9• 1 .5 Werner's Theory of Coordination Compounds

Alfred Werner (1892) prepared and isolated s complex compounds by the action of ammonia

00 evebr~ co alt chloride. He characterised these compounds as follows.

Compound Colour Name accordine to colour CoC13 · 6NH3 Yellow Luteo complex --CoC13 · SNH3 Purple Purpureo complex CoC13 · 4NH3 Green Praseo complex CoC13 · 4NH3 Violet Violeo complex

He studied the properties of these compounds in detail. In order to explain the properties of these compounds, he proposed a theory commonly known as Werner's theory of coordination compounds. This theory is briefly described below.

Postulates of Werner's theory : The main postulates of Werner's theory are as follows : (i) The central metal atom or ion present in a complex

exhibits two types of valencies-primary valency and s_gcondary valency. ::,p (a) Primary valency : This type of valency (linkage)

is ionisable and corresponds to the oxidation state of the central metal ion. It is always satisfied by a negative ion. The primary valencies are shown

CO no» ~..,,-tied--: tfnes. The ions attached to the central I>)' do through pri111ary valencies can iont

etal ton se di 1ution, Th. iJl SO darY vale~ : ids type of valency

) 5,ct,'1 ) .

5 non,fonisable an corresponds to th

(1J oiJtlcage ! 0

n,tmber of the central metal atom e caordinauocentral metal atom or ion has a f'

0~

ioO· Eve_ry f secondary valencies (this nu~bxe mtJer o d' . er,

~ufact, refers to the :lhoor mat1don number of the in I ..,.,etal ion). ~ e secon ary valencies a

0rra ,.. . . re

ce . ed either by negative ions or by neutral sansfi Ies They are represented by thick Ii

01ecu · h d th nes J1l th groups attac e to e central metal i nd e bl . . . on a h them are una e to iomse m the solutio (:hroug . h n. l metal atom or wn as a tendency to satisfy

•') rhe centra rimarJ and secondary valencies. In order to (O all of it~ P equirement, the central metal atom or ion

t thl.S r · · rne,~, attach one or more nelga~ve wns through both rna~ and secondary va encies. Thus, some of the pn.m~rYions may play a dual role. Such ions do not

negatne . . nise in solution. . . . io ndary valencies are directional, i.e., they point

••') The seco . . Th . . (ill in definite ~irectwns \n space. e primary valencies are

non-directional. \.. 'J;-res of some Coordination Compounds on

structu , B sis of Werner s Theory the r~e structures of some complexes derived on the basis

r's theory are discussed below. ofWerne . . . 1. coc1

3. 6NH3: In thIS complex, c_obalt 1s m +3 oxidation

Therefore its primary valency is 3. Experimentally, it

state. ' ' has been observed_ that o~e mole of the complex gives three ]es of er ions m solution. Therefore, the three Cl atoms

~~gative ions) present in. it should be li~ked to the central cobalt by primary valencies. The coordmation number of cobalt is six. Therefore, it must have six secondary valencies. Obviously, the six NH3 molecules (neutral molecules) must be attached to cobalt by these secondary valencies. Thus, on the basis of Werner's theory, the complex CoC13 • 6NH3 should be represented as shown in Fig. 9 .1.

NH3

H3N"' Cl

~ / NH3 Cl Co

/ ~NH3

NH Cl 3

NH3

Fig. 9.1 Werner'~ ~"--·-L----- _.£' r,_r,1~ r •• ;r.::- ' this 2• CoCl3 · 5NH3 : "The oxidation s~ate of cobalt in p. complex is +3. Therefore it should have three 1ie7 dvalencies and all the 'three Cl atoms should five ~ to cobalt by these valencies. The remaining Becond 3 molecu~es should be attached to cobalt by

ary valencies. Since the coordination number

of cobalt is six, It mutt haw • In order to satisfy all the lb one Cl atom (already attached by a Pl'llnll, also be linked by a secondary valency should have the structure as shown In': t1la atom linked by both primary and secondary a. 9 2. not io~ise in the ~olution. Therefore, this~-ionise 1n the solution as follows : --··•rJQ ......

CoC13 · SNH3 ~ [CoCl • SNH3)2+ + 2Cl

NH3

NH3~ /NH3 Cl ···· · Co ···· Cl

H3N/ :~NH3 Cl

Fig. 9 2 Werner's structure of CoCl- 51\Tf-T-> This has been confirmed conductometrically. One

mole of the complex is found to give two moles of ci- ions and one mole of [CoCl · SNH3]

2+ ion.

