2.14: Complexation Equilibria: Overview
Complexation reactions take place when dative or coordinate covalent bonds form between metal ions and ligands. The compounds formed in these reactions are called coordination compounds. The number of bonds formed between the metal ion and the ligands is called its coordination number. Generally, most metal ions in an aqueous solution are solvated by water molecules and thus exist as aqua complexes.
The equilibrium constant of the complexation reaction is represented as the formation constant Kf, also known as the stability constant Ks. Conversely, the dissociation of the complex is the reverse of the complex formation, and the dissociation equilibrium constant is called the dissociation constant Kd or instability constant Ki, which is equal to the reciprocal of the formation constant Kf.
Complexation generally happens stepwise, where a metal ion initially complexes with one ligand, followed by the second ligand, until it satisfies its coordination number. The equilibrium constants for each reaction step are the stepwise formation constants represented as Kf1, Kf2, ...., Kfn. The overall formation or cumulative constant β is the product of the stepwise formation constants, or simply Kf.
The stepwise formation constant values decrease steadily due to statistical, coulombic, and steric factors. Statistical factors come into play after the first ligand attaches to the metal ion, which decreases the number of available sites for the next ligand and therefore decreases the probability of the next ligand binding. Coulombic factors also come into play when the metal ion complexes with the first ligand, because the positive charge on the metal ion decreases, thus decreasing the coulombic attraction for the next ligand. Steric factors have the most effect when a ligand is bulky, as the steric repulsions created by this ligand affect the ease with which the subsequent ligands interact with the metal ion.
