Mo(NMe₂)₄ and the tridentate, dipyrrolyl ligand H₂dpma^mes were found to form 5-coordinate Mo(NMe₂)₂(dpma^mes) (1), which exhibits spin-crossover behaviour in solution. The complex is a ground state singlet with a barrier of 1150 cm⁻¹ for production of the triplet in d₈-toluene. The complex reacts with 1,1-disubstituted hydrazines or O-benzylhydroxylamine to produce nitrido MoN(NMe₂)(dpma^mes). The mechanism of the 1,1-dimethylhydrazine reaction with 1 was examined along with the mechanism of substitution of NMe₂ with H₂NNMe₂ in a diamagnetic zirconium analogue. The proposed mechanism involves production of a hydrazido(1–) intermediate, Mo(NMe₂)(NHNMe₂)(dpma^mes), which undergoes an α,β-proton shift and N–N bond cleavage with metal oxidation to form the nitrido. The rate law for the reaction was found to be −d[1]/dt = k_obs[1][hydrazine] by initial rate experiments and examination of the full reaction profile. This conversion from hydrazido(1–) to nitrido is somewhat analogous to the proposed mechanism for O–O bond cleavage in some peroxidases.