The relative binding abilities of PY3 (PMe3, PMe2Ph, PMePh2, PPh3, P(OMe)(3), P(OMe)(2)Ph, PEt3, P(OEt)(3), P(OEt)Ph, and dmpe) toward Re-V were evaluated. The equilibrium constants for the reactions, MeRe(NAr)(2){P(OMe)(3)}(2) + PY3 = MeRe(NAr)(2)(PY3)(2) (1) + P(OMe)(3), decrease in the order PMe3 > dmpe > PMe2Ph > P(OMe)(2)Ph similar to PEt3 > P(OEt)(3) > PMePh2 > P(OEt)Ph-2 > PPh3. Both electronic and steric factors contribute to this trend. The equilibrium constant increases as the basicity of PY3 increases when the steric demand is the same. However, steric effects play a major role in the coordination, and this is the reason that the affinity of PEt3 toward Re-V is less than that of PMe2Ph, A mixed-ligand complex, MeRe(NAr)(2){P(OMe)(3)}(PY3), was also observed in the course of the stepwise formation of 1. The large coupling constant, (2)J(PP) greater than or equal to 491 Hz, between the two phosphorus atoms suggests a trans geometry for the phosphines. Compound 1 catalyzes the oxidation of PY3 by molecular oxygen. Kinetic studies suggest that the reaction of 1 with O-2 is first-order with respect to [O-2] and inverse-first-order with respect to [PY3]. A mechanism involving a peroxorhenium intermediate MeRe(NAr)(2)(eta (2)-O-2) is proposed for the catalytic processes. The reactivity of MeRe(NAr)(2)(eta (2)-O-2) toward triaryl phosphines parallels that of the known compound MeReO2(eta (2)-O-2).