Complexation Equilibria: Overview

In complexation reactions, metal cations form complexes with compounds called ligands with unshared pairs of electrons that satisfy the metal ion coordination number.

The metal cation is a Lewis acid, an electron pair acceptor, whereas the ligand is a Lewis base, an electron pair donor, and the bond formed between them is the dative or coordinate covalent bond. 

The equilibrium constant of a complexation reaction is called the formation constant K_f or the stability constant K_s.

The higher the value of the formation constant, the greater the stability of the complex.

In general, the complexation reactions occur in a stepwise fashion, and the equilibrium constant for each step is called the stepwise formation constant.

However, for the whole reaction, the cumulative or overall formation constant β is the product of all the stepwise formation constants.

In contrast, the dissociation reaction of the complex is the opposite of its formation equilibria, where the equilibrium constant is called the dissociation or instability constant, expressed as the reciprocal of the formation constant.