Isomerism in Coordination Complexes - Practice Questions & MCQ

Geometrical Isomerism
This isomerism is due to ligands occupying different positions around the central metal atom or ion. The ligands occupy positions either adjacent or opposite to one another. This type of isomerism is also known as cis-trans isomerism. When two same ligands are at right angle(90^o), the form is cis- form and when they are present diagonally at 180^o to each other, the form is termed as trans- from. Geometrical isomerism is very common in coordination number 4 and 6 complexes.

Another type of geometrical isomerism occurs in octahedral coordination entities of the type  like []. If three donor atoms of the same ligands occupy adjacent positions at the corners of an octahedral face, we have the facial (fac) isomer. When the positions are around the meridian of the octahedron, we get the meridional (mer) isomer.

Optical Isomerism
Optical isomers are mirror images that cannot be superimposed on one another. These are called as enantiomers. The molecules or ions that cannot be superimposed are called chiral. The two forms are called dextro (d) and laevo (l) depending upon the direction they rotate the plane of polarised light in a polarimeter (d rotates to the right, l to the left). Optical isomerism is common in octahedral complexes involving didentate ligands.

Linkage Isomerism
Linkage isomerism arises in a coordination compound containing ambidentate ligand. A simple example is provided by complexes containing the thiocyanate ligand, NCS^–, which may bind through the nitrogen to give M–NCS or through sulphur to give M–SCN. Jørgensen discovered such behaviour in the complex [Co(NH₃)₅(NO₂)]Cl₂, which is obtained as the red form, in which the nitrite ligand is bound through oxygen (–ONO), and as the yellow form, in which the nitrite ligand is bound through nitrogen (–NO₂).

Coordination Isomerism
This type of isomerism arises from the interchange of ligands between cationic and anionic entities of different metal ions present in a complex. An example is provided by [Co(NH₃)₆][Cr(CN)₆], in which the NH₃ ligands are bound to Co³⁺ and the CN^– ligands to Cr³⁺. In its coordination isomer [Cr(NH₃)₆][Co(CN)₆], the NH₃ ligands are bound to Cr³⁺ and the CN^– ligands to Co³⁺.

Ionisation Isomerism
This form of isomerism arises when the counter ion in a complex salt is itself a potential ligand and can displace a ligand which can then become the counter ion. An example is provided by the ionisation isomers [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄.

Solvate Isomerism
This form of isomerism is known as ‘hydrate isomerism’ in case where water is involved as a solvent. This is similar to ionisation isomerism. Solvate isomers differ by whether or not a solvent molecule is directly bonded to the metal ion or merely present as free solvent molecules in the crystal lattice. An example is provided by the aqua complex [Cr(H₂O)₆]Cl₃ (violet) and its solvate isomer [Cr(H₂O)₅Cl]Cl₂.H₂O (grey-green).

The stability of a complex in solution refers to the degree of association between the two species involved in the state of equilibrium. The magnitude of the equilibrium constant (stability or formation) for the association, quantitatively expresses the stability. Thus, if we have a reaction of the type:

then the larger the stability constant, the higher the proportion of ML4 that exists in solution. Free metal ions rarely exist in the solution so that M will usually be surrounded by solvent molecules which will compete with the ligand molecules, L, and be successively replaced by them. For simplicity, we generally ignore these solvent molecules and write four stability constants as follows:

where K₁, K₂, ....K_n, etc., are stepwise stability constants. The overall stability constant (β) of the formation of species ML_n from M and L can be given as:

The stepwise and overall stability constant are therefore related as follows: