raction

(2) Electr on-Electr on r epulsion.

Table 2.1 Electronic configurat ion of second and third row transit ion ser ies

Y Zr N b Mo Tc Ru Rh P d Ag Cd

[Kr] 4d15s

2 4d25s

2 4d45s

1 4d55s

1 4d55s

2 4d75s

1 4d85s

1 4d105s

0 4d105s

1 4d105s

2

La Hf Ta W Re Os Ir P t Au Hg

[Xe] 5d16s2 5d24s2 5d34s2 5d44s2 5d54s2 5d64s2 5d76s2 5d106s0 5d106s1 5d106s2

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8 2 CHE MIS TRY

Physical Properties: These are given in Table 2.2 and 2.3

Size of atoms and ions

The atomic and ionic radii decreases as we move from left to right. However, there is

a slight increase in radii at the end of series. It is due to increase in repulsion due to almost

comp letely filled d-orbital (Table 2.1).

On descending one of the main groups of elements, the size of atoms increases because

extra shells of electron present. The elements of the first group in the d-block show the

expected in crease in size ScYLa. However, in t he subsequent groups t here is an increasein th e ra dius 12pm between th e first and second n umber, but har dly any increase between

the second and third elements. This trend is shown in both in the covalent and ionic radii

(Table 2.2). There are 14 elements called lathanides between La and Hf. In these elements

penultimate 4f shell of electrons is filled. There is a gradual decrease in size of the 14

lanth anides elements from Ce to Lu. This is called lantha nide contr action. The lan tha nide

cont ra ction cancels almost exactly the n orma l size increase on descendin g a group of tr an sition

elements. The covalent radii of Hf and the ionic radii of Hf4+ are actually smaller tha n t he

corresponding value of Zr.

37 38 39 40 41 42 43 44 45 46 47 4 8 4 9

Sr Y Zr N b M o Tc Ru R h Pd Ag C d

Atomic number

4 .5

4 .0

3 .5

3 .0

2 .5

2 .0

1 .5

1 .0

0 .5

0 .0

3rd IE

2nd IE

1st IE

IE( M J m o l )1

Fig. 2.1 The variation in ionization energies (E) with atomic number

for the second row of the d-block.

Ionization Energy

Due to almost same size of second and third row transition series, they show almostsame value of ionization energy (Fig. 2.1). A particular feature of third IE against atomic

num ber is discontinu ity between Tc and Ru in second r ow an d Re an d Os in t hird r ow. This

reflects the additional energy is required to break into half filled subshells.

Table 2.2 Some important physical propert ies of the second transit ion ser ies

Property Y Zr N b M o T c R u R h Pd A g Cd

At om ic n u m ber 39 40 41 42 43 44 45 46 47 48

O ut er e le ct r on ic 4d15s2 4d25s2 4d45s1 4d55s1 4d55s2 4d75s1 4d85s1 4d105s0 4d105s1 4d105s2

configuration

(Cont.)

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 8 3

Atom ic size (pm ) 180 157 141 136 130 133 134 138 144 149

Ion ic ra diu s (pm ) 090 080 070 081 069 050 126 097

(Y3+) (Zr 4+) (Nb5+) (Ru 3+) (Rh 3+) (Pd2+) (Ag+) (Cd2+)

Crys ta l s t ruc ture h cp hcp, bcc bcc bcc hcp hcp fcc fcc fcc h cp

Den sit y (g/m l) 5.5 6.5 8.6 10.2 11.5 12.4 12.4 12.0 10.5 8.7

Melt in g poin t (C) 1490 1860 1950 2620 ~ 2100 2450 1970 1550 960 320

Boi ling poin t (C) 2900 ~ 2900 3300 ~ 4800 > 2700 > 2500 2200 1950 760

Sta ble oxida t ion +3 +4 +5 +6 +4 +3 +3 +2 +1 +2

s ta tes

First ionisation

energy (kJ mol1 ) 636 669 652 694 698 724 745 803 732 866

E lect r on ega t ivit y 1.2 1.4 1.6 1.8 2.9 2.9 2.2 2.2 1.9 1.9

Heat of fusion

(kJ mol1) 11.28 16.7 26.7 27.6 23 25.5 21.7 16.7 11.3 6.1

Heat of vaporiz-

ation (kJ mol1) 389 502 535 502 603 577 381 286 180

Table 2.3 Some important physical propert ies of the third transit ion ser ies

Property La H f T a W R e Os Ir Pt A u Hg

At om ic n u m ber 57 72 73 74 75 76 77 78 79 80

Ou ter elect ron ic 5d16s2 5d26s2 5d36s2 5d46s2 5d56s2 5d66s2 5d76s2 5d96s1 5d106s1 5d106s2

configuration

Atom ic size (pm ) 188 157 143 137 136 134 135 138 144 155

Ion ic r adiu s (pm ) 106 080 073 068 068 078 066 052 137 110

(La 3+) (Hf 4+) (Ta 5+) (W6+) (Re7+) (Os4+) (Au +) (Hg2+)

Crys ta l s t ruc ture h cp, fcc h cp bcc bcc hcp hcp fcc fcc fcc rh om bic

Den sit y (g/m l) 6.2 13.3 16.6 19.3 20.5 22.7 22.6 21.5 19.3 13.6

Melt in g poin t (C) 890 2200 3030 3370 3200 2700 2450 1774 1060 39

Boilin g poin t (C) 3450 5100 5425 5930 5900 7530 4190 4130 2600 357

St a ble oxida t ion +3 +4 +5 +6 +2 t o+7 + 4, +6 +3, +4 +2, +4 +1, +3 +2

s ta tes

First ionisation

energy (kJ mol1 ) 541.3 531 577 770 761 841 887 866 891 1008

E lect r on ega t ivit y 1.1 1.3 1.7 1.7 1.9 2.2 2.01 2.2 2.4 1.9

Heat of fusion

(kJ mol1) 10 21.7 28.4 33.7 33.0 26.8 27.6 21.7 12.7 2.37

Heat of vaporiz-

ation (kJ mol1) 339 648 753 774 636 690 648 565 363 58

OXIDATION STATES

The element s of second an d th ird r ow do not sh ow identical pyram id of oxidation sta tes

as the first row. In a iron family Os and Ru show oxidation states upto (VIII) for example

OsO4 and RuO4 Table 2.4.

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8 4 CHE MIS TRY

The h ighest oxidat ion st ate available to an element is usua lly foun d am ong its compounds

with two most electronegative elements, fluorine and oxygen, so that an examination of thebinary fluorides an d oxides of the tr ansition elements should reveal their maximum chemically

attainable oxidation states. The stoichiometric oxides and fluorides of the second and third

row elements are given in Tables 2.5 and 2.6. Binary compounds with less electronegative

element chlorine might be expected to show a slightly different range of oxidation states.

The elements second and third rows they show maximum oxidation state for example MO 4(M = Os or Ru) with oxygen and Re(+VII) in ReF 7 with fluorine Tables 2.5 and 2.6.

MAGNETIC PROPERTIES

One important characteristic of the heavier elements is that they tend to give low-spin

compounds, which means that in oxidation states where there is an odd number of d-

electrons there is frequently only one unpaired electron, and ions with an even number of

d-electr ons are often diamagnet ic. There a re t wo reasons for this int rinsically great er t endencyto spin pairing. First, the 4d and 5d orbitals are spatially larger than 3d orbitals so that

double occupat ion of an orbital pr oduces significant ly less int er electr onic repulsion. Second

a given set of light at oms produces longer sp littings of 5d than 4d orbitals an d in both lar ger

splitting than for 3d orbitals.

Table 2.4 Oxidation states of 4d and 5d e lements .

Second transit ion series

Y Zr N b Mo Tc Ru Rh Pd Ag Cd

+1

+2 +2 +2 +2

+3 +3 +3 +3 +3 (+3) (+3)+4 +4 +4 +4 +4 +4

+5 +5 +5

+6 (+6) +6 +6 +6

(+7) (+7)

(+8)

Third transit ion series

L a Hf Ta W Re Os Ir Pt Au Hg

+1 +1

(+2) (+2) (+2) +2 +2

+3 (+3) (+3) (+3) +3 (+3) +3

+4 (+4) +4 +4 +4 +4 +4

+5 +5 +5 +5

+6 +6 +6 +6

+7

+8

* The values in the parentheses are for the less common states.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 8 5

Table 2.5 Oxides and hal ides of the second row

Y Z r N b M o T c R u R h Pd A g Cd

+II O N bO Rh O P dO AgOx

Cd O

F PdF2 AgF 2 Cd F2

Cl Zr Cl2 [Mo6Cl 8]Cl 4c PdCl2 CdCl2

Br Zr Br 2? [Mo6Br 8]Br 4c PdBr 2 CdBr 2

I Zr I2? [Mo6I8]I4c Pd I2 CdI2

+III O Y2O3 Ru 2O3b Rh 2O3 (P d2O3)

h? (Ag2O3)

F YF 3 (NbF 3)c MoF 3 Ru F 3 Rh F 3 Pd[PdF 6]

Cl YCl3 ZrCl3 NbCl3c MoCl3

m RuCl3 RhCl3

Br YBr 3 ZrBr 3 NbBr 3c MoBr 3 RuBr 3 RhBr 3

I YI3 Zr I3 Nb I3c MoI 3 Ru I3 Rh I3

+IV O Zr O2 NbO2 MoO2m TcO 2m Ru O2 Rh O2 (PdO2)

h

F Zr F 4 NbF 4 MoF 4 Ru F 4 Rh F 4 Pd F4

Cl Zr Cl4 NbCl4m MoCl4

m TcCl4 RuCl4

Br Zr Br 4 NbBr4m MoBr 4

I Zr I4 NbI4m MoI 4?

+ V O Nb2O5 Mo2O5

F NbF 5 MoF 5 TcF 5 Ru F 5 (RhF 5)

Cl NbCl5 MoCl5

Br NbBr 5

I NbI5

+VI O MoO3 TcO3 (RuO3)h

F MoF 6 TcF 6 Ru F 6 Rh F 6

Cl (MoCl6) (TcCl6)?

Br

I

+VII O Tc2O7

F

Cl

Br

I

Ot h er Nb6F14c Ru O4 Ag 2O

com po- Nb6I14c AgF

un ds AgClAgBr

AgI

Table 2.6 Oxides and hal ides of the third row

La H f T a W R c Os Ir Pt A u H g

+II O (Ta O) (P t O)h H gO

F HgF 2

Cl HfCl2? W6Cl12c [ReCl2] (I r Cl2)? PtCl2 HgCl2

(Contd.)

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8 6 CHE MIS TRY

Br H fBr 2? W6Br 12c (ReBr 2) P r Br 2 HgBr 2

I W6I12c (ReI2) PtI2 HgI 2

+III O La 2O3 Re 2O3h Ir 2O3

h (P t 2O3)h? Au 2O3

F LaF3 (TaF3)c Ir F3 Au F 3

Cl LaCl3 HfCl3 TaCl3c W6Cl18

c Re 3Cl9c IrCl3 PtCl3? Au Cl3

Br La Br 3 HfBr 3 TaBr3c W6Br 18

c Re 3Br 9c IrBr3 PtBr 3? Au Br 3

I LaI3 HfI3 W6I18c Re3I9

c Ir I3 PtI3?

