9.3 ISOMERISM IN COORDINATION COMPOUNDS The phenomenon in which two or more substances

hm ing the same molecular formula possess either different chemical structures or different spatial arrangements of atoms or groups is called isomerism and such substances are called isomers.

Isomerism is quite common in coordination compounds. They exhibit both types of isomerism, i.e., structural isomerism as \Veil as stereoisomerism. The structural isomerism comes into existence due to their different ionisation behaviour, difference in the position of ligands and due to the different modes of linkage of ligands. On the other hand, the stereoisomerism originates due to the different ste:reochernical disposition of ligands around the central metal atom or ion. Coordination compounds may show both types of stereo isomerism, i.e., geometrical as well as optical isomerism.

Various types of isomerism shown by complex compounds is described below.

9.3.1 Structural Isomerism In structural isomerism, two or more complex

c~mpounds having the same molecular formula possess different structural arrangements. Structural isomerism shown by coordination compounds may be of following types.

. 1. Ionisation isomerism : When coordination compounds having the same molecular formula show a difference in their

ionisation behaviour and furnish different types solution, the phenon1enon is called ionisation

Ionisation isomerism comes into existence an exchange of groups between coordination and spheres. For example, two distinct compounds ha,; formula Co(NH3) 5(Br)(S04) are as follows:

[CoBr (NH3 )5 ] S04 pentaamminebromidocobalt OII) sulphate

(red-violet)

and [Co (S04 ) (NH3 ) 5 ] Br

pentaamminesulphatocobalt (lll) bromide (red)

,: 11

The ionisation behaviour of these complexes is different. They ionise in solution as given belo,,:

[CoBr(NH3)5 ]S04 ~ [CoBr(NH3)5f~ - ~-

[Co(S04)(NH3)5]Br ~ [Co(S04 )(NH3hr - Bl'-

. The complex [CoBr(NH3) 5]S04

is red ,iola lts aqueous solution gives a vvhite precipitate of Ba.50 with BaC12, thus confirming the presence of sof ions in the solution. On the other hand. the comples [Co(S04) (NH3) 5]Br is red in colour and its aqueoussolutidl gives a light yellow precipitate of A~Br ,vith AgNO,, dlilG confirming the presence of Br ions \n its solution .

Some other pairs sho,ving this type of isomerism• Ci) [PtCl2(Nll3) 4 1Br2 and [PtBr2 (NH3)~Cl2

Coordination Compounds

(ii) [C0Cl2(NH3) 4]N0 and

(iii) [Pt(0H) 2 (NH3) ~ SO [CoCl(N02)(NIJ3)4 lCI

4 4 and [Pt(S04) (NIJ3)4J i 4· Coordination Isomerism : This type of isomensm

5 shr~wn hy the c:r,mplcx1·s in which both positive and

2. Hydrate isomerism . T,he (OH)2 · ·water m l l ·

both as ligands as well as simpl l O ecu es can act

negatJvC' parts ·ire I . , 1 . . . d c comp ex species. I ,e tsomerurn anse.s

. . e mo ecules oif

crvstallzsatz.on. When two complexes diffi . . water of

• • z er zn thzs a exhibit hydrate isomerism. For example .th spect, they

ue to the interchange of ligands between the coordination

spheres oif · · •

this ty Pf ~sitzve and negative parts. Some pairs showing

pe o isomerism are given below.

CrCl3 . 6H20, following three isomers ' wkn1 the formula are own t .

0) J Cl o exist

[Co(NH3)6] [Cr(C204)3] a1fc1"'"[o;cc2O4)3) [Cr(NH_y_& .)

[Co(NH3)6] [Cr(CN)6J and ~l[Cr{NH3)6J

[Cu(NH3)4J [PtC14] and [Pt(NH3)4] [CuCl4]

[Cr(H2 6 3 hexa~quachromium (III) ·

chlonde (violet) . _In all these cases, the ligands have been changed

within the coordination spheres. For example, in the last

case, the first complex has cation [Cu(NH3) 4]2+ and anion

[PtCl4] 2-. Its coordination isomer has cation [Pt(NH3)4]2

+

and anion [CuC14J2- . Thus, the ligands coordinated to Cu

and Pt have been changed.

