A complex in general is any species build by specific association of molecules or ions by donor-acceptor interactions. In aqueous solutions the most important complexes are those build between a metal ligands and cation, which can be ions (e.g. halides, cyanide, oxalate) or neutral molecules (e.g. ammonia, pyridine). The ligand behaves as a donor and replaces one or more water molecules from the primary solvation sphere, and thus a complex is different from an ion pair, which forms through completely electrostatic interactions in solvents of low polarity. Although complex formation is generally characteristic of transition metal ions it is by no means confined to them.
Several steps of complex formation can be possible, and the successive equilibrium constants for the reactions and so on are known as the stepwise formation constants K1, K2.... The overall equilibrium constant for the reaction