+IV O H fO2 Ta O2 WO 2m Re O2

m OsO2 Ir O2 PtO2

F HfF 2 WF 4 Re F 4 Os F 4 Ir F4 PtF 4

Cl HfCl2 TaCl4m WCl4 ReCl4

m OsCl4 (IrCl4) PtCl4

Br H fBr 2 TaBr 4m WB r 4 ReBr 4 OsBr 4 PtBr 4

I HfI2 TaI4m WI 4? ReI4 Os I4 Pt I4

+ V O Ta 2O5 (W2O5) (Re2O5)

F TaF5 WF 5d Re F 5 Os F 5 (IrF 5) (PtF 5)4

Cl TaCl5 WCl5 ReCl5 OsCl5

Br TaBr 5 WB r 5 ReBr 5

I TaI5

+VI O WO3 ReO3 (OsO3)h (IrO3) (P tO3)

h

F WF 6 Re F 6 Os F 6 Ir F6 PtF 6

Cl WCl6 (ReCl6)?

Br WBr 6

I

+VII O

F Re 2O7

Cl ReF 7 (OsF 7)

Br

I

Other OsO4 Pt 3O4 Au 2O Hg 2F2m

compounds OsCl3.5 AuCl Hg2Cl2m

Au I Hg2Br 2m

Hg2I2m

ZIRCONIUM AND HAFNIUM

Zirconium occurs widely over th e ear th s crust but not in very concent ra ted deposits.

The major minerals are baddeleyite, ZrO2

and Zircon, ZrSiO4. Hafnium is found in Nature

in all zirconium minerals in the range of a percent of the Zirconium content extraction

methods. Zr is made by Kroll process. Hf always occurs with Zr. There chemical properties

are almost same. Separation of two elements is difficult. But now they can be separated

effectively by ion exchan ge or solvent extr action. Zr a nd H f are sepa ra ted by solvent extr action

of their nitrates into tri-n butyl phosphate or thiocyanates into methylisobutyl ketone.

Alternatively the elements can be separated by ion exchange of an alcoholic solution of

tet ra chlorides on silica gel column s. On eluting t he column with a n a lcohol/HCl mixtu re, th e

Zr comes off first.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 8 7

Properties and Uses

Like titan ium both Zr an d Hf are meta l, har d and corr osion resistan t an d their m elting

point 1855C and 2222C respectively. They ar e resista nt to acids and t hey ar e best dissolved

in HF where the formation of anionic fluoro complexes is important in the stabilization of

solutions. Zr reacts with air at high temperature to give a mixture of nitride, oxide and

oxide-nitride Zr2ON 2.

Zr is used for cladding. Zr is also used to make alloys with steel and Zr/Nb alloy is an

import ant superconductor. The very high absorption of thermal n eutr ons by Hf is tur ned to

good use. Hf is used to make control rods for regulating the free neutron levels in the

nuclear reactors used in submarines.

Compounds of Zr and Hf

Zr and Hf show oxidation states (+II), (+III) and (+IV). (+IV) is the stable oxidation forboth the metals. So most of compounds are known in (+IV) oxidation states.

Oxides

On a ddition of hydroxide t o Zirconium (IV) solutions a gelatinous pr ecipitat e ZrO2.n H 2O

is formed. Where n is variable. ZrO2 and HfO2 are stable white solids, non-volatile and

rendered refractory by strong ignition. On strong heating ZrO 2 becomes very hard and its

high melting point of 2700C and its resistance to chemical attack make it useful for making

high tempera tur e crucibles and furnace lining. If the solids ha ve been pr epared dr y or h ave

been heated, they do react with acids.

ZrO2 and H fO2 ar e basic. Like TIO2+, ZrO2+ exists in solut ion an d form polymeric species

in solut ion. ZrO(NO3)2 forms an oxygen bridged chain structure and soluble in water. If the

ZrO2 and HfO2 are fused with the appropriate quantities of other metal oxides at 1000C2500C. Zinconates and hafnates are formed. These are mixed oxides.

Halides

All the halides of the type MX4 (where M = Zr or Hf and X = F, Cl, Br, I) are known.

ZrCl4. It can be prepared by the chlorination of heated Zirconium, Zirconium carbide

and a mixture of ZrO2 and charcoal. It is white solid, subliming at 331C.

It fumes in m oist a ir a nd h ydrolyzed vigorously by water . Hydrolysis pr oceeds only part

way at room temperature, affording the stable oxide chloride.

ZrCl H O4 2+ 9 ZrOCl H O + 2HCl2 2.8

ZrCl4 and HfCl4 also combine with donors such as ethers, POCl3 and CH3CN and withCl ions to form six-co-ordinate species.

ZrCl Cl4 +2 ZrCl6

2

ZrCl4 and HfCl4 also combine with diarsines to form ZrCl4 (dias)2 which has the

dodecahedral type eight-coordinate structure. ZrCl4 reacts with carboxylic acid at 100C to

give Zr(RCO2)4. The compoun ds ZrBr4, ZrI4, HfBr4 are similar to ZrCl4. ZrF4 is a wh ite solid

subliming at 903 which un like oth er h alides, is insoluble in donor solvents. Hydra ted ZrF 4.1

or 3H 2O can be obtained from HF-HNO3 solution.

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8 8 CHE MIS TRY

Zr4+ is relatively large, highly charged and spherical with no partly filled shell to give

it st ereochem ical pr eferen ces. Ther efore, Zr(iv) compounds exhibit high coordin at ion nu mberfor exa mple, Li2ZrF6, CuZrF6.4H2O, ZrF7

3. ZrF73 ha s two different str uctur es (1) pent agonal

bipyramid Na 3ZrF7 (2) capped trigonal prism (NH 4)3 Zr F7. [Zr2F 12]4, Cu 2Zr F8.12H2O and

Cu 3Zr 2F 14 are also known. With oxygen ligands high coordination number and varied

stereochemistry are also prevalent. Thus M6I [Zr(OH)2(CO3)6].nH2O contain OH bridged

binuclear u nits in which each Zr a tom is in dodecah edra l 8-coordinat ion a nd ZrO7 pentagonal

bipiram ids are found in ZrO2 (OH) OMO2O4(OH)5.

AQUEOUS CHEMISTRY AND COMPLEXES

ZrO2 is more basic as compared to TiO2, therefore it is virtually insoluble in an excess

base. The a queous chemistr y of Zr4+ is well known since it is doubtful th at hydr olysis of Zr 4+

takes place. The hydrolyzed ion is often known as Zirconyl ion and written ZrO2+. The most

important Zirconyl salt is ZrOCl2.8H 2O which crystallizes from dil. HCl and contain[Zr4(OH)8]H2O16]

8+.

In acid solutions except concentration. HF, where ZrF 62 and HfF 6

2 are present.

[Zr 3(OH)4]8+ and [Zr4(OH)8]

8+ are exist in 12M perchloric acid.

C h e l a t i n g a g e n t s u c h a s E D T A a n d N T A f o r m c o m p l e x e s w i t h Z r I V, t h e

[Zr(N(CH2COO)3)2]2 ion has been shown to be dodecahedral.

M(acac)2X2 and M(acac)3X type of complexes were known with acetylacetonate. The

former have cis octahedral whereas the latter one 7-coordinate. Zr IV forms complexes or

with carboxylate, Zr(OCOR)4, the tetratries (acetylacetonate), the oxalate. It also forms

covalent compounds with NO3.

ZrCl N O4 2 5+ 430

4

+Zr (NO XN O YN O 4 NO Cl3 2 5 2 4 2) . .LOWER OXIDATION STATES

The chemistry of lower oxidation states is so far limited to non-aqueous chemistry of

lower halides and some complexes thereof.

The ha lides of Zr in tr ivalent st ates su ch a s ZrCl3, ZrBr 3 and ZrI3 ar e well known. These

can be prepared, by reduction of tetrahalides with H 2, 2ZrCl4 + H2 2Zr Cl3 + 2HCl. HfI3is isotypic. ZrX3 compounds react r eadily at r oom t emperat ure with bases such as Pyridine,

bipyridine, phenant hroline a nd CH 3 CN t o form ZrX3.2Py, 2ZrX3.5CH3CN. The trihalides of

Hf are nonstoichiometric HfI3.0 to HfI3.5 .

The lower chlorides of Zr and Hf have recently been carefully studied. ZrBr 2 and ZrI2are well known.

Organometallic Compounds

Most of the Organ ometallic compoun ds of Zr an d Hf ar e similar t o their Ti an alogs. The

general formula is (4-C5H 5)2Zr X2.

NIOBIUM AND TANTALUM

Niobium and tantalum occur together. Niobium is 1012 times more abundant in the

eart h crust tha n t ant alum. The ma in commer cial source of both are t he columbitytant alite]

(FeMnNb2O6) Fe,MnTa2O6) series of minerals. The most important mineral is Pyrochlorite

CaNaNb2O6F. However 60% of Ta is recovered from the slag from extracting Sn. The ores

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 8 9

are dissolved either fusion with alkali or in acid. Formally the separation of Nb and Ta was

achieved by treatment with a solution of HFNb forms a soluble K2 [NbOF 5] and Ta formsinsoluble K2[TaF7]. Separation is now performed by solvent extraction from dil. HF to

methyl isobutyl ketone. The metals are obtained either by reducing pentaoxide with Na, or

by electrolysis of molten fluoro complexes such as K 2[NbF7].

Properties and Uses

Nb and Ta are silvery coloured metals with high melting points. The pure Nb and Ta

are m oderat ly soft a nd ductile, but tr aces of impur ities make th em ha rder a nd britt le. They

are extremely resistant to corrosion due to the formation of oxide film on the surface. At

room temperature they are not affected by air, water or acids other than HF with which

they form complexes. Nb and Ta both dissolve in fused alkali. Nb and Ta do not form

positive ion. Thus though Nb and Ta are metals, their compounds in the (+V) state are

mostly, volatile and readily hydrolyzed. Nb2O5 and Ta2O5 are amphoteric. They have onlyweak acidic properties. Niobates and tantalates are only formed by fusing with NaOH.

Nb is used in various stainless steels, and Nb/steel is used to encapsulate the fuel

elements for some nuclear reactors. A Nb/Zr alloy is a superconductor at low temperatures,

and is used to make wire for very powerful electromagnets. Ta is used to make capacitors

for the electronics industry. Because it is not rejected by the human body it is valuable for

making metal plates screws and wire for repairing badly fractured bones. TaC is one of the

highest melting solids known (is 3800C).

Compounds

Nb and Ta react with all of the halogens on heating to give pentahalides MX5. The

range of halides is given in Table 2.7. All the halides are volatile, covalent and hydrolyzed

by water.

Table 2.7 Halides o f Niobium

+III +IV +V

NbF 3 NbF 4 NbF 5

NbCl3 NbCl4 NbCl5

NbBr 3 NbBr4 NbBr5

NbI 3 NbI 4 NbI 5

-- Ta F5

Ta F 3 TaCl4 TaCl5

TaCl3 TaBr 4 TaBr 5

TaBr 3 Ta I4 Ta I5

+V Halides

Nb and Ta form penta halides. These may be formed by direct reaction of the elements

or by the reaction oxides.

M O F2 5 2+ MF5 ,

NbCl or TaCl F5 5 2+ MF5

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9 0 CHE MIS TRY

NbCl or TaCl HF5 5

+ MF5

NbF 5 and TaF5 form cyclic tetramers with four octahedra joined in this way (Fig. 2.2).

