Mechanisms for magnetic exchange in some binuclear halide complexes of chromium(III) and molybdenum(III)

Magnetic exchange has been studied in a series of compounds of formula AI3MIII2X9 (A = univalent cation, M = Cr or Mo, X = Cl or Br) over the range 90-450°K. By correlating the variations observed with X-ray data on bond angles and metal-metal distances in the M2X93- binuclear entity, information on the relative contributions of the different mechanisms for spin-coupling has been obtained. In the case of the molybdenum compounds direct exchange predominates. Superexchange through the bridging halogens constitutes a minor proportion of the total exchange but the relative contributions of the ferromagnetic and antiferromagnetic superexchange are affected by structural variations. For the corresponding chromium compounds, in which the total exchange is smaller by a factor of 50 or more, superexchange is comparable in magnitude with direct exchange.