The order of rate of substitution of chloride by pyridine (in ethanol) in the following complexes

Concept:

Substitution reaction:

• A substitution reaction is a type of chemical reaction, in which one functional group is replaced by another functional group.
• Metal complexes in which the rate of ligand displacement reaction is very fast and hence show very high reactivity is called labile complexes.
• Metal complexes in which the rate of ligand displacement reaction is very slow and hence show less reactivity are called labile complexes.

Explanation:

• In complexes, I, II, and III the metal ion involved is Ni, Pd, and Pt. Ni, Pd, and Pt are the elements of the 3d, 4d, and 5d series respectively. 
• The electrons present in d orbitals cannot shield the valence electrons and hence experience a much more effective nuclear charge on it.
• From 3d to 5d orbital, the effective nuclear charge on the valence electrons increases. Hence moving down the group from 3d to 5d series along a group, the inertness increases.
• So, from Ni to Pt, the inertness increases, and hence the rate of substitution of chloride by pyridine decreases.

Conclusion:

• Hence, the order of rate of substitution of chloride by pyridine (in ethanol) in the following complexes is I > II > III.