Stability of a complex in solution means the measure of resistance to the replacement of a ligand by some other ligand. This stablility can be expressed in terms of equilibrium constant. Let the reaction between metal and ligand be represented as Ma+ + nLx- ⇌ MLnb+ Stability or Dissociation constant (K) = [ML+n][M+][L−]n The reciprocal of the stability constant K is known as instability constant or dissociation constant Ki=1K=[M+][L−]n[ML+n] Factors affecting the stability of a complex ion (i) Nature of metal ion : Greater the charge and smaller the size of the ion, more is its charge density and greater will be stability of the complex. (ii) Nature of ligand : More the basicity of ligand, more is its tendency to donate electron pair and therefore, more is the stability of the complex.