In complexation reactions, metal cations are the electron pair acceptors, and the ligands are the electron pair donors. The stability of the metal complexes depends primarily on the complexing ability of the central metal ion and the nature of the ligands. Generally, the complexing ability of the metal ion depends on the size and charge of the ion. As the metal ion size increases, the stability of the metal complexes decreases, provided that the valency of the metal ion and the ligands remain the same. On the other hand, as the charge on the metal ion increases, the stability of the complexes increases.
Metal ions are categorized into classes A and B based on their preferred ligands, specifically, ligands described in terms of soft or hard acids and bases. Soft bases have easily oxidizable donor atoms with high polarizability and low electronegativity whereas hard bases have donor atoms that show low polarizability and high electronegativity. Smaller, more highly charged metal ions with unsaturated outer orbitals are categorized as Class A and prefer to bind to hard bases. In contrast, larger, less charged or uncharged metal ions with filled outer orbitals are categorized as Class B and prefer complexation with soft bases. In terms of ligand effects on the stability of complexes, smaller, more highly charged ligands tend to form more stable complexes, whereas larger ligands tend to form less stable complexes because of the effects of steric repulsion. Ligands with vacant p or d orbitals can form pi bonds with certain metal ions, which in turn exhibits back bonding behavior toward the ligand, both of which serve to stabilize the complex further.
