A dissociative mechanism for phosphine exchange in quadruply bonded bimetallic complexes

The exchange of two phosphine ligands in the metal dimer [Mo 2Cl4{H2P-(CH2)2-PH 2}2] was studied by means of DFT calculations with the B3LYP functional. A dissociative mechanism was fully characterized (minima, transition states) for the reaction that transforms the α isomer with two chelating diphosphines into the β isomer with two bridging diphosphines. It is essentially a two-step mechanism, each step involving the breaking of a Mo-P bond at a metal centre, followed by the formation of a new Mo-P bond with the other metal centre. This Mo-P bond exchange processes are found to be accompanied by the migration of a chloride ligand in the opposite direction. The activation energy computed at the unrestricted UB3LYP-bs level (24 kcal mol-1) is found to be in the range of experimental values.