3. CoCl3 · 4NH3 and CoCl3 · 3NH3 : Following the arguments as given above, the structures of these complexes can be written as shown in Fig. 9.3.

Cl ' '

. Cl (a)

Cl

Cl (b)

Fig. 9.3 Werner's structure of (a) CoC13-4NH3 and Cb) C0Cl3•3NH3.

"fference between D uble Salts and Coord_· ____ Compounds

Both double salts as well as coordination comp ... are molecular or addition compounds and are form .. the combination of two or more salts in stoichiometrk simple molecular) ratio. However, the two differ in ionisation behaviour in aqueous solutions as de~\. ahead.

... - lose their identity In

el(8lllPleS illustrate the nature = ...._ [llohr'• ,alt, PeS04. (NH4hS04 . 6H O ..

tfJ aY t,e obtained by mixing the satu/ t : 8 double tl.,ttd-~ sulphate and ammonium sulphat: :n IOlutlonll .1fel1°'1 ... jxture. d COOlin1 • ,itrant ••· ~~eS04 + (NH4)S04 , 61120 >

FeSO (NLJ 4 . r 4 )2SO · 6H O M h

, 4 2 o rssalt

"',,, n dissolved in water, Mohr's salt loses its 'd . vv ,.e . . . 1 enttty . •ates into its constituent 10ns as follows

_A d1ssoc1 • 8J>' peSO/NH4)zS04 · 6H20 -

Fe2+ + 2NH: + 2so~- + 6H2O

rhe aqueous solution of this salt gives the test of Fez+, + d so~- ions. Thus, Mohr's salt is a double salt

NI-14 an ' . . (iiJ potash alum, K2SO4 · Al2(SO4h_ · 24H2O1s another

Ple of a double salt. It may be obtamed by mixing the

exam . f . 1 at

ed solutions o potassmm su phate and aluminium ~~ . . sulphate and evaporatmg the resultant solution.

K2SO

4 + Al2(SO4)3 + 24H20 ~

K2SO 4 · Al2 (SO 4 )3 · 24H2O Potash alum

When potash alum is dissolved in water, it loses its identity and dissociates into its constituent ions as shown

below. K2S04 · Al2(SO4h · 24H20 --t

2K+ + 2Al3+ + 4SO~- + 24H20

Thus, the solution of potash alum responds to the test

of Ir, Al3+ and so~- ions. Some other examples of double salts are carnallite

(KCl · MgC12 · 6H2O), ferric alum [(NH4)iS04 · Fe2(SO4h. 24H20], etc.

2. Coordination compounds : As described earlier, coordination compounds are those molecular or addition compounds which retain their identity in aqueous solutions and ~how properties entirely different from those of their constituent ions. Following examples illustrate the nature of COordina · . tion compounds.

C (t) Potassium fierrocyanide K4 [Fe ( CN) 6] is a OOrd' · ' .. aso1u:at1on compound and may be obtained by m1xm?

the on of ferrous sulphate with a solution of KCN until

Prect · . Pltate of ferrous cyanide formed gets dissolved. PeS04 + 2KCN ~ Fe (CN)z + K2S04

Ferrous cyanide (ppt.)

, Pe(~)a._

J, C "'4,!.~~ • 'y- ~As xplatn d

· follows. K 4 ( 1

4 e{CN)6]

4KCN

arlt t It dla-...,,.. --·:aa•·--~

4K + (P F nocyan

1~/te ferrocyanide ion is a complex ion and retaim identity in solution. It does not dissociate further to give Fe2 + and CN- ions. Thus, an aqueous solution of potasstum ferrocyanide does not give the test of Fe

2+ and CN ions ..

(ii) Potassium tetracyanidonickelate (11), K2[Ni(CN) 4 ] is also a coordination compound and may be obtained by dissolving the white precipitate of nickel cyanide in the excess of potassium cyanide solution. Ni(CN)2 + 2KCN > K2 [Ni (CN)4]

Potassium tetracyanonickelate (II)

When dissolved in water, it dissociates as follows.

K2 [Ni(CN)4 ] ::::==~ 2K+ + [Ni (CN)4 ]2

-Tetracyanonickelate

(II) ion

The tetracyanidonickelate (II) ion is a complex ion and does not dissociate further into Ni

2+ and CN- ions.

Therefore., the aqueous solution of this complex does not give the tests of Ni2+ and CN- ions.