[Cr(Hz0)sCU Clz.H20 pentaaquachloridochrom.

(III) chloride monohydra;~m

(blue green)

[Cr(H20)4Cl2] Cl.2Hz0 tetraaquadichloridochromium

(III) chloride dihydrate (dark

green)

These isomers differ from each oth · er m number

of water molecules _coordinated to central chromium

5. Ligand isomerism: This type of isomerism arises when

the ligand present in the complex can exist in more than one

isomeric form. For example, the ligand diaminopropane

can exist both as 1, 2-diaminopropane as well as 1,

atom. They possess different colours and may furth b

,i:u . d b d . er e

uwerentlate Y con uctivity measurements. 3-diaminopropane.

Some other complexes showing hydrate isomerism CH2 -CH-CH3 I I

are (i) [C0Cl(H20) (NH3)4] C12 and [CoC12(NH3) 4J

Cl-H20

(ii) [CrCl2(H20)2 (pyh] Cl and [CrC13 (H20) (py)2J · H20.

3. Linkage or structural salt isomerism : This isomerism

arises when the ligands have two different atoms available

for coordinating with metal ion, i.e., ligands are ambident.

In one isomer one donor atom of the ligand coordinates with

the central metal atom, while in the other isomer the second

donor atom coordinates with the metal atom. For example,

in N02 ion, both N and 0 atoms can coordinate with the

central metal atom. Thus following pair of compounds

exhibits linkage isomerism:

NH3 2+

H3N \, J, ✓N02 /Co

H3N t '\,.NH3

NH3 NH3

2+

NH2 NH2 1, 2-diaminopropane Cpn)

NH2 NH2 1, 3-diaminopropane Ctn)

When these ligands coordinate to a metal atom or ion,

ligand isomers are obtained. For example, complexes

[Co(pn)2Cl2r and [Co(tnhCl2J+

are ligand isomers.

9.3.2 Stereoisomerism

Coordinate bonds are directional in nature and

give rise to the phenomenon of stereoisomerism in

coordination compounds. In this type of isomerism, different

atoms or groups of atoms occupy different spatial positions

around the central metal atom. The isomers thus obtained

are called stereoisomers.

Stereoisomerism is very common in coordination

compounds and is studied extensively. Stereoisomerism is

of two types : geometrical isomerism and optical isomerism.

Both are exhibited by complex compounds. A brief account

of these isomerism is given below.

[AJ Geometrical Isomerism

Pentaamminenitrocobalt (III) ion pentaanuninenitritocobalt (III) ion

(yellow brown) (red)

Pen Although both pentaamminenitroc~balt (III) and

Yi taanuninenitritocobalt (III) ions contain N02 group,

£ et tbey differ in colour and behaviour towards acids. The

Ollner · · d 8cids 18 . yellgw brown in colour and 1s stable towar s

a,.;-,_' While the latter is red and is easily decomposed by

-~to . .

8. ~ve IUtrous acid. . . _

&rQq ~ly, coordination compounds contammg SCN

:. as ligand also show linkage isomerism.

Geometrical isomerism comes into existence bv

the different spatial arrangements of groups around th~

central metal atom. Similar groups may either be arranged

on the same side or on opposite sides of the central metal

atom. This gives rise to two types of isomers called cis and

trans i~omers. When similar groups are arranged on the

sgme szde of the central TJietal atom, we have cis isomer,

and when the similar .gr.Dll.p.S ore spatially placed Q.n the

OJ!POSite sides, we have trans isomer.

, Geometrical isomerism is comrrion in coordination

compounds and depends upon the coordination number --yr ex&nple

fer{SCN)(H;0) 5J2+ and [Cr(NCS)(H20)sJ2+_

I 1 I Ill' vconll'll y of of the central metal atom am a so upon " . . f 1 • . 1

, , 0 1· iso111cns111 illl' the complex. Vanous cases o t 11s ) pc ·

discussed below.

