摘要 : The dinuclear phosphido-bridged molybdenum complexes [(CO)_4Mo(#mu#-PR_2)_2Mo(CO)_4][R=Ph or Me) react with P(n-Bu)_3 to give the corresponding mono and bis-phosphine derivatives.A kinetic study of the first substitution in decali... 展开 The dinuclear phosphido-bridged molybdenum complexes [(CO)_4Mo(#mu#-PR_2)_2Mo(CO)_4][R=Ph or Me) react with P(n-Bu)_3 to give the corresponding mono and bis-phosphine derivatives.A kinetic study of the first substitution in decalin indicates a CO-dissociative mechanism involving the coordinatively unsaturated intermediate [(CO_4)Mo(#mu#-PR_2)_2Mo(CO)_3].The overall substitution rate depends on the rate of CO dissociation,k_1,and on the rate of bimolecular attack by CO,k_(-1),and by P(n-Bu)_3,k_2,on the reactive 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 (#DELTA#_1~*=125(Ph)versus 141 (Me)kJ mol~(-1)),which may reflect a weakening of the Mo-CO bond in the presence of a more electron-withdrawing ligand 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 favoured with respect to the large P(n-Bu)_3;this suggests that the steric crowding 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. 收起