(i) Coordination compounds which possess chirality exhibit optical isomerism similar to organic compounds.
(ii) The pair of two optically active isomers which are mirror images of each other are called enantiomers.
(iii) Their solutions rotate the plane of the plane polarised light either clockwise or anticlockwise and the corresponding isomers are called 'd' (dextrorotatory) and 'I' (levorotatory)  forms respectively.
(iv) The octahedral complexes of type \(\left[\mathrm{M}(\mathrm{xx})_{3}\right]^{\mathrm{n} \pm}, \left[\mathrm{M}(\mathrm{xx})_{2} \mathrm{AB}\right]^{\mathrm{n\pm}}\) and \(\left[\mathrm{M}(\mathrm{xx})_{2} \mathrm{~B}_{2}\right]^{\mathrm{n\pm}}\) exhibit optical isomerism.
Examples:
(i) The optical isomers of \(\left[\mathrm{Co}(\mathrm{en})_{3}\right]^{3+}\) 
(ii) The coordination complex \(\left[\mathrm{CoCl}_{2}(\mathrm{en})_{2}\right]^{+}\) has three isomers, two optically active cis forms and one optically inactive trans form.

(i) It does not explain the colour of the complex.
(ii) It considers only the spin only magnetic moments and does not consider the other components of magnetic moments.
(iii) It does not provide a quantitative explanation as to why certain complexes are inner orbital complexes and the others are outer orbital complexes for the same metal. For example, [Fe(CN)₆]^4- is diamagnetic (low spin) whereas [FeF₆]^4- is paramagnetic (high spin).

[Ni (H₂O)₆]²⁺

It has two unpaired electrons. So there is d-d transition. Hence it is green coloured.
[Ni(CN)₄]^2-

There is no unpaired electrons. So it is colourless, as there is no d-d transition.

In an Octahedral complex, the d-orbitals of the central metal ion, divide (1) into two sets of different energies. The separation in energy is the crystal field splitting energy.
The d-orbitals lying along the axes dx², dy² and dz² orbitals will experience strong repulsion and raise in energy to a greater extent than the orbitals with lobes directed between the axes (d_xy, d_yz, and d_zx)Thus the degenerate d-orbitals now split into two sets and the process is called crystal field splitting.

(i) When excess of liquid ammonia is added to an aqueous solution of copper sulphate gives tetra ammine copper (II) sulphate is formed.
CUso₄ + 4NH₃ \(\rightarrow\) [Cu(NH₃)₄]SO₄
(ii) The co-ordination entity is [Cu(NH₃)₄]²⁺