Substitution of Mn(CO)5 Is Associative

Article abstract of DOI:10.1021/ja00306a016

The substitution of Mn(CO)5 radicals by triphenylphosphine (PPh3) or triphenylarsine (AsPh3) in hexane has been studied by observing the manner in which the competition between CCl4 and PPh3 or between CH2Br2 and AsPh3 is affected by changes in concentrations of the reactants.In both cases the substitution process shows kinetics behavior indicative of an associative process.The second-order rate constants are 1.7 (+/- 0.2) E7 and 6.5 (+/-0.8) E4 M^-1 s^-1 for PPh3 and AsPh3, respectively, in hexane at 24 deg C.Less extensive measurements provide analogous bimolecular rate constants for substitution by P(n-Bu)3, P(i-Pr)3, and P(O-i-Pr)3 of 1.0 (+/-0.1) E9, 6.7 (+/-0.7) E7, and 3.1 (+/-0.3) E7 M^-1 s^-1, respectively.There is no evidence in the results for a concurrent dissociative loss of CO from Mn(CO)5.On the basis of the data for AsPh3, which exhibits the smallest associative rate constant, the first-order dissociative rate constant must be less than about 90 s^-1.