Kinetics and substitution mechanism in the reaction of the seven-coordinate complexes bis-mu-diphenylphosphido- and bis-mu-dimethylphosphido-octacarbonyldimolybdenum (Mo-Mo) with tri-n-butyl-phosphine

The dinuclear phosphido-bridged molybdenum complexes [(CO)(4)Mo(mu -PR2)(2) Mo(CO)(4)] (R = Ph or Me) react with P(n-Bu)(3) to give the corresponding m ono and bis-phosphine derivatives. A kinetic study of the first substitutio n in decalin indicates a CO-dissociative mechanism involving the coordinati vely unsaturated intermediate [(CO4)Mo(mu -PR2)(2)Mo(CO)(3)], The overall s ubstitution rate depends on the rate of CO dissociation, k(1), and on the r ate of bimolecular attack by CO, k(-1), and by P(n-Bu)(3), k(2). on the rea ctive intermediate. The nature of the substituents at the phosphido bridge markedly affects the value of k(1) which is higher for the phenyl compared with the methyl group. This is mainly due to a lower activation enthalpy (D eltaH(1)* = 125 (Ph) versus 141 (Me) kJ mol(-1)), which may reflect a weake ning of the Mo-CO bond in the presence of a more electron-withdrawing ligan d in trans position. The values of the competition rate ratio k (- 1)/k(2), always largely greater than unity, show that attack of the small CO is fav oured with respect to the large P(n-Bu)3; this suggests that the steric cro wding observed on the molecular structure of the starting seven-coordinate complexes should play an important role also on the reactivity of their six -coordinate intermediates. (C) 2001 Elsevier Science B.V. All rights reserv ed.