White phosphorus (P(4)) reacts with three-coordinate molybdenum(III) trisamides or molybdaziridine hydride complexes to produce either bridging or terminal phosphide (P(3)(-)) species, depending upon the ancillary ligand steric demands. Thermochemical measurements have been made that place the MoP triple bond dissociation enthalpy at 92.2 kcal.mol(-)(1). Thermochemical measurements together with computational analysis rule out simple P-atom abstraction from P(4) as a step in the phosphorus activation mechanism. Kinetic measurements made by the stopped-flow method show that the reaction between the monomeric molybdenum complexes and P(4) is first-order both in metal complex and in P(4). Cyclo-P(3) complexes can be obtained when ancillary ligand steric demands are small, but kinetic measurements rule them out as monometallic intermediates in the P(4) activation mechanism. Also studied by calorimetric, kinetic, and in one case variable-temperature NMR methods is the process of mu-phosphide bridge formation. Post-rate-determining steps of the P(4) activation process were examined in a search for minima on the reaction's potential energy surface, leading to the proposal of two plausible, parallel, bimetallic reaction channels.

Although the kinetics and mechanism of metal-mediated oxygen atom (oxo) transfer reactions have been examined in some detail, sulfur atom (sulfido) transfer reactions have not been similarly scrutinized. The reactions [M(IV)(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-) + Ph(3)AsQ --> [M(VI)Q(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-) + Ph(3)As (M = Mo, W; Q = O, S) with variable substituent X' have been investigated in acetonitrile in order to determine the relative rates of oxo versus sulfido transfer at constant structure (square pyramidal) of the atom acceptor and of atom transfer at constant structure of the atom donor and metal variability of the atom acceptor. All reactions exhibit second-order kinetics and entropies of activation (-25 to -45 eu) consistent with an associative transition state. At parity of atom acceptor, k(2)(S) (0.25-0.75 M(-1)s(-1)) > k(2)(O) (0.023-0.060 M(-1)s(-1)) with M = Mo and k(2)(S) (4.1-66.7 M(-1)s(-1)) > k(2)(O) (1.8-9.8 M(-1)s(-1)) with M = W. At constant atom donor and X', k(2)(W) > k(2)(Mo) with reactivity ratios k(2)(W)/k(2)(Mo) = 78-184 (Q = O) and 16-89 (Q = S). Rate constants refer to 298 K. At constant M and Q, rates increase in the order X' = Me less, similar OMe < H < Br < COMe < CN; increasing electron-withdrawing propensity accelerates reaction rates. The probable transition state involves significant Ph(3)AsQ...M bond-making (X' rate trend) and concomitant As-Q bond weakening (bond energy order As-O > As-S). Orders of oxo and sulfido donor ability of substrates and complexes are deduced on the basis of qualitative reactivity properties determined here and elsewhere. This work complements previous studies of the reaction systems [M(IV)(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-)/XO where the substrates are N-oxides and S-oxides and k(2)(W) > k(2)(Mo) at constant substrate also applies. The reaction order of substrates is Me(3)NO > (CH(2))(4)SO > Ph(3)AsS > Ph(3)AsO. This research provides the first quantitative information of metal-mediated sulfido transfer.