Solid NbCl5 and TaCl5 are dimeric with two octahedra joined by sharing two corners

(Fig. 2.2b). All the pentahalides can be sublimed under an atmosphere of the appropriate

halogen. Pentafluorides react with F to give MF 6.

MF F5 +

MF6

If the conc. of F is higher than the complexes [NbOF 5]2 [NbF 7]

2 and [TaF7]2 a re

formed. The structure of seven-coordinate species are capped trigonal prism. Ta forms

[TaF8]3 with a squa re an tiprism whereas, Nb forms [NbOF6]

3. This inability of Ta to form

oxohalides has been used to separate Nb and Ta. Fluoride are white but other halides are

coloured due to change transfer.

2.56

C l C l

C l C l C l

C lC lC l

N b N b101 3

2.25

(b )(a )

C l C l

A

A

Fig. 2.2 The tetrameric structures of NbF 5 and TaF 5. The dinuclear structure

of crystalline Nb2Cl

10. The octahedra are distorted.

(+IV) Halides

The tetrahalides are formed by the reduction of pentahalides with H 2, Al, Nb or Ta.

NbX or TaX5 5 H Al, Nb or Tareduce with

42

MX,

NbF 4 is black paramagnetic nonvolatile solid made up of regular octahedra joined in

chain by their edges. Other tetrahalides are brown-black solids and are diamagnetic. This

suggests extensive metal-metal interaction. NbI4 the stru ctur e is a chain of octah edra joined

by their edges. NbCl4 is similar. Tetrahalides tend to disproportionate.

2TaCl4400 +C 3 5TaCl TaCl

They hydrolyze by water

4 5TaCl H O4 2+ + +Ta O TaCl 10HCl2 5 3

(+III) Halides

All the trihalides are known except TaI3. They are reducing, have a d1 configuration.

They ar e brown or black in colour . The tr iha lides of Nb an d Ta a re t ypically nonst oichiomet ric.

In NbCl3 the Nb ions occupy octahedral holes in a distorted hexagonal close packed array

of Cl ions in such a wa y tha t n iobium at oms in thr ee adjacent octah edral ar e close enough

to bonded together into metal cluster.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 9 1

Lower Halides

High temperature reduction of the pentahalides. NbX5 and TaX5 with Na or Al give a

series of lower h alides such a s M6Cl14, M 6I14, N b6F15, Ta 6Cl15, Ta 6Br 15 and Ta6Br 17. These are

all based on the [M6X12]n+ unit. For example

Nb Cl6 14water

AgNO

6 12

3

AgCl + Nb Cl +

22

These are known as cluster. The cluster can be oxidized:

M X6 122+

+ +

M Cl M Cl6 12 6 123 4

Oxides

The metals all react with O2 at elevated temperatures and give pentaoxide M2O5. The

main oxides formed are shown in Table 2.8.

Table 2.8 Oxides

(+II) (+III) (+IV) (+V)

NbO NbO2 Nb2O5

TaO TaO2 Ta2O5

Pentaoxides of Nb2O5 and Ta2O5 are commonly made by ignition of other Nb or Ta

compound s in air . Nb2O5 and Ta 2O5 ar e colourless (d). Nb2O5 and Ta 2O5 react with HF. They

form niobates a nd t etalates when fused NaOH. These precipitate t he hydra ted oxide at P H

7 and 10 respectively and the only isopoly ion found in solution in [M 6O19]8.

Organometallic Compounds

Nb and Ta form cyclopentadienyl compounds such as [Nb(5C5H 5)2 (1C 5H 5)2].

[Nb(5C5H5)2 (1.C5Hd5)2 Cl3] an d [Nb(

5-C5H5)2 (1-C5Hd5)2Cl3.

MOLYBDENUM AND TUNGSTEN

Mo an d W are quit e rar e. Abun dan ce of Mo an d W in t he ear th s cru st by weight is 1.2 ppm .

Molybdenu m occur s chiefly as m olybden ite MoS2, but a lso as m olybdates su ch as Wulfenite

(PbMoO4) or MgMoO4. Tungsten is found almost exclusively in the form of tungstate, the

chief ore being Wolframite (FeWO4 and MnPO4), scheelite (CaWO4) and stolzite (PbWO4).

The sm all am ount s of MoS2 in ores ar e concent ra ted by th e foam flota tion process. This

is converted into MoO3. It is reduced to metal with hydrogen. Reduction with carbon should

be avoided because it form carbides instead of metal.

Tungsten ores are concentrated by mechanical and magnetic processes and the

concentrate attacked by fusion with NaOH. The cooled melts are leached with water, giving

solutions of sodium tungstate from which hydrous WO3 is precipitat ed on a cidificat ion. Th e

hydrous oxide is dried and reduced to metal by hydrogen.

Properties and Uses

The metals are hard and have very high melting and low volatility (Tables 2.2 and 2.3).

The melting point of W is next to carbon. In the powder form in which they are first

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9 2 CHE MIS TRY

obtained both m etals ar e dull grey, but when converted int o the ma ssive stat e by fusion a re

lustrous silver white substances of typically metallic appearance in properties. They haveelectrical conductance 30% that of Ag. They are extremely refractory. The melting points

of Mo and W are 2610C and 3418C respectively.

These metals do not react with air at room temperature. However, on strong heating

both form oxide of the type MO3 (M = Mo or W).

2Mo + 3O2 2MoO3

They also combine with Cl2 to give MCl6. They react with F 2 at room temperature to

form MF 6.

Mo + 3Cl2 MoCl6

Neither metal is readily attacked by acids. Con. HNO 3 initially attacks Mo but metalsur face is soon passivat ed. Both met als can be dissolved in a mixtu re of con. HN O3 and HF.

W dissolves slowly. Aqueous a lkali does not r eact with th e met als h owever, oxidizing alkaline

such as fused KNO3NaOH or Na 2O2 attack them rapidly.

As a result of lan th an ide contra ction, there is a close similarity in t he size and properties.

The difference in properties is greater as compared to Zr and Hf. Thus Mo and W can be

easily separated by using conventional scheme i.e., qualitative analysis. WO3(H 2O)n is

precipitated in the 1st group a nd m olybdates ar e reduced by H 2S in IInd-group where MoS

is precipitated out.

The chief uses of both the metals are in the production of alloy steel, even small

amount s cause tr emendous increases in h ardn ess and str ength. High speed steels which a re

used to mak e cutt ing tools and r emain h ard even a t r ed heat. Tun gsten is also used for lam p

filaments. The element give hard, refractory and chemically inert industrial compounds with

B, C, N or Si on direct reaction at high temperatures. Tungsten carbide is also used for

tipping cutting tools.

Compounds +VI oxidation State

A number of the stable compounds of Mo and W are known in (+VI) oxidation state.

Oxides

Oxides of MoO3 and WO3 are known in (+VI) oxidation state. They are formed by

heating the metal in air.

Mo + Air MoO3They are acidic in nature therefore, except HF they are not attacked by acids. They

dissolve in NaOH by forming MoO42 and WO4

2 ions. MoO3 is a white solid at room

temperature but becomes yellow when hot and melting at 795C to a deep yellow liquid. It

is anhydride of molybdic acid, but it does not form hydrates directly. MoO3 has a ra re type

of layer structure in which each molybdenum atom is surrounded by a distorted octahedron

of oxygen atoms. WO3 is lemon yellow solid. M.P. 1473C.

MoO3 and WO3 differ from CrO3 in several ways:

1. MoO3 and Wo3 are stable and having no oxidizing properties.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 9 3

2. They are insoluble in water.

3. They have high melting points .

Cr O3 (197C) MoO3 (795C) an d WO3 (1473C)

4. Their colour and str uctures ar e different.

Mixed Oxides

Several mixed oxides can be made by fusing MoO 3 or WO3 with group I or II oxides.

These contain chains or rings of MoO 6 or WO6 octahedra. moist WO3 turns slightly blue on

exposure to U.V. light. Mild reduction of aqueous suspensions of MoO 3 and WO3 or acidic

solutions of molybdates K2Mo4O13 or t ungstate K2W4O13 also gives a blue colour. The blue

oxide so produced are thought to have Mo or W in oxidation states of (+VI) and (+V) and

contain some OH instead of O2 to balance the charge.

Halides and Oxyhalides

The MF6 type compounds are volatile colourless and diamagnetic. MoF 6 and WF6 are

quite sta ble. They show low m elting points (MoF6, 17.4C an d WF6 1.9C) and ea sily hydrolyzed.

MoF 6 is reduced easily and attack organic matter while WF 6 is less active. MoCl6 wa s

claimed in 1967 as a black powder very sensitive to water and prepared by the action of

SOCl2 on MoO3. WCl6 is formed by the direct chlorination of metal. It is moderatly volatile,

monomeric in vapour an d soluble in organic solvents su ch as CS2, CCl4 Alcohol etc. It reacts

slowly with cold water but ra pidly with h ot water to give tun gstic acid. It is us ed as cata lyst

for alkylation of benzene. W Br 6 can also be obta ined by direct halogena tion of met al. It

is dark blue solid.

Molybdenyl chlorides are covalent and readily decomposed by water. Tungstenyl chloride

hydr olyze less rea dily.

(+V) Oxidation State

Oxides

MO2O5, a violet solid soluble in warm acids. It is prepared by heating the required

quantity of finely divided molybdenum with MoO3 at 750C. On addition of NH 3 a brown

precipitated MnO(OH)3 is formed. On heating it gives Mo2O5.

Halides

Treatment of molybdenum carbonyl with fluorine diluted in nitrogen at 75C gives a

product of composition Mo2F9. On heating Mo2F 9 at 150C gave the nonvolatile MoF 4 as a

residue a nd volatile MoF 5 which conden ses in a cooler r egions of the a ppar at us. MoF5 is alsoobtained by the reactions.

5MoF Mo(CO)6 6+25

6 6 +C 5MoF CO

Mo + 5MoF6 6MoF5

Mo + F2 ( )d il400 C 5MoF

WF 5 is obtained by quenching the products of reaction of W with WF 6 at 8001000C.

It disproportionates above 320C into WF4 and WF6. Crystalline MoF 5 and WF5 have the

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9 4 CHE MIS TRY

tetrameric structure common to many pentaflourides. Heating of Mo with Cl 2 gives Mo2Cl10.

This is soluble in benzene and other organic solvents. It exists as monomeric MoCl 5 insolution, but dimerize to Mo2Cl10 in t he solid. Mo2Cl10 is used as the starting point for making

other Mo compounds (scheme 1 Fig. 2.3). It is rapidly hydrolyzed by water, and removes O

from oxygenated solvents, forming oxychlorides. Mo2Cl10 is paramagnetic (M = 1.6 B.M.),

indicating that there is one unpaired electron and thus no metal-metal bonding.

MoO Cl (OPPh ) (orange)2 2 3 2+ MoOCl (OPPh ) (green)3 3 2

M oO C l b ipy3

P h P O3

bipy, H Oin CCl

2

4

M e S O2

M o C l2 10

+ M C l

SO ( l iq.)2

SO ( l iq.)2

[MoOCl ]3 n

M e S O2

M oO C l (O SM e )3 2 2

N H C lin MeCN+ H O

4

2

N H [M oO C l M eC N ]4 4

10 H C lM

reduce inH C l

[MoOCl ]52

[MoOCl ]4

+

[MoCl ]62

(sol.)

(insol.)