1. Four Coordination Compounds Complexes with coordination numbct four arc ei

thcr

tetrahedral or square planar in shape. Tetrahedral complexes can not show geometrical isomerism because all the four ligands lie at tlte same distance from central metal ato11~.

Therefore. no case is obsen1ed where a tetrahedral complex is

found to show this type of isomerism. Geometrical isomerism is very common in square

planar complexes. However, all square planar complexes can not exhibit geometrical isomerism. Square planar complexes of the type Ma4, Ma3b and Mab3 are unable to exist in cis and trans forms (where a and b represent monodentate ligands). Geometrical isomerism is shown

only by the following types of complexes. (i) Complexes of the type (Ma2b2)n+ : Complexes of

rhe type (Ma2b

2t± can exist in cis and trans forms. Cis

form is obtained when both the groups a and both the groups b occupy neighbouring positions, while trans form is obtained when similar groups occupy trans positions

around the central metal M. For example, the cis and trans forms of complex

diamminedichloridoplatinum (II), [PtCl2 (NH3)z] are

shown in Fig. 9 .4.

H3~,'~--;;7;CI

, Pt , v_~ I I I

Cl -------- NH3

Cis-isomer Trans-isomer

Fig. 9.4 Cis and trans isomers of [PtCl2(NH3)2l

Another complex of this type is [Pt(N02) 2 (NH3)zJ. It

can also exist in cis and trans forms in the same way as

shown above. (ii) Complexes of the type (Ma2bc)n± : In this case, cis

isomer is obtained when similar groups a are adjacent to each other and the trans form is obtained when they are

opposite to each other. For example, the cis and trans forms of complex

diamminebromidochloridoplatinum (II), [PtBrCl(NH3) 2]

exist as shown in Fig. 9.5.

Fig. 9.5 Cis and trans forms of [PtBrCl(NH3)2J,

(iii) Complexes of the type (Mabcd)n± : When all the blr .ri,mds are different, three geometrical isomers are

possible in square planar complexes. An example of complex is [PtBrCl(Nl 1~1) (Py)]. It can exist in the

three forms (Fig. 9.G).

H3N;~·7:e, / Pt I

I I

Cl ------ _,: Py

Fig. 9.6 Three geometrical isomers of fFtBrClCNHa)(Py

(iv) Complexes of the type [M(AB)z]n± : In the complex

[M(AB)z1'1±, M is the central metal atom, while AB is an unsymmetrical bidentate ligand. An example of this type "of complex 1s [Pt (gly)zf, where gly stands for glycino, (NH

2CH

2C007 ligand. The cis and trans forms of this

complex are shown in Fig. 9.7. JH2 ,'~-----7.0\

CH2 ,' Pt/ 1 co

\ ,' / : I

CfoV-----~•'N fH2 H2

Cis-form Trans-form

Fig. 9.7 Cis and trans forms of [Pt(glyh].

2. Six Coordination Compounds Complexes with coordination number six are

octahedral in shape. A regular octahedron contains eight faces and six equivalent vertices. In an octahedral complex the metal is placed at the centre and six ligands occupy their

positions at the vertices. Octahedral complexes of the type Ma

6, Ma

5b and Mab5 do not show geometrical isomerism.

because in these complexes different spatial arrangements of ligands are not possible. Geometrical isomerism in all other types of octahedral complexes is very common. Some_ of the important types of octahedral complexes shoWlng

geometrical isomerism are as follows. (i) Complexes of the type [Ma4b2]n+: In these complexdes r ans

four monodentate ligands a and two monodentate ig l,f b are octahedrally attached to the central metal. atom (1.

Cis-isomer is obtained when ll8ands b QfC!IPY ad1~cen~,a ~' while trans-isomer is obtaine~lig@-1 • are opposite to each other, i.e., they occup 1, 6 ositi

0~;.

