Spin crossover in cobalt(II) systems

Cobalt(II), a d(7) ion, can exist in a low spin doublet state or a high spin quartet state. Both spin-state configurations are known in four-, five- and six-coordinate complexes. For each of these coordination numbers, systems are known where the two spin states may be thermally interconverted. For four-coordinate and some five-coordinate systems these interconversions are best regarded as configurational equilibria; in other instances a spin transition is involved. The features of these transitions show similarities to, but also marked differences from, those in iron(II) systems. The smaller change in spin associated with the transition (DeltaS=1) and the operation of a Jahn-Teller effect in the doublet state species confer special characteristics on the cobalt(II) systems.