N Hethanol+ bipy

4+ in(HCl)

1 2 H ClM

1 2 H ClM

M oO ,3

M oO 42

MoO(OH)3

MoCl63

H O2

H Cl[M o O C l2 3 8

4]

acac

b ipy M o O C l2 2 4 2 2 3 4M o O ac ac

SO ( l iq.)+ MCL

2

Zn/Hg/HCl

Fig. 2.3 Some preparations and reactions of molybdenum pentachloride and of

oxomolybdenum compounds.

Green WCl5 and black WBr 5 are prepared by direct halogenation, the condition being

critical, especially the temperature.

(+IV) Oxidation State

Oxide

Molybdenum (IV) oxide MoO2 is obtained by r educing MoO3 with h ydrogen or NH 3 below

470C (above th e temp era tu re r eduction pr oceed to meta l) an d by reaction of Mo with stea m

at 800C. It is a brown violet solid with a coppery luster , in soluble in nonoxidizing m inera l

acids but soluble in con. HNO3 with oxidation of the Mo(IV) Mo(VI). The structure is

similar to that of Rutile but so distorted that strong MoMo bonds are formed. WO 2 is

similar.

Halides

The tetrahalides include MoF 4 and WF 4. The MoF 4 is obta ined on dispr oport iona tion of

Mo2O9. Both can be prepared by the reduction of hexahalides with hydrocarbon i.e., C6H6 a tis 110C. Both are nonvolatile. MoCl4 which is very sensitive to oxidation and hydrolysis

exists in two forms.

MoCl5Hydrocarbons

4MoCl

On heating -MoCl4 at 250C in the presence of MoCl5, it is changes to -form. -MoCl4has partial spin pairing through MoMo interactions whereas the -form has an hcp away ofCl atoms with Mo atoms so distributed in octahedral interstices. That no MoMo bond is

formed. WCl4 is best obtained by redu cing WCl6 with Al in a t herm al gradient. It disproport iona te

at 500C to WCl2 + 2WCl5. MoBr4, WBr4 and WI4 all exist but are not well known.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 9 5

+III Oxidation State

Mo(III) and W(III) do not form oxides, but all the halides are known except WF 3(Table 2.9a). These compounds do not contain simple ions. Mo(III) compounds are fairly

stable. However, they oxidize in air and hydrolyse in water. They react with halide ion to

form octahedral complexes.

MoCl Cl3 +3

MoCl6

3

Two forms of MoCl3 are known.

Reaction of MoCl3 ar e given F ig. 2.5. One with cubic close pa cking of chlorat e at om, the

other based on hexagonal close packing.

W(III) compounds are unstable. WCl3 is really W6Cl18 and forms a cluster compound

[W6Cl12]6+. W6Br 18 also forms a cluster compound [W6Br 8]

6+.

+II Oxidation State

Mo and W do not form difluorides, but other six (+II) halides Fig. 2.4 are known. They

ar e usu ally made by r eduction or t her ma l decomposition of higher h alides. They do not exist

as simp le ion bu t form clust er compoun ds inst ead. MoBr2 is really [Mo6Br 8]Br 4.2H 2O. All Six

dihalides ha ve same str ucture ba sed on cluster [M6X8]4+ unit ion an octahedral cluster of six

metal atomes (Fig. 2.5). There is a strong M-M bonding. These compounds are diamagnetic.

Mo6Cl12 is not reducing whereas W6Cl12 is redu cing in solution. Mo2(CH 3COO)4 is a compoun ds

which is also diamagnetic due to quadrupole interaction.

M o C l2 10M oC l3H 2

400C

Fuse KCl

MoCl6

KCl inIC l

Reflux

benzeneMoCl4

MoCl6

R N C lin MeCN

4

CC l4

250M oO 2

C l2 500

M oC O C l2

750M o C l6 12

M o600C

C l2 inCCl , 2504

[M o C l6 8 ] s alt s4+

Fig. 2.4 Preparation of molybdenum chlorides and chloro complexes.

M II X

Fig. 2.5 [M6X8]4+ clusters with X bridges over each face of the octahedron of metal ions.

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9 6 CHE MIS TRY

Table 2.9 Halides of Group VI (mpC)

S tate Fluorides Chlorides Brom ides Iod ides

+6 MoF6 (MoCl6)

colour less (17.4) black

bp 34

WF6 WCl6 WBr 6colour less (1.9) dark blue (275) dark blue (309)

bp 17.1 bp 346

+5 MoF5 MoCl5yellow (67) black (194)

bp 213 bp 268

WF5 WCl5 WBr 5yellow dark green (242) Black

bp 286

+4 MoF4 MoCl4 MoBr4 MoI4?

pa le green black black

WF4 WCl4 WBr 4 WI4?

red-brown bla ck black

+3 MoF3 MoCl3 MoBr3 MoI3brown (>600) very dark red green (977) black (927)

(1027)

WCl3 WBr 3 WI3red black (d > 80)

+2

MoCl2 MoBr2 MoI2yellow (d > 530) yellow-red

(d > 900)

WCl2 WBr 2 WI2yellow yellow brown

MOLYBDATES AND TUNGSTATES

On acidification the solution of molybdates or tungstate, they condensed and give a

ra nge of various polymolybdat es or polytun gstat e. Below pH 1 a h ydrat ed oxide MoO3.2H 2O

yellow or WO3.2H2 (White) are precipitated out. The formation of polyacids is a prominent

feature of the chemistry of Mo and W. The polyanions contain MoO 6 or WO6 octahedra.When one joined together in a variety of ways by sharing of edges but not faces. The poly

acids of Mo and W are classified into two main groups.

1. Isopolyacids, where the a nions which condense together are all of the sam e type

for example all MoO6 groups or all WO6 groups.

2. Het eropolyacids, where two or m ore different t ypes of anion conden se togeth er for

example molybdates or tungstate groups with phosphate, solicate or borate groups.

The isopolyacids Fig. 2.6a of Mo and W are not completely understood. It is quite

difficult to study them because extent or hydration and protonation of various species in

solution are not known. The relationship between the stable species so far known is.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 9 7

MoO Mo O Mo O MoO 2H O4

pH 6

7 24

ph 1.5

8 26

pH < 1

3 2

4 6 2 9 4

.

Nor m a l m olybda t e P olym olybda t es Polymolybda tes h ydra ted oxide

The structures of polymolybdates are confirmed by X-ray crystallography.

Tungstate

[WO ]42 pH 6.7

Biol OH [H W O ]21

56

Slow[W O ]41

1012

pH = 3.3

[H W O ]213

3 6

pH < 1

WO 2H O23.

or

[W OH) ]1010

12 36O (

H+

[H O ]406

2 12W

Heteropolyacids are formed if a molybdate or tungstate solution is acidified in the

presen ce of phosphat e, silicat e or met al ion. The second an ion pr ovides a centr e round which

the MoO6 or WO6 octahedra condense, by sharing oxygen atom with other octahedra and

with the central group. The central groups are oxoanions such as PO43, SiO4

4 and BO43

but other elements including Al, Ge, Sn, As, Sb, Sc, Te I and many of the transition

elements will serve as the second group. The ratio of MoO 6 or WO6 octahedra to P, Si, B

or other centr al a tom is u sua lly 12:1, 9:1 and 6 : 1. A well known examp le of heter polymolydat e

as (NH4)3 [Mo12O36.P O4]. The structure of several heteropolyacids are known (Fig. 2.6).

Fig. 2.6 The structues of two heteropoly anions: (a) 12-molybdophosphate or 12-tungstophosphate;

(b) and (c) details of coordiantion of three MO6 octahedra with one corner of the heteroatom

tetrahedron.

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9 8 CHE MIS TRY

Tungsten Bronze

The reduction of sodium tungstate with hydrogen at red heat gives a chemically inert

substance with a bronze like appearance. Similar compounds are obtained by vapour phase

reaction of alkali metals with WO3. They ar e now made u p by heating Na 2WO4 with W metal.

Tungsten bronze are nonstochiometric substances of general formula MInWO3 (O < n

< 1). The colour s var y great ly with composition from golden n is 0.7 yellow for n 0.9 toblue-violet n 0.3. Tungsten bronze with n > 0.3 ar e extr emely inert a nd h ave semimetallicproperties, especially metallic in cluster and good electrical conductivity which the charge

carriers are electrons. Those with n < 0.3 are semiconductors. They are insoluble in water

and resistant to all acid except HF. They can be oxidized to tungstate (VI) by oxygen in the

presence of base.

4 4NaWO NaOH + O3 2+ +4 2Na WO H O2 4 2

Str uctura lly, the sodium tun gsten bronzes ma y be regarded as defective. M1WO3 phases

having the perovskite structure. In a defective phase M InWO3, there are (1 n )WVI atoms

and (1 n ) of Na sites of the pu re N aWO3 phase a re u noccupied. It appear s th at completely

pure NaWO3 has not been prepared, although phase with sodium enrichment upto perhaps

n = ~0.95 are known. The cubic structure changes to rhombic and then triclinic for n < ~

0.3. In the limit ofn = 0 we have of course WO3, Thus the actual range of composition of

the tungsten bronze is approximately NaO 3WO3 to Na 0.95WO3.

The semimetallic properties of the tungsten bronzes are associated with the fact that

no distinction can be ma de between WV and WVI atoms in t he latt ice, all W atoms appearing

equivalent. Thus the n extra electrons per mole the distributed throughout the lattice,

delocalized in energy bonds some what similar to those of metals.

Mo also forms bronze similar to W, but a high pressure is required to form them and

the Mo compounds are less stable.

Preparation of some carbonyl containing molybdenum arsine and phosphine is given in

Fig. 2.6(a).

Mo(CO)6diars

M o (C O )II 3 diars X2

Br.I

22

Mo(CO)4 diarsI i n ho tC H C l2

3I i n ho tCC l2

4

M o (C O )II 2 diars I2

( = 1.98 B.M.)

M o ( C O )III 2 diars I3

( = 1.40 B.M.)

Mo(CO)6 Mo(CO)2 diphos2 [Mo (CO)I 2 diphos ] I2 3diphos I2

( = 1.60 B.M.)

Fig. 2.6(a) Preparation of some carbonyl-containing molybdenum arsine and phosphine complexes.

TECHNETIUM AND RHENIUM

Technetium does not occur in nature and was the first manmade element. All the isotopes

ar e ra dioactive. 99Tc is one of the fission products of uranium. It is a emitter with half-life of2.1 105 years. It is obtained in kilogram quantities from spent fuel rods from reactors at

nu clear power st at ion. The r ods may cont ain 6% Tc. These rods must be stored for several year s

to allow the short lived radioactive species to decay. Tc can be extracted by oxidation to Tc2O7

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 9 9

which is volatile. Altern at ively solutions can be separ at ed by in excha nge an d solvent extraction.

It can be dissolved in water, forming TcO4 ion a nd crystallize as NH4TcO4 or KTcO4. They canbe reduced with H2 to give metal. Tc has no commercial use

97Tc and 98Tc can be made by

neutron bombardment of Mo, small amounts of Tc compounds are sometimes injected into

patients to allow radiographic scanning of the liver and other organs.

Rhenium is a very rare element, and occurs in small amounts in molybdenum sulphide

ores. Re is recovered as Re2O7 from the flue dust from roasting these ores. This is dissolved

in NaOH, giving a solution containing ReO4 ion. The solution is concentrated and then KCl

added t o precipitate KReO4. The metal is obtained by reducing KReO4 or NH4ReO4 with H2. It

is used to make Pt-Re alloy which one used as catalyst for making low lead or lead free petrol.