An example of this type of complex is [ i(NH~4 ill

The cis and trans forms of this complex cation are sh~Wll t

Fig. 9.8. In the cis-isomer, the two Cl- are at two adJ3~

positions (any) and hence it is designated as c~_isornt y) trans isomer, the two ci- are in opposite pos1oons all

and therefore it is designated as trans isomer.

Compounds

--Nj~~--7:111

, c1 + H3N cj I NH3 • / / ,,,~-7 ~, ,' Co ,'

.... ------'-----1c1 V I ~ NH .... 1 H3N 1

N~ a Cis-isomer Trans-isomer

Fil• 9.8 Cis and trans isomers off Cr>C'l -,(NH 1 )41""

(ii) com~lexes of the type [~cf.311-.J]n± : An important example of this type of complexes 1s [CrC13 (NH3)J]. Its cis and rrans fonns are shown in Fig. 9.9.

Cl(1) l f t,1

3N , .,, Cl (2) H3N ~ - ..,,____,__..::::,,. -- ,

Ci (1)

J Ci (6)

Cis-rsomer Trans-isomer

Fig. 9.9 Cis FP1.d trans isomers o~ [CrC13(NH3)3].

In the cis-isomer, the three Cl- are on one triangular face and the three NH3 molecules are placed on the opposite rriangular face. This isomer is designated as facial (Jae) isomer. In trans-isomer, Cl- are placed on the edges of the .octahedron, while NH3 molecules are present on the opposite edges. This isomer is termed as meridonal (mer) or peripheral isomer. Another example of this type of complexes is (Rh [PyhCl3].

(iii) Complexes of the type [M(AA)2a2]n± : In this type of complexes, central metal atom M is attached to two symmetrical bidentate chelating ligands AA and two monodentate ligands a. The two letters A and A in AA

H2N-CH . ] ... C + I ~ 't,;H2 I I

c_;~t::;;;r, r?,',,~l-:;7 t~:, a~-c~~H, j l:t~-1~ },H,

.,..CH2 H2N-cH2 Cl

OR

N~ +

c;~-/-7N ~ -c~

N--✓. Cl Cis-isomer Trans-isomer

Pig. 9.10 Cis and trans-isomers of [CoC12(en)z(.

+

!!:te the two similar coordinating atoms. These i'!.:v--,,. .. 11••es~ exist in cis and trans forms and exhibit the

on of geometrical isomerism.

(

An example of this type of complex is [CoC12(enz)t. The cis and trans forms of this complex are shown in Fig. 9.10.

Other complexes of this type are [Co(en)z(NO3)zi+, flr(C2O4) 2Cl2 1

2 c1nd I lr(C2O4) 2Cl2]3

. These exist in cis and trans forms in the s;ime way as shown above.

lltf'rasm Optical isomerism is shown by those compounds which

posssess chirality, i.e., which do not possess any element of symmetry. Presence of an element of symmetry makes a molecule symmetric and renders it optically inactive. When a molecule does not possess any element of symmetry, its mirror image is not superimposable on the molecule itself. This makes the molecule optically active. Such an asymmetric molecule can exhibit the phenomenon of optical isomerism. The two forms of the molecule which are mirror images of each other are called enantiomers. One form rotates the plane of plane polarised light in clockwise direction, while the other in anticlockwise direction. The former is called the d-form, while the latter is termed as l-form.

The phenomenon of optical isomerism is quite common in coordination compounds. The complexes whose molecules are asymmetric exhibit optical isomerism. The optical isomerism of various types of complexes is discussed below.

1. Four Coordination Compou r .Js (i) Square planar complexes : In square planar

complexes, all the four ligands and central metal atom lie in the same plane. Therefore, they possess a plane or axis of symmetry and are generally not chiral in nature. This is why optical isomerism is not common in square planar complexes and is very rarely observed.

(ii) Tetrahedral complexes : Organic compounds having asymmetric tetrahedral carbon atoms are generally optically active. Therefore, it is expected that a tetrahedral complex with four different ligands attached to a central metal atom or ion, i.e., a complex of the type [Mabcd] n± should exhibit optical activity. However, it has not been possible to resolve optically active d- and l-forms of such a complex due to its labile nature .

