The oxygen atom transfer reactivity of the dioxo-Mo(VI) complex, Tp(iPr)MoO(2)(OPh) (Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate), with a range of tertiary phosphines (PMe3, PMe2Ph, PEt3, PBu3n, PEt2Ph, PEtPh2, and PMePh2) has been investigated. The first step in all the reactions follows a second-order rate law indicative of an associative transition state, consistent with nucleophilic attack by the phosphine on an oxo ligand, namely, Tp(iPr)MoO(2)(OPh) + PR3 -> Tp(iPr)MoO(2)(OPh)(OPR3). The calculated free energy of activation for the formation of the OPMe3 intermediate (Chem. Eur. J. 2006, 12, 7501) is in excellent agreement with the experimental Delta G double dagger value reported here. The second step of the reaction, that is, the exchange of the coordinated phosphine oxide by acetonitrile, Tp(iPr)MoO(OPh)(OPR3) + MeCN -> Tp(iPr)MoO(OPh)(MeCN) + OPR3, is first-order in starting complex in acetonitrile. The reaction occurs via a dissociative interchange (I-d) or associative interchange (l(a)) mechanism, depending on the nature of the phosphine oxide. The activation parameters for the solvolysis of Tp(iPr)MoO(OPh)(OPMe3) (Delta H double dagger = 56.3 kJ mol(-1); Delta S double dagger = -125.9 J mol(-1) K-1; Delta G double dagger = 93.8 kJ mol(-1)) and Tp(iPr)MoO(OPh)(OPEtPh2) (Delta H double dagger = 66.5 kJ mol(-1); Delta S double dagger = -67.6 J mol(-1) K-1; Delta G(double dagger) = 86.7 kJ mol(-1)) by acetonitrile are indicative of l(a) mechanisms. In contrast, the corresponding parameters for the solvolysis reaction of Tp(iPr)MoO(OPh)(OPEt3) (Delta H double dagger = 95.8 kJ mol(-1); Delta S double dagger = 26.0 J mol(-1) K-1; Delta G double dagger = 88.1 kJ mol(-1)) and the remaining complexes by the same solvent are indicative of an Id mechanism. The equilibrium constant for the solvolysis of the oxoMo(V) phosphoryl complex, [Tp(iPr)MoO(OPh)(OPMe3)](+), by acetonitrile was calculated to be 1.9 x 10(-6). The oxo-Mo(V) phosphoryl complex is more stable than the acetonitrile analogue, whereas the oxo-Mo(IV) acetonitrile complex is more stable than the phosphoryl analogue. The higher stability of the Mo(V) phosphoryl complex may explain the phosphate inhibition of sulfite oxidase.