It is a lso used a s catalyst for h ydrogenat ion a nd deh ydrogenat ion r eactions. Due to its high m .p.

(3180C) it is used in thermocouple, electric furnace windings and mass spectrometer filaments.

Properties

Tc and Re are less reactive. They do not react with H 2O or non-oxidizing acids. They

do not dissolve in HCl and HF, but they react with oxidizing acids, such as conc. HNO3 and

H 2SO 4 forming pertechnic acid HTeO4 and perhenic acid HReO4. Tc and Re undergoes

similar reaction with H 2O2 and bromine water. They get tarnish slowly in moist air, but

powder metal is more reactive. Heating with O2 gives Tc2O7 and Re2O7 which are low

melting point (119.5C) and 300C respectively) and volatile. On heating with F 2 gives MF 6and MF 7 (M = Tc or Re).

Compounds

Tc and Re form compoun ds in +VIII, +VI, V, VI, III, II oxidat ion sta tes . Some compound s

are also known in lower oxidation states.

+VII oxidation states

Many compoun ds such as M 2O7, M2S7 MO3 ion oxhalides hydrides a nd ReF 7 are k nown

in (+VII) oxidation states. The oxides Tc2O7 and Re2O7 are formed when metals are heated

in a ir or oxygen. Both ar e yellow solids. Tc2O7 and Re2O7 have m elting point 120C an d 220C

respectively. Tc2O7 is more oxidizing than Re2O7.

Both oxides are soluble in water and form HTeO 4 and HReO4 ions which are colourless.

Another form pur henic and kn own a s TcO4 and ReO4

are tetra hederal. TcO4 and ReO4

react

with H2S to form Tc2S7 and Re2S7 respectively. TcO4 and ReO4

are stable in acidic metals.

KTcO K K TcH

KReO K K [ReH

4en

2 9

4

en

2 9

+

+

[ ]

]TcO4

and ReO4 ar e colourless because the char ge tra nsfer band occur at higher energy

in the UV region. However solution HTcO4 (Red solid) and HReO4 yellow-green. These

colour s ar ise becau se the t etra hader al ReO4 ion becomes less symm etr ical when un dissociat ed

HOReO3 is formed. Raman spectrum shows lines due to acid.

Halides

On heating Re with fluroine to form ReF7. Tc forms only TcF6. Several oxohalides are

form ed such a s ReOF5, ReO2F3, ReO3F, TeO3F a nd TcO3Cl. These a re pale yellow or colour less

compounds. They exist either as low melting solid or liquid.

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1 0 0 CHE MIS TRY

(+VI) State

Re(+VI) is known as the red coloured oxide ReO3, but the existance of TcO3 uncertain.

The structure of ReO3 is shown in Fig. 2.7a. Each metal is octahedrally surrounded by

oxygen at oms. The h alides TcF6, ReF 6 and ReCl6 are known. The fluorides ar e prepar ed by

direct reaction of elements with fluorine, chlorides are prepared by treating fluorine with

BCl3. Fluoride and chlorides are yellow and green-black in colour. These compounds have

low melting points r an ging from 18C to 33C. They show magnet ic moment lower t ha n spin

only value due to strong spin-orbit coupling.

Fig. 2.7(a) The structure of ReO3.

They are sensitive to waterReFe H O6 2+ + +HRe O Re O

(VII)4

(IV)2 HF

(V) Oxidation State

Tc is reluctant to form the (+V) state and Re(+V) compounds readily hydrolysed by

water and at the same time they disproportionate.

3Re Cl H O(V) 5 2+ 8 +HReO ReO (H O)4 2 2 nReCl5 is dimeric. The oxide Re2O5 is also known.

(+IV) state

It is second most stable state for Tc and Re.

The oxides TcO2 and ReO2 can be prepared by using the following methods.

(1) By burning th e metal in a limited supply of oxygen.

M + O2 MO2(2) By hea t ing M2O7 with M.

M O + M2 7 MO2

(3) Therma l decomposition of NH4MO4

(4) By reducing TcO4 and ReO4

with Zn/HCl. The hydrated oxides so form can be

dehydrated by heating.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 1 0 1

TcO2 and ReO2 are black and brown respectively. TcO2 is insoluble in alkali but ReO2

reacts with fused alkali, forming ReO32. Both the oxides have distorted rutile structure.

The su lphides TcS2 and ReS2 are k nown. These are obtained by heating hepta sulphides

with sulphur in vacuum . They have the advant age over h eterogenous platinum metal catlysts

ion that they are poisoned by sulphur compounds.

Halides

The kn own h alides of the element s ar e shown in Ta ble 2.9(b). TcCl4 has a solid stru ctur e.

It is permagnetic and there is metal-metal bonds. All four ReX4 halides are known. ReCl4can be prepared by the action of SOCl2 on ReO2.XH 2O. However, the other methods are

more important.

2ReCl SbCl ReCl SbCl

3ReCl Re Cl ReCl

2ReCl Cl C = CCl ReCl C Cl

4 3 4 5

5 3 9 4

5 2 2 4 2 6

+ +

+

+ +

2

6

2

Table 2.9 Halides of Group 7

S tate Fluorides Chlorides Brom ides Iod ides

+7 ReF 2yellow

mp 48.3, bp 73.7

+6 TcF6 TcCl6yellow green

mp 37.4, bp 55.3 mp 25

ReF 6 ReCl6yellow red-green

mp 18.5, bp 33.7 mp 29 (dichroic)

+5 TcF5

yellow

mp 50, bp (d)

ReF 5 ReCl5 ReBr5yellow-green brown-black dark brown

mp 48, mp 220 (d 100)

bp(extrap) 221

+4

TcCl4 (?TcBr 4)

red(subl > 300) (red-brown)

ReF 4 ReCl4 ReBr4 ReI4blue purple-black da rk r ed black

(subl > 300) (d 300) (d above r t )

+3

[ReCl3]3 [ReBr3]3 [ReI3]3dark red red-brown lust rous bla ck

(subl 500) (d) (d on warming)

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1 0 2 CHE MIS TRY

It is metastable and reactive and has a structure based on cubic close packed chlorine

atoms. The adjacent octahdera have a common face and the Re-Re distance is only 237 pm,indicative of a bond.

Man y complexes ar e kn own [ReCl6]2 (TcCl6)

2 are octah edral an d obtained by reduction

of KMO4.

MO4

+KI

Con. HCl

6MCl 2

ReCl6 is hydrolysed by water

ReCl H O6 22

+ ReO (H O)2 2. n

A number of other halo-complexes are known of these metals ion. For example [MF 6]2

[MBr6]2 and [MI

6]2. They can be prepared by the reaction of hexachlroide with appropriate

ha logen a cid. ReF6 is stable in wat er. On tr eatm ent of [MI6]2 with KCN. Cyano complexes

i.e., [Tc(CN)6]2 and [Re(CN)6]

2 are formed. However Re is oxidized by CN and forms

[ReV(CN)8]3.

+(III) States

Tc(III) is unstable but Re2O3.(H 2O)n and the heavier halides are known. The chloride,

bromide, and iodide have been str uctura lly char acterized an d th eir tr ue m olecular formu las

are Re3X9. They are not isomorphous but all consist of Re3X9 units connected by sharing of

X atoms as shown in Fig. 2.7b. Re3X9 units are metal-atom cluster compounds. The Re-Re

dista nces are 248 pm an d th e M-M bonds a re order 2. The simplest explana tion of th e double

bonds between Re atoms is that each Re has nine atomic orbitals available for bonding (five

d, one s and three p). The metal is surrounded by five ligands, leaving four unused orbitals.Assuming the unused orbitals are pure d or mainly d in character, there are 12 atomic

orbitals for Re-Re bonding. If these are delocalized over the three atoms there will be six

bonding MOs, corresponding t o double bonds between each of the t hr ee Re at oms. Since all

the electrons are paired, the clusters should be diamagnetic and this has been proved

experimently.

Fig. 2.7(b) The cluster structure of Re3Cl9.

When Re3Cl9 or Re3Br 9 are dissolve in con. HCl or HBr, one, two or three halides ions

may be attached to the isolated Re3X9 unit. For example K+[Re 3Br 10], K2[Re 3Br 11] and

K3 [Re3Br 12].

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 1 0 3

Re(II) is not ionic and is formed only few complexes such as [Re (pyridine)2Cl2], ReCl2

(dians) and ReCl(N2) (diphos)2]+.

RUTHENIUM AND OSMIUM

Ru and Os are very rare. They are found in metallic state together with the platinum

metals and the coinage metals (Cu, Ag and Au). The main sources are traces found in NiS/

CuS ores mined in South Africa, Canada and USSR. The largest sources are South Africa

45%, the USSR 44%, Canada 4%, the USA 2% and Japan 0.8%.

Abun dan ce of th e elements in t he ear th s cru st by weight is Ru (.0001 ppm) an d OS

(0.005 ppm).

Ru and Os are obtained from the anode slime which accumulates in the electrolytic

refining of Ni. This contains a mixture of platinum metals together with Ag and Au. The

elements Pd, Pt, Ag and Au are dissolved in aquaregia and the residue contains Ru, Os, Rhand Ir. After a complex separaion Ru and Os are obtained as powders and powder forming

techniques are used to give the massive metal.

Properties and Uses

Ru and Os are unaffected by mineral acids below ~ 100C and are best dissolved by

alkaline oxidizing fusion for example NaOH + Na 2O2, KClO3 etc.

Os is oxidized to OsO4 by aqua-regia. The effect of lanthanide contraction is less

pronounced in this part of the periodic table. Therefore, the similarities between the second

and third row elements are not so close as one found in the earlier transition groups.

Density of Os 22.57 g cm 3 and Ir is 22.61 g cm 3. These elements are both scarce and

expensive. Ru is used to alloy with Pd and Pt, and Os is also used to make hard alloys. All

these metals have specific catalyst properties.

Compounds

Ru an d Os form RuO4 andOsO4 which are in the (+VIII) state. Ru(III) and Os(+IV) are

the most stable states. Ru(+V), Os(VI) and Os(VIII) are also reasonably stable.

Thus the usual trend is observed that on descending a group, the higher oxidation

states become more stable.

(+VIII) state

OsO4 is prepared either by burning finely divided metal in O2, or by treating it with

concentr ated HNO3. RuO4 is prepared by oxidation with perma gnet or bromate in H 2SO 4. It

is less stable. Both are yellow coloured volatile solids with melting points of 25C and 40C

respectively. Both th e oxides ar e toxic, smell like ozone an d a re st rongly oxidizing. They ar e

slightly soluble in water but are soluble in CCl4. Aqueous solution of OsO4 are used as a

biological stain because the organic matter reduces it to black OsO 2 or Os. OsO4 vapour is

harmful to the eyes for this reason. OsO 4 is also used in organic chemistry to add double

bonds and give us glycols. The tetraoxides do not show basic properties and HCl reduces to

trans [OsO2Cl4]2, [OsCl6]

2 and [Os2OCl10]2. RuO4 dissolves in NaOH solution and liberats

O2. Ru (VIII) reduced to peruthenate (+VII) ion and ruthenate (+V) ion.

4RuO4 + 4OH 4RuO4

+ 2H 2O + O2

4RuO4 + 4OH 4RuO42 + 2H 2O + O2

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1 0 4 CHE MIS TRY

When RuO4 is heated with HCl (g) and Cl2 hygroscopic crystals of (H 3O)2 [RuO2Cl4] is

produced. The ion is hydrolysed by water.

