Isomerism In Co-ordination Compounds

Isomerism In Co-ordination Compounds

Isomers: Two or more forms of compounds having the same compositions are called isomers.

Types of isomers

(i) Structural isomers

(ii) Stereo isomers

(i) Structural isomers:

Co-ordination compounds having same compositions but different bonding / attachments.

Types of structural isomers

(a) Ionization isomers

(b) Hydration isomers

(c) Linkage isomers

(d) Co-ordination isomers

(a) Ionization isomers

Structural isomers in which ion exchanged between ligands and anion.

Example:

$\left[Co{\left(N{H}_3\right)}_{4}ClBr\right]Cland\left[Co{\left(N{H}_3\right)}_{4}C{l}_2\right]Br$

$\left[Pt{\left(N{H}_3\right)}_{3}Cl\right]N{O}_{2}and\left[Pt{\left(N{H}_3\right)}_{3}N{O}_2\right]Cl$

(b) Hydration isomers

Structural isomers in which water exchanged between ligands and hydrate.

Example:

$\left[Cr{\left(H_{2}O\right)}_6\right]C{l}_3,\left[Cr{\left(H_{2}O\right)}_{5}Cl\right]C{l}_{2}and\left[Cr{\left(H_{2}O\right)}_{4}C{l}_2\right]C{l}_2.H_{2}O$

$\left[Co{\left(N{H}_3\right)}_5\left(H_{2}O\right)\right]C{l}_{3}and\left[Co{\left(N{H}_3\right)}_{5}Cl\right]C{l}_2.H_{2}O$

(c) Linkage isomers

Structural isomers in which ligands can form at more than the atomic site (anibidentate)

Example:

:CN^- and :NC^-

${\left[Cr{\left(H_{2}O\right)}_{5}CN\right]}^{2+}and{\left[Cr{\left(H_{2}O\right)}_{5}NC\right]}^{2+}$

(d) Co-ordination isomers

Structural isomers differ in the distribution of ligands in the cation and the anion.

$\left[Co{\left(N{H}_3\right)}_6\right]and\left[Cr{\left(N{H}_3\right)}_6\right]\left[Co{\left(CN\right)}_6\right].$

$\left[Cr{\left(N{H}_3\right)}_6\right]\left[Co{\left(C_2{O}_4\right)}_3\right]and\left[Co{\left(N{H}_3\right)}_6\right]\left[Cr{\left(C_2{O}_4\right)}_3\right]$

(ii) Stereo isomers

Space isomerism arises on account of the different positions and arrangements of ligands in space around the metal ion.

Type of stereo isomers

1. Geometrical isomers

2. Optical isomers

1. Geometrical isomers

Stereo isomers having different orientations of ligands within a given geometry. Square planar and octahedral structures with two different ligands can have like ligands adjacent (cis) or opposite (trans) to one another.

Type of square planar complex:

(a) MA₂B₂ (b) MA₂BC (c) M(AB)₂

For types MA₂B₂:

For type MA₂BC:

For type M(AB)₂:

Type of octahedral complex

(i) $\left[M{A}_4{B}_2\right]or\left[M{A}_2{B}_4\right]$

(ii) $\left[M{\left(AA\right)}_2{B}_2\right]or\left[M{\left(AA\right)}_{2}BC\right]$

For MA₄B₂ types:

For M(AA)₂B_­2 type:

2. Optical isomers

Type of stereo isomers having non identical mirror images with the same geometric arrangement of ligands.

Some important points regarding optical isomers:

(i) A set of two isomers are called enantiomers.

(ii) Enantiomers differ in optical properties – rotate plane polarized light in opposite directions.

(iii) Optical isomers are mirror images of one another that can not be super imposed.

(iv) Check for optical isomers by checking super positions of mirror images.

(Rotate to super impose)

Optical Isomers