The molybdenum dinitrogen complex trans-[Mo(N-2)(2)(dppe)(2)] (2) reacted with 2-2.5 equiv of various beta-ketonitriles at room temperature to afford the (nitrido)(nitrile-enolato) complexes trans-(Mo(N)(NCCR1-COR2)(dppe)(2)] (4; R-1 = H, R-2 = 4-MeOCOC6H4, 4-ClC6H4, 4-Tol, 4-MeOC6H4, 2-C4H3O, 2-C4H3S, Pr-i; R-1 = CN, R-2 = Me, Ph; dppe = Ph2PCH2CH2PPh2 via the C=N triple bond cleavage of the nitriles on the molybdenum center. On the other hand, the reaction of complex 2 with ? equiv of pivaloylacetonititrile at room temperature led to the isolation of the (alkylideneamido)(nitrile-enolato) complex trans-[Mo(NCHCH2COBul(NCCHCOBul)(dppe)(2)] (5k), which further underwent the cleavage of the C=N double bond of the alkylideneamido ligand to give the corresponding (nitrido)(nitrile-enolato) complex trans-[Mo(N)(NCCHCOBul)(dppe)(2)] together with 4,4-dimethyl-1-penten-3-one. Furthermore, treatment of 2 with large excess amounts of 3-chlorobenzoylacetonitrile followed by anion metathesis with [NHEt3][OTf] (OTf = OSO2CF3) resulted in me isolation of the cationic (imido)(nitrile-enolato) complex trans-[Mo{NCH2CH2CO(C6H4Cl-4)}{NCCHCO-(C6H4Cl-4)}(dppe)(2)][OTf] (6c(+)[OTf](-)). The solid-state structures of 4h . 1.5C(2)H(4)Cl(2) (R-1 = CN, R-2 = Me), 5k . C6H6, and 6c(+)[OTf](-) were determined by single-crystal X-ray analyses. The detailed NMR analysis of the reaction of 2 with aroylacetonitriles revealed that the (alkylideneamido)(nitrile-enolato) complexes trans-[Mo-NCHCH2COR)(NCCHCOR)(dppe)(2)] (5) act as the key intermediates for the C=N triple bond fission. and the rate constants for the conversion of the complexes 5 into the nitrido complexes 4 showed good correlation with the Hammett sigma(p) or sigma(a) constants for the aroyl substituents, when positive rho values were obtained (rho(p), 1.42; rho(a), 0.41). A reaction mechanism for the nitrido complex formation is proposed, which includes (1) the substitution of a dinitrogen ligand in 2 with a beta-ketonitrile molecule, (2) the fast protonation of the nitrite ligand by a second beta-ketonitrile molecule leading to the formation of complex 5, (3) the relatively slow proton shift from the a-position of the carbonyl group to the amido carbon in the alkylideneamido ligand to form an enolated imido ligand. and (4) the fast elimination of a vinyl ketone from the imido ligand giving the nitrido complex 4 as the final product.