2 2Cs RuO Cl H O RuO RuO 4CsCl + 4HCl2 3 4 2 4 2+ + +

If RuO4 in dil. H 2SO 4 reduced with N a 2SO3 green solution [RuO2(SO4)2]2 are obtained.

It can also be formed by mixing Ru IV solution with RuO4 in dil H 2SO 4.

Ru Ru RuIV VIII VI+

However, OsO4 is more stable and is not reduced by NaOH, but form tra ns Osmet ic ion.

OsO 2OH OsO OH)4 4 2+ [ ( ]2

This reacts with NH3 to give unusual nitride complex [OsO3N] and reduced trans

[OsO2(OH 4]2 by E + OH (+VI and +VII) state.

As discussed above the fusion of Ru and its compounds with alkali give RuVII

O4

orRu VIO4

2 ion. The tetrahedral RuO42 ion is moderately stable in alkali solutions. It is

paramagnetic with two unpaired electrons. Some reactions are given in Fig. 2.8a, RuF 6 is

the highest halide of Ru. It is prepared by heating the elements and quenching. RuF 6 is

unstable, but in contrast OsF 6 is stable.

C l

VariousRu spec iesIV

K [ Ru O C l ]2 2 10HCl KCl

e

[RuCl ]62

Cl2

H g[R u Cl (H ]5 2

2O

yellow re d

Zn

H (Pt)2

[R u C l ]5 122

blue

R uO 4

R u s pec iesIV

HClO 4H O 22 H C l

H Cl

evap

C aC l2

Cs [RuO Cl ]2 42re d Yellow

Ru complexesIV

dilHC l boil

Ru C l 3 H O 3 2.

trans- [RuCl (H O) ]4 2 2

dark green

O 2 e

Conc .H Cl

H O(fast)

2

[RuCl ]63

re d

trans- [RuCl (H O) ]2 2 4+

yellow

Fig. 2.8(a) Some reactions of ruthenium chloro complexes.

(+IV) States

By burning Ru or Os metals in air to give RuO2 as a blue-black solid and OsO2 as acoppery coloured solid. Os also forms a stable tetrafluoride and tetrachloride.

(+III) State

A number of Ru(III) compluxes are known as compared to Os (III) complexes. Both

elements form [MII I (NH 3)6]3+ and Ru forms a range of mixed halogen-ammonia complexes.

If RuO4 is added to con. HCl and evaporat ed a dar k red m ater ial formu lated RuCl3.3H2O is

formed. Reactions of RuCl3.3H2O are given in Fig. 2.8b. This is readily soluble in water. It

contain not only Ru II I species but also some polynuclear Ru IV species. Ru (III) forms basic

acetat e [Ru 3O(CH3COO)6L3]+. The chloro species of Ru (III) cata lyse th e hydr at ion of alkynes.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 1 0 5

RuH (PPh)2 3(N )2N 2

N2

AlEt3

R uH (PPh )2 43PPh3

RuHCl(PPh )33

HN Et

2

3

RCN

SCNM

e

2

2

RuCl (RCN) (PPh )22 2 3

Ru(S CNMe ) (PPh )22 2 3

Ru(ocac) (PPh )22 3NoBHEtOH

4 acacH

RuCl (CO)(PPh )22 3

PPh3

PPh 3

EtOH

N TSEtOH

CO.alcPPh 3

boil

boil

RuCl . 3H O3 2

PEt P

h2

boil,a

lc

Ru Cl (PEt Ph) ]C l2 3 2 6

EtOH, KOH

PEtPh2EtO

H

RuHCl(CO)(PEt Ph)2 3

mer-RuCl (PEt Ph)3 2 3

EtOH

RuCl (NH ) (PEt Ph)2 3 2 3

RuCl (NO)(PPh )3 3 2

Zn, THF

RuCl (NO)(PPh )3 2

Ru(CO) (PPh )23 3

PPH

,alc

3

MaOH

,CO

Cl2ZnTH F

R u (C O )3 12

RuHCl(CO)(PPh )33

N TS

RuCl (CO)(NO)(PPh )3 2

RuCl (PPh )2 33

Fig. 2.8(b) Some reactions of RuCl3.3H.

Tertiary Phosphine Complexes

Both element s ha ve an extenisve chemistry with these -acid ligands. Some represen ta tiverea ctions ar e shown for Ru in F ig. 2.8(c). The complexes RuH Cl (PPh 3)3 and RuH 2(PPh 3)3 are

of interest in that they are highly active catalysts for the selective homogeneous hydrogenation

of alkenes.

RuH (N ) (PPh )2 2 3 3N 2

AlEt3RuClH(PPh )3 3

C ORuClH(CO)(PPh )3 3

N 2

R uH (PPh )2 3 4PPh 3

NaBHbenzene

4

H . Netbenzene

2 3

RuHCl (PPh )3 32acacH

Ru(acac) (PPh )3 32

PPh

EtOH

3

RuCl . (3H O)3 2

PPhEt2M eO C H C H O H

boil2 2

[Ru C l (PPhEt ) ]C l22 3 6

PPhEt2

EtOHmer-RuCl (PPhEt )2 2 3

Fig. 2.8(c) Some reactions of tertiary phosphine complexes of ruthenium. Note that the use of

different phosphines may give different products.

A characteristic of ruthenium complex ammine chemistry is the formation of highly red

or brown coloured species usually known as ruthenium reds. If commercial ruthenium

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1 0 6 CHE MIS TRY

chloride is treated with NH 3 in air for several days, a red solution is obtained. It can also

be prepared by reducing Ru II I chloro complexes by etha nol and then tr eated with amm oniaat 90C.

The structure of red species appears to be that of linear trinuclear ion with oxygen

bridges between the metal atoms.

[(NH3)5 RuII IORu II I (NH3)4ORu (NH3)5]

6+

Reaction of ammine complexes of Ru are given in Fig. 2.9.

H 2

[RuCl (H O)(CO)]4 22

[Ru(NH ) (N O)]3 5 22+

C r2+ N O2

N 2

H O2

boil

6M HCl

C l2

NH OH4Cl (long)2

[RuCl (CO)]52

C ON H C l

aq .4

N H2 4

RuC l . 3H O 3 2

boilZn

N H O HN H C l

4

4

SO ,HCl

2

H , NO+[Ru(NH ) ]3 6

3+

[Ru(NH ) N ]3 5 22+

C l25

(brief)

2

[Ru(NH ) ]3 62+

[Ru(NH ) (NO)]3 53+

NH OH4

C O

R CN[Ru(NH ) H O ]3 5 2

2+

[Ru(NH ) Cl ]3 52+

t rans - [Ru(NH ) Cl(NO)]3 5

Zn + RCN

N H O H4

HClO 4

H O

[Ru(NH ) (CO)]3 52+

[Ru(NH ) (NCR)]3 52+

[Ru(NH ) (NO)(OH)]3 42+

[Ru(NH ) (H O)]3 5 23+

Fig. 2.9 Some reactions of ruthenium ammines in aqueous solutions.

Low oxidation states

The (II) state is rare but found in carbonyl Ru(CO)42. Zero valent state occurs in

carbonyls Ru (CO)5 Os(CO)5 Os 2(CO)9, Ru 3(CO)2, OS3(CO)2. (Fig. 2.10)

Fig. 2.10(a) Structures of carbonyls .

RHODIUM AND IRIDIUM

The elements ha ve odd at omic numbers a nd h ave low abunda nce in t he eart hs crust.

Rh (0.0001 ppm) and Ir (0.001 ppm) are rarely found in earth crust. Trace amounts of Rh

and Ir are found in the metallic state together with the platinum metals and the coinage

meta ls in th e NiS/CuS ores mixed in th e South Africa, Cana da a nd USSR. The largest source

were South Africa 45% the USSR 44% Canada 4% the USA 2% and Japan 0.8%. Rh and Ir

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 1 0 7

are obtained from the anode slime which accumulates in electrolytic refining of Ni. This

contains a mixture of the platinum metals together with Ag and Au. The element Pd, Pt,Ag and Au are dissolved in aqua-regia and the residue contains Ru, Os, Rh and Ir. After a

complex separation Rh and Ir are obtained as powders. (Extraction scheme of Rhodium and

Iridium is given in Fig. 2.10(b)).

Fig. 2.10(b) Flow diagram for the extraction of rhodium and iridium.

Properties and uses

Rh an d Ir a re ha rd m etals. They ar e noble and un reactive. Ir has h ighest density, 22.61

gcm 3. Rh and Ir are r esistan t t o acids, but react with O2 and halogens at high temperature.

Both the elements form a large number of coordination compounds.

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1 0 8 CHE MIS TRY

Compounds

The trend for the elements in the second half of the d-block not to use all their outer

electrons for bonding in the maximum oxidation state is continued. The most stable state

for these elements are Rh (+III), Ir (+III) and Ir (+IV). Simple ionic compounds of these

elements are uncommon. The halides formed are shown in Table 2.10.

Table 2.10 Halides of rhodiu m a nd ir idum (mpC)

Oxida t ion st a t e F luor ides Ch lor ides Br om ides Iodides

+6 RhF6

black (70)

IrF6

yellow (44)

bp 53

+5 [RhF5]4

dark red

[IrF5]4

yellow (104)

+4 RhF4

purple-red

IrF4 IrCl4 ? IrBr 4 ? Ir I4 ?

dark brown

+3 RhF3

RHCl3

RhBr3

RhI3

red red red-brown bla ck

IrF3 IrCl3 IrBr3 Ir I3

black red red-brown dark brown

(+V) and (+VI) states

Rh (+V) and Ir (+V) exist as pent afluorides (RhF 5)4 and (IrF 5)4. These are very reactive

and are readily hydrolysed. The only complexes known are Cs[RhF 6] and Cr[IrF 6].

Rh F6 and IrF 6 are the only compounds known in (+VI) oxidation states. They can be

prepared by direct reaction. These compounds are not stable. However, IrF 6 is more stable

than RhF 6.

(+IV) State

Ir (IV) is one of the most stable states but Rh (+IV) is unstable and forms few compounds.

Both metals form tetrafluorides. RhF 4 can be prepared from RhCl3 and BrF 3. IrCl4 is not

very stable. IrO2 is form ed by burn ing the meta l in air, but RhO2 is only formed by st rongly

oxidizing Rh (+III) in alkaline solution, for example NaBiO 3. Rhodium form s only few complexes

for example K2[RhF 6] and K2[RhCl6]. The complexes react with water to form RhO 2 and

liberate O2. However, a number of complexes of iridium are known. For example K 2[IrCl6],

[IrCl3(H2O3)Cl], [IrCl4(H2O)2] and K [IrCl5.H 2O]. The oxalat e complex [Ir (ox)3]2 can be r esolved

in d and l optical isomes.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 1 0 9

(+III) state

Almost all halides of Rh and Ir are known in trivalent state. RhF 3 is prepared by

fluorinating RhCl3 and IrF 3 by reducing IrF 6 with Ir. Other halides are prepared by direct

rea ction. All the ha lides are insoluble in water an d also un rea ctive. Probably th ey have layer

latt ice. Rh2O3 is prepared by burning the metal in air however, hydrated Ir 2O3 is prepared

with difficulty. It is prepared by adding alkali to Ir (III) solution under an inert atmosphere

because it oxidizes to IrO2.

