The terminal phosphorus monoxide complex (OP)Mo(N[(t)Bu]Ar)(3), 1 (Ar = 3,5-Me(2)C(6)H(3)), undergoes an O-for-PSiR(3) metathesis reaction with the niobium phosphinidene complex (i)Pr(3)SiPNb(N[CH(2)(t)Bu]Ar)(3), 2, to generate the oxoniobium complex ONb(N[CH(2)(t)Bu]Ar)(3), 3, and the diphosphenido complex (i)Pr(3)SiPPMo(N[(t)Bu]Ar)(3), 4. The structure of 4, as determined by X-ray crystallography, contains a "singly bent" diphosphenido moiety, suggesting that the diphosphenido ligand serves as a 3e(-) donor to a formally d(2) metal center. This bonding characterization was supported by DFT calculations and is unique among known diphosphenido complexes. Diphosphenido 4 was found to react over time to produce products consistent with a bimolecular degradation pathway where the terminal phosphide complex PMo(N[(t)Bu]Ar)(3), 5, serves as a stable leaving group. Mixtures of 4 and PPh(3) were observed to set up an equilibrium (K(eq) = 0.7) between 4, PPh(3), and the products of phosphinidene transfer, 5 and (i)Pr(3)SiP=PPh(3).