Thermodynamic and structural features of chloro-complex formation of bivalent transition metals in dimethylsulfoxide

Heats of formation of complexes of the type MCl+ have been determined calorimetrically for M=Mn, Co, Ni, and Cu in dimethylsulfoxide (DMSO), at 25°C. Combined with the stability constants of the complexes they complete the thermodynamic characteristics of these systems. It appears that the complexes are formed in markedly endothermic reactions and owe their considerable stabilities to the large positive reaction entropies. The exceptionally large entropy of formation of the CuCl+ complex indicates its essentially pentacoordinate structure, CuCl(DMSO) 4 + , while the other complexes are octahedral, MCl(DMSO) 5 + , with a probable contribution of the outer-sphere ion-pair for CuCl+. The characteristic features of complex formation in dimethylsulfoxide, as compared with water, are discussed and ascribed to the relatively large molar entropy of this liquid.