Benzil bis(semicarbazone), H2L1, reacts with common rhenium(V) nitrido complexes such as [ReNCl2(PPh3)(2)] or [ReNCl2(PR2Ph)(3)] (R = Me, Et) under the release of one semicarbazone unit, cyclization, and formation of stable triazine-3-onato complexes of rhenium(V). The resulting 5,6-diphenyltriazine-3-one, HL2, acts as monodentate or chelating, monoanionic ligand depending on the reaction conditions applied. Complexes of the compositions [ReNCl(L-2-kappa N-2,kappa O)(PR2Ph)(2)] (R = Me, Et) or [ReN(L-2-kappa N-2,O)(L-2-kappa N-2)(PPh3)(2)] were isolated. The N 2 nitrogen atom is the preferred binding site of the monodentate form of the ligand. This contrasts the behavior of the analogous thione HL3, which preferably coordinates to nitridorhenium(V) centers via the sulfur atom. HL3 is readily formed by the abstraction of methanol from 5-methoxy-5,6-diphenyl-4,5-dihydro-2H-[1,2,4]triazine-3-thione, (H2LOCH3)-O-3. In the presence of [ReNCl2(PPh3)(2)] or [ReNCl2(PR2Ph)(3)] complexes (R = Me, Et), this reaction yields stable complexes of the composition [ReN(L-3-kappa N-2,kappa S)(L-3-kappa S)(PR2Ph)(2)] (R = Me, Et, Ph) in good yields. Reduction of the metal atom and formation of the seven-coordinate [Re(PPh3)(L-3-kappa N-2,kappa S)(3)] was observed during reactions of (H2LOCH3)-O-3 with [ReOCl3(PPh3)(2)] or [ReO2I(PPh3)(2)], while no rhenium complexes could be isolated during similar reactions with H2L1, although cyclization of the bis(semicarbazone) and the formation of (H2LOEt)-O-2 were observed.