In [Ni(NH³)²Cl²], Ni²⁺ is in sp³ hybridisation,thus tetrahedral in shape. Hence the four ligands are not different toexhibit optical isomerism. In tetrahedral geometry all the positions are adjacent to one another
∴geometrical isomerism is not possible.

The octahedral coordination compounds of the type MA³B³ exhibit fac-mer isomerism.

In octahedral field the crystal field splitting of d- orbitals of a metal ion depends upon the field produced by the ligands. In general ligands can be arranged in a series in the order of increasing fields and splittings which they produce around a central metal ion. A portion of the series is given below.cyanide > ethylene - diamine > ammonia > pyridine > thiocyanate > water > oxalate >hydroxide > fluoride > chloride > bromide >iodide.

Out of the given ligands water, ammonia, cyanide and oxalate, we can find from the above series of ligands that the maximum splitting will occur in case of cyanide (CN^–)i.e. the magnitude of Δ⁰ will be maximum in case of [Co(CN)⁶]³⁺.

Non superimposable mirror images are called opticalisomers and may be described as “chiral’. They are also called enantiomers and rotate plane polarised light in opposite directions.