Complex ions typically exist in complex equilibrium involving its central metal ion and the ligands.
- A metal ion in solution does not exist in isolation, but in combination with ligands (such as solvent molecules or simple ions) or chelating groups, giving rise to complex ions or coordination compounds. These complexes contain a central atom or ion, often a transition metal, and a cluster of ions or neutral molecules surrounding it.
- Complex-Ion Equilibria
- In general, chemical equilibrium is reached when the forward reaction rate is equal to the reverse reaction rate and can be described using an equilibrium constant, K. Complex ion equilibria are no exception to this and have their own unique equilibrium constant. This formation constant, Kf , describes the formation of a complex ion from its central ion and attached ligands. This constant may be caled a stability constant or association constant.
- Hard and Soft Acids and Bases
- The thermodynamic stability of a metal complex depends greatly on the properties of the ligand and the metal ion and on the type of bonding. Metal–ligand interaction is an example of a Lewis acid–base interaction. Lewis bases can be divided into two categories: hard bases contain small, relatively nonpolarizable donor atoms (such as N, O, and F), and soft bases contain larger, relatively polarizable donor atoms (such as P, S, and Cl).
- Stability of Metal Complexes and Chelation
- Ligands like chloride, water, and ammonia are said to be monodentate (one-toothed, from the Greek mono, meaning “one,” and the Latin dent-, meaning “tooth”): they are attached to the metal via only a single atom. Ligands can, however, be bidentate (two-toothed, from the Greek di, meaning “two”), tridentate (three-toothed, from the Greek tri, meaning “three”), or, in general, polydentate (many-toothed, from the Greek poly, meaning “many”), indicating that they are attached to the metal at two, th