Spin crossover in chromium(II) complexes

The chromium(II) complex [CrI2(depe)2], where depe is 1,2-bis(diethylphosphino)ethane, is unusual in that it is high spin at room temperature, but on cooling undergoes a sharp transition to the low spin form at ca 170 K. Other complexes of chromium(II) halides and depe or 1,2-bis(dimethylphosphino)ethene (dmpe) are low spin. The structure of the high spin form of [CrI2(depe)2] has been determined.