A adequate number of the complexes of Rh (III) and Ir (III) are knwon. For example

[RhCl6]3, [Rh(H2O)6]

3+ an d [Rh(NH 3)6]3+. The chloro complexes are prepared by heating finely

divided Rh or Ir with alkali metal chloride and chlorine.

2Rh + 6NaCl + 3Cl2 2N a 3 [RhCl6]

Hydra ted complexes [Rh(Cl6)

5

. 12H 2O is of Red colour. (Red)On boiling with water it gives [Rh(H 2O)6]

3+. On addition of NaOH it gives Rh 2O3.H 2O.

If a limited amount of HCl in added to [Rh(H 2O)6]3+ then RhCl3.3H2O is obtained however,

if excess HCl is added then [RhCl6]2 is regenerated. The complexes are diamagnetic end of

octa hedr al geomet ry. RhCl3.3H 2O exists in two fac and mer isomers. Reactions of RhCl3.3H2O

ar e given in Fig. 2.11(a) Few complexes of these element s ar e known which ar e not octa hedra l

for example [RhBr 5]2 and [RhBr 7]

4. Few complexes are also known having M-M bond.

[(R3As)3 Rh(HgCl)]+ Cl (Rh-Hg) an d [Ir2Cl6(SnCl3)4]

4

Fig. 2.11(a) Some reactions of rhodium trichloride.

Rh (III) and I r (III) form basic aceta tes [Rh3O(CH3COO)6L3]+ which ha ve unusual stru ctur e.

Rh at oms form a t riangle with an O atom a t t he centr e. The six acetat e groups a ct a s bridges

between the Rh atomstwo acetate groups across each edge of the triangle. Thus each Rh

atom linked to four acetate groups and the central O and the sixth position of octahedron

is occupied by water or another ligand. The magnetic moment is reduced due to partial

pairing ofd-electrons on the three metal atoms by means of d -p bonding through O. A

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1 1 0 CHE MIS TRY

nu mber of hydr ide complexes [Rh(R3P)3.HCl2]2+ and [Ir(R3P)3.H.Cl2]

2+, [Ir(R3P)3.H 2Cl]4+ and

[Ir(R3P)3H 3]6+ ar e also known.

(+II) State

Few compounds ar e kn own of Rh(II) an d Ir (II) becau se on redu ction of Rh (III) an d Ir (III)

gives metal instead of Rh(II) or Ir(II). The existance of RhO is uncertain and IrCl2 exists as

a polymer. On warming the solution of R-HCl3.3H2O of sodium acetate in methanol a

dimeric diacetate is formed HOH 3C.Rh.(R.COO)4.Rh.CH 3OH. The four carboxylate groups

bridge the two Rh atoms. This has a M-M bond of 239 pm. Some complexes of phosphine

are also known. Rhodocene [Rh II (n 5C 5H 5)2] is a less stable compound. It tends to dimerize.

(+I) State

There is a fairly extensive chemistry of Rh (+I) and Ir (+I) complexes with -bonding

ligands such as CO, PH3, PR3 and alkene. Two are the important compounds of Rh (I) andIr(I) i.e. [Rh(Cl) (PP h 3)3] Wilkinson cata lyst and tra ns [IrClCO(PPh 3)2].

1. Wilkinson Catalyst. This red-violet compound which is readily obtained by refluxing

eth an olic RhCl3.3H2O. Fig. 2.11 (b) with an excess P Ph 3 was discovered in 1965. It u nder goes

a variety of reactions, most of which involve either replacement of a phosphine ligand with

CO, CS, C2H4 or O2 giving tr an s products or oxidat ive addit ion with H 2 or MeI to form RhII I.

However, its importance arises from its effectiveness as a catalyst for highly selective

hydrogenations of complicated organic molecules which are of great importance in the

phar maceutical industries.

Rh(acac)(C H )2 4 2

acac

[Rh C l (C H ) ]2 42 2 2C H2 4EtOH

RhCl .3H O3 2

[Rh Cl (SnCl ) ]42 2 3 4

SnClEtOH

3

CO, 1 atm

100[Rh(CO) Cl]2 2

Rh(acac)(CO) 2H ,CO, 100 atm.2

60

Rh (CO)2 12

Rh (CO)6 16

h5-C H Rh(CO)5 5 2

C H N o5 5

R SHRh (SR) (CO)2 2 4

ExcessPPh ,EtOH

3

RhCl(PPh )3 3RhH(PPh )3 4N H2 4

PPh3

C H2 4

RhCl(C H )(PPh )2 4 3 2 RhCl(CS)(PPh )3 2 RhCl(CH )(CO)(PPh )3 3 2 Rh(C F H)(CO)(PPh )2 4 3 2

RhH(CO)(PPh )3 3RhCl(CO)(PPh )3 2

PPh3

CO, RCHO

RCOCl, etc.

CO, BH4

C S ,PPh ,MeOH

2

3 CH I3 C F2 4

EtOH

2.11(b) Some preparations and reactions of rhodium (I) compounds.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 1 1 1

The simplified mechanism is given in Fig. 2.12.

Fig. 2.12 The catalytic cycle for the hydrogenation of an alkene, catalysed by [RhCI(PPh 3)3] in

benzene : possible coordination of solvent molecules has been ignored and the ligand PPh 3 has

been represented as P throughout, for clarity.

The complex trans [Rh(CO) (H) (PPh 3)3] has also been used in-oxo process and is an

important catalyst in the hydrogenation of alkenes (Fig. 2.13).

Fig. 2.13 The catalytic cycle for the hydroformylation of an alkene catalysed by trans-

[RhH(CO)(PPh 3)3] . The tertiary phosphine ligand has been represented as P throughout.

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1 1 2 CHE MIS TRY

Vaskas compound

This yellow compound can be prepared [IrCl(CO)(PPh 3)2] by the reaction of triphenyl

phosphine and IrCl3 in solvent such as 2-methoxy ethanol which acts both as reducing agent

an d su pplier of CO. It was discovered in 1961 by L. Vaska a nd J .W. di Luzio and r ecognized

as ideal m at erial for t he st udy of oxidative addit ion r eactions, since it pr oducts ore genera lly

stable and readily characterized. It is containly the most throughly investigated compound

of Ir I. It forms octahedral Ir II I complexes in oxidative addition reactions with H 2, Cl2, HX,

MeI and RCO3H. Spectr al studies indicate t hat in all cases the ph osphine ligands ar e tra ns

to each other. The 4 remaining ligands (Cl, CO and two components of the reactant,)

therefore lie in plane and 3 isomers are possible.

It readily absorbs O2 an d becomes ora nge colour ed. The O2 may be removed by flusing

with N2. This reversible oxygena tion ha s been st udied as a model for oxygen carr ying ability

of haemoglobin.

Lower oxidation states

Numerous compounds of Rh and Ir are known in which the formal oxidation state of

metal is Zero, 1 or even lower. Many these compounds contain CO, CN or RNC as ligands.

The (1) state is found in the tetrahedral complexes [Rh(CO)4 and K[Ir(PF3)4]. The zero

oxidation sta te occurs a s carbonyls such as Rh 4(CO)12 and Ir 4 (CO)12. All have M-M bond s an d

contain a cluster of four metal atoms Fig. 2.14. Rh 6(CO)11 is also known and has usual

structure.

Fig. 2.14 Molecular structures of some binary carbonyls of Rh and Ir. (a) Ir 4(CO)12 . (b) M4(CO)12 ,

M = CO, Rh (c) Rh 6 (CO)16.

PALLADIUM AND PLATINUM

Pd and Pt are rare elements, but they are appreciably more abundant than the other

platinum group meta ls. Even th rough Pd is slightly more abundan t th an P t, but production

of Pt is greater than that of Pd.

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 1 1 3

The platinum group m etals occur as t races in th e sulphide ores of Cu an d Ni. They are

obtained as concentrates as anode sludge from electrolyte process for the major metals. Theplatinum group metals are also obtained from the Cu/Ni alloy produced in the separation

of the sulphide matte of Cu 2S/Ni2S3. The sulphide matte is cooled slowly giving an upper

slivery layer of Cu2S and a lower black layer of Ni2S3 which can be separated mechanically.

A small amount of metallic Cu/Ni alloy is also formed. This dissolve any of the paltinum

group metals present, and is used as a source of these rare and expensive elements.

Separat ion of the platinium meta ls is complex, but in th e last st ages (NH 4)2 [PtCl6] and

[Pd(NH 3)2Cl2] ar e ignited t o given th e respective met als. The meta ls are obtained a s powders

or sponges and are fabricated into solids objects by sintering. Extraction scheme for Pd and

Pt is given in Fig. 2.15.

Pt metal

concentrates

Aquaregia

Soln of HAuClH P d C l H P tC l

4

2 4 2 6

FeCl2

Soln of H PdCland H PtC l

2 4

2 6

N H C l4

ppt of (NH ) PtCl4 2 6

Ignite

Impute Pt sponge

Dissolve inaqua regia,evaporatewith NaCl

Impure Na PtC l2 6

Aq NaBrO 3

Soln of Na PtC l2 6

N H C l4

ppt of (NH ) PtCl4 2 6

Ignite

Pt metal

Rh/Ir hydroxides

Pt metal

ppt of pure [Pd(NH ) Cl ]3 2 2

Ignite

Dissolve inNH OH, ppt withhydrochloricacid

4

ppt of impure [Pd(NH ) C l ]3 2 2

Excess NH OH,then hydrochlor icacid

4

Soln. of H PdCl2 4

ppt of Au

Residue of Ru, Rh,Ir, AgCl

Fig. 2.15 Flow diagram for the extraction of palladium and platinum.

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1 1 4 CHE MIS TRY

Properties and uses

Palladium and platinum are both rare and expensive. They are noble and not very

reactive, but are slightly more reactive than the other platinum group m etals. Both are u sed

as catalyst. The most common oxidation state as Pd(II) and Pt(II) and Pt(IV). These are not

ionic. Pd dissolve slowly in concentr at ed H Cl in t he p resence of O2 or Cl2, and fairly readily

in conn . HNO3 giving [PdIV (NO3)2 (OH)2]. Pt is the most resistant to acids, but dissovle in

aqua-regia, giving chloroplatinic acid H 2[PtCl6]. Both P d and P t ar e rapidly att acked by fused

alka li meta l oxides and per oxides for example Na 2O and Na 2O2. Red heat Pd r eacts with F 2,

Cl2 and O2. Pt is less reactive but at red heat it reacts with F 2, and at a high temperature

and pressure it reacts with O2. Both absorb hydrogen. The amount absorbed depend on the

physical stat e of gaseu s H2. Pt (II) an d Pt (IV) form a n extr emely large nu mber of complexes.

Pd and Pt find extensive chemical uses as catalyst.

A new increasing use of Pt is three way catalytic convertors. PdCl 2 is used in Wakerprocess for converting C2H 4 to CH3CHO. Pd is used for hydrogenations such as phenol to

cyclohexanone and also for dehydrogenations. Pt is very important as a catalyst in the oil

indu str y in th e reform ing of hydrocar bon. Pt /PtO is used a s Adams catalyst for redu ctions.

Pt was used as cata lyst for m anu facture of H2SO 4 (Contact Process) and HNO3 (Ostwalds)

Pt is also used to mak e appar atu s to han dle HF a nd t o seal into soda glass to allow electrical

connections to pass through the glass. This is important in making electrodes, thermionic

valves etc.