Thus, optical isomerism is not very common in both square planar as well as tetrahedral complexes. Only a very few four coordination complexes are known which exhibit optical isomerism.

2. Six Coordllldti011 Compo11nds Optical isomerism is more common in six coordination

complexes, i.e., in octahedral complexes. Optical isomerism of some important types of octahedral complexes is discussed below.

1. Octahedral complexes containing only monodentate ligands : Octahedral complexes of the type [Ma2b2c2],

[Ma2b2cd], [Ma2bcde] and [Mabcdej] do not possess any element of symmetry and should be optically active. However, none of these complexes could be resolved till

• For details, see§ 10.2.7 in Unit 10. I

now. Theoretically, octahedral complexes containing o~ly monodentate ligands are optically active and should ('X15t

ind- and l-fonns, but the paucity of adequate cxpcrimci~tal techniques to resolve them makes their optical isomerism of little value.

2. Octahedral complexes containing one or more symmetrical bidentate chelating ligands : Octahedral complexes containing all the monodcntate ligands could not be resolved. However, if an octahedral complex contains one or more bidentate chelating ligands, it is possible to resolve it into its optically acti\'e forms. Some examples of such complexes are given below. +

(i) Complexes of the type [M(AAht- : In the complexes of this type, three symmetrical bidentate chelating ligands M are coordinated to the central metal atom M. Such complexes do not possess any element of symmetry and are optically active. Moreover, these complexes can be resolved into optical isomers. 3_

An example of this type of complexes is [Cr(Cz04)3] • It is optically active and has been resolved into d-and l-forrns (Fig. 9.11).

>1"'J7'0 3- f(J~:;;t1 3-

l : /r I ,' Cr ,' ox \v---r~)o o~r~~v

a-ox ox-a d-form Mirror I-form

Fig. 9.11 Optically active forms of complex [Cr(C20 4)i-. {OX refers to bidentate oxalato ligand.)

Other examples of this type are [Co(enh] 3+,

[Co(pnhJ3+, [Pt(enh] + and [Cd(PnhJ 2

+. The optical

isomers of [Co(enh)]3+ are shown in Fig. 9.12.

r ·,-en 3+ -N 3+

1 ,,N."'<r7N N1~+7 N1 { / Co f i Co i en

l NL.; .. ~N ~+~v N-en en _N

d-form Mirror I-form

Fig. 9.12 Optically active forms of [Co(en)3]3+.

(ii) Complexes of the type [M(AA)za2r± : The ~omplexes in which two symm:trical bidentate chelating ligands AA and two monodentate ligands a are coordinated to ~entral_ metal atom M also exhibit the phenomenon of optical ISOmensm and can be resolved into their optical isomers.

An example of this type of complexes is [CoCl2 (en)zt. It exhibits both geometrical as well as optical isomerism. Its cis form is unsymmetrical while the

.,pm form is symmetrical because it contains ~ plane of s,mDletiy. ~ence, optical isomerism is shown by cis form ~ 1Wds form has been resolved into d- and Z-forms.

Nootan ISC ~ ......

d d I-forms along with the optically~ The • an ---.ve .....__ form arc shown in Fig. 9.13. , ~

d-form cis

t . 1

I 1~7/· ~. a J L

M.rror

Optically active forms (cis)

Cl Optically Inactive

trans form

-form CIS

Fig. 9.13 Optically active (cis) and optically inactive (trans) forms of the complex [C0Cl2(en ,2~·.

(iii) Complexes of the type [M(AAhab t± : In this case AA are symmetrical bidentate chelating ligands, while a and b are monodentate ligands. Such complexes exist in three forms, two are optically active (d- and Z-fonns) and the third one is inactive meso form. An example of this type of complexes is [CoCl(en)i(NH3)]

2+. Its three forms are shown in Fig. 9.14.

d-form

2+

Mirror

Optically active forms

NH3

Cl Optically inactive

meso form

I-form

2+

Fig. 9.14 Optically active and meso forms of compld [CoCl(en)2 (NH3)]2+.