**Abstract**

Substitution reactions are a type of reactions where one functional group or ligand is substituted by another. They could be electrophilic or nucleophilic, depending upon whether the reagent is involved. Complex compounds could be involved in a number of substitution reactions such as ligand exchange, solvent exchange, complexation or anation reactions, solvolysis, acid and base hydrolysis, inter- and intramolecular isomerization, racemization, and metal ion reaction. Hard-soft acid–base principle (HSAB) contributes to better understanding of the mechanism of nucleophilic substitution reactions of transition metal complexes. Metal–ligand bonds in transition metal compounds are closely related to the HSAB nature of metals and their preferred ligands. Also, the principle is qualitatively useful to predict the preference of the metal for the ligand in bioinorganic reactions.

**Keywords:** substitution, complexes, hard-soft acid–base principle, metal–ligand bonds, bioinorganic reactions