(+V) and (+VI) States

These are only found for Pt, and are rare. The (+V) state is represented by PtF 5, which

is tetrameric and has the same structure as many transition metal fluorides. The [Pt F 6]

ion also contains Pt (+V) and was first formed by reacting Pt F 6 with O2 to give compoundO2

+ [Pt F 6]. The only examples of the (+VI) state which are known for certain are PtO3 and

PtF6.

(+IV) State

Pd oxide (PdO2) is known only in hydrated form. However, Pt forms stable oxides in

both anh ydrous a nd h ydrated forms. The anh ydrous oxide is insoluble but the hydrat ed form

dissolves in acids and alkalies. Only tetrafluoride of palladium (PdF4) is known whereas all

th e halides of Pt (Pt X4) are known. Direct reaction of Pd and F 2 gives P dF 3 [P dII (Pd IV F 6)]

and Pd F 4 whereas Pt gives PtF 4, Pt F 5 and Pt F 6. PtCl4 is formed either direct reaction

or by heating H 2[PtCl6].

P t H [PtCl ] P tCl 2HCl

aqua-regia

2 6

Heat

4 +Pd (+IV) forms a few octahedral complexes [Pd X6]

2 where X = F, Cl or Br and [Pd X4(NH 3)2]. They are reactive. [PdF 6]

2 hydrolyses rapidily in water whereas t he other h alides

complexes are decomposed by hot water, giving [Pd II X4]2 and halogen.

However, Pt (+IV) form s a la rge nu mber of octa hedr al compexes for exam ple [Pt(NH 3)6]4+,

[Pt(NH3)5Cl]3+, [Pt(NH 3)4Cl2]

2+ and [PtCl6]2. Similar series of complexes exist with a wide

range of ligands such as F , Cl, Br , I, OH, acetylacetone, CN etc. Chloroplatinic acid is

commercially the most common Pt compound. It is prepared either by dissolving Pt in aqua-

regia or conn-HCl with Cl2. It forms red crystals of H 2[PtCl6].2H 2O. Sodium or potassium

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S E CO ND AND TH IRD TRANS ITIO N S E RIE S 1 1 5

salts ar e common st art ing mat erial for m aking other plat inum (IV) compounds. Platinum is

unusual in that it forms alkyl derivatives by a Grignard reaction.

4Pt Cl4 + 12CH3MgI [(CH 3)3 Pt I ]4 + 8MgCl2 + 4MgI2But actual compound is [(CH 3)3 Pt(OH)4.

(+III) State

The compounds of Pd (III) are rare and it is doubtful that the Pt (III) exists. The

complexes N a[PdF 4] and NaK2 [PdF6] have been r epeat ed. Some compounds of mixed oxidation

states are known for example Pd 2+ [Pd F 6] etc.

(+II) State

Pd (II) and Pt (II) exist a s oxides, halides, nitrat es and sulphat es. Genera lly anh ydrous

solids are not ionic. PdO is found in ah ydrous form wherea s Pt O is only known as an un sta blehydrat ed form. E xcept P tF 2, all the ha lides are kn own. PdF 2 is ionic. All the complexes for

example [Pd (OH)4]2 ar e diama gnetic. It is du e to large cryst al field splitting ener gy. All the

dihalides are molecular and are diamagnetic. The chloride of Pd (II) and Pt are prepard from

the elements. They exist in two different forms depending upon the conditions used.

Pd + Cl2>550

C

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1 1 6 CHE MIS TRY

An imporant reaction occurs between PdCl2 and alkenes. With ethene complexes such

[Pd(C2H4)Cl3], [Pd(C2H4)Cl2]2 and [Pd(C2H 4)2Cl2] are form ed. Similar compoun ds ar e knownfor Pt. For example Zeises salt K[Pt(C2H 4)Cl3]. H2O yellow coloured compound. It is known

since 1825. It is of square planar geometry with 3Cl at the corners and H 2C = CH 2 a t the

other corner. Bonding in such type of complexes will be discussed latter.

The very importnat medical use of Pt (II) compounds is the use of cis-isomer of

[Pt(NH3)2Cl2] as an anticancer drug for treating several types of malignant tumours. The

trans-isomer is ineffective. The cis isomer is called cis-platin and is highly toxic.

The complexes between PdCl2 and alkenes are decomposed by water, giving ethanal

C2H 4 + PdCl2 + H 2O CH 3CHO + Pd + 2HCl

This reaction forms the basic of the Waker Proces for the production of CH 3CHO. The

Pd is converted back into PdCl2 in situ by CuCl2.

Pd + 2CuCl2 PdCl2 + 2CuCl

PdCl2 catalyse the reaction between ethene, CO and H 2O.

CH 2 = CH 2 + CO + H 2 CH 3CH 2COOH

Magnus green salt h as th e formu la [Pd(NH 3)4]2+, (Pt Cl4)

2+ and th e square planar anion

an d cat ions a re sta cked on top of each other. Similar st ru cture is reported for K2[Pt(CN)4].3H2O.

Lower Oxidation State

Pd and Pt do not form complexes with phosphines such as [Pd(PPh 3)3], [Pt(PPh3)3],

[Pd(PPh3)4] and [Pt(PPh 3)4]. However [Pd(CO) (PPh 3)3] and [Pt(CO)(PPh 3)2] are known. A

series of clust er compoun ds su ch as [Pt 3(CO)6]n2 ar e form ed by reducing [PdCl6]

2 in alkaline

solution under an atmosphere of CO. Similar complexes of Pd are not known.

SILVER AND GOLD

Silver is found a s sulph ide ores Ag2S (argent ite), as th e chloride AgCl (horn silver) an d

as the native metal. There are three process of extraction.

1. It is obtained m ostly as a biproduct from the extra ction of Cu, Pb or Zn. It m ay be

extracted from the anode slime formed in the electrolytic refining of Cu and Zn.

2. Zinc is used t o extract silver by solvent extraction from molten lead in Pa rkes

process.

3. Silver and gold are extracted by making soluble cyanide complexes.

2Ag2S + 8CN + O2 + 2H 2O 4Ag(CN)2

+ 2S + 4OH

2Ag(CN)2 + Zn 2Ag + [Zn(CN)4]2

Historically gold has been found as lumps of metal in the ground called nuggets. Gold

occurs mainly as grains of metal disseminated in quartz veins. Many of these rocks have

weathered with t ime. The gold and powdered rock ar e washed away in str eams a nd accumu late

as sediments in river beds. The grains of gold can be separated from silica by panning i.e.

swirling them both with water. Gold is very dense and settles to the bottom.

Now a days rocks containing traces of gold are crushed and extracted either with

mer cury or with sodium cyanide. Gold dissolve in mu rcur y forming an am algam . By distillation

gold is recovered.

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It can also be extracted by cyanide process

4Au + 8NaCN + 2H 2O + O2 4Na[Au(CN)2] + 4NaOH Complex

This complex is soluble and separated from the rest of the rock. The gold can be

obtained by adding Zn powder.

[Au(CN)2] + Zn Zn(CN)4 + 2Au (s)

The m ain u se of Ag ar e as AgCl and AgBr in ph otograp hic emulsions, for jewellery and

silver ornaments, for batteries and silvering mirrors. Gold is used in jewellery. It is alloyed

with a mixture of Cu and Ag. These alloy retain the golden colour but are harder. The

proportion of gold in the alloy is expressed in carats. Pure gold is 24 carats and contain

9/24, 18/24, 22/24 carats gold respectively.

Small amounts of gold are used to make corrosion-free electrical contacts. For exampleon comput er boards, thin film of this metal reflects u nwant ed heat from sun in th e summ er.

The liquid gold is used to decorate picture frames, glass and ceramic ornaments.

The met als in t he group ha ve the h ighest electrical and ther mal conductivities known.

They are the most melleable and ductile. The higher enthalpy of sublimation and higher

ionization energy are the reason why Ag and Au tend to be unreactive i.e. show noble

character. The metals have posit ive E values and are thus below hydrogen in the

electrochemical series. Thus they do not react with water or liberate H 2 with acids. Ag

dissolve in dil. HNO3 an d hot Con. H2SO 4 wherea s Au is inert to all acids except a qua -regia.

2Ag (s) + 2H 2SO4 (l) Ag2SO 4 (s) + SO2 (g) + 2H 2O (l)

3Ag (s) + 4HNO3 3AgNO3 (aq) + 2H 2O (l) + NO (g)

The HNO3 acts as a n oxidizing agent a nd t he chloride ions as complexing agent . Ag andAu both ar e inert toward O2. Ag reacts with H2S to give sulphide but Au does not rea ct with

H2S.

2Ag + H 2S Ag2S + H 2Compounds. The m ost common oxidat ion st at es ar e Ag (+1) an d Au (+III) an d th e two

elements differ widely in their Chemistries. The univalent Au disproportionate in the water.

(+III) State

A black oxide Ag2O3 is obta ined by a nodic oxidat ion of Ag+ in a lkaline solution. H owever,

in the presence of complexing agent in alkaline solution AgII I complexes are obtained. The

separation of AgI and AgII I can be made by the reaction.

4AgO + 6KOH + 4KIO3 2K5H2 [Ag(IO6)2]The complexes of AgII I are also prepared by the oxidation of Ag2O in strongly alkaline

solut ion by S2O82. For example K6H[Ag(IO6)2].10H2O and Na 6H 3[Ag(TeO6)2].18H2O.

Salts of the tetrafluoroaurate ion are obtained by the action of BrF 3 on a mixtu re of gold

and an alkali chloride, K [AuF 4] is isomorphous with K [BrF 4] and has a square AuF 4 ion.

When gold is dissolved in aqua-regia or Au 2Cl6 is dissloved in HCl and the solution of

AuCl4 is evaporated. Chloroamic acid can be obtained as yellow crystals [H 3O]

+ [AuCl4]

.3H 2O.

Au + HNO3 + HCl H3O+ [AuCl4]

.3H 2O AuCl3

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Other water soluble salts such as KAuCl4 and NaAuCl4.2H 2O are readily obtained. A

number of other complexes are also known. For example, [AuCl2Py2]Cl, [Au(phen)Cl2]Cl,[Au(CN)4]

, [Au(NO3)4], [Au(dian)2I2] I an d [Au(dian)2I]

2+.

Gold (III) alkyls are also usually stable only when other ligands such as triphenyl

phosphine are present as in (CH3)3 AuPPh 3.

AuCl4 decompose to metal quite readily on heating

[AuCl4] + OH Au (OH)3 dehydrate Au2O3 15 C Au + Au 2O + O2

Hydrate gold oxide is amphotene.

H[Au(NO3)4]HN O3 Au2O3.n H 2O NaOH NaAuO2.H 2O

H2SO4

H[Au(SO4)2]Au2Cl6 is dimeric.

Ag (+II) is known as the fluoride AgF 2. This is brown solid and is prepared by heating

Ag in F 2. AgF 2 is a st rong oxidizing agent an d a good fluorina ting. It d ecomposes on heat ing

AgF2heat Ag F +

1

2F 2

Ag (II) is more stable in complexes such as [Ag(Py)4]2+, [Ag(dipy)2]

2 and [Ag(o-phen)2]2+

which form stable salts with nonreducing an ion such as N O3 and ClO4

. They are usually

prepared by oxidizing a solution of Ag+ and the ligands with K2S2O8. The complexes are

square planar a nd para magnetic.

A black oxide of form ula AgO is formed by st rong oxidat ion of Ag2O in alkaline solution.

Au (II) is foun d in d