The kinetics of the reactions of trans-[M(N2)2(Ph2PCH2CH2PPh2)2] or trans-[M(N2)2(Et2PCH2CH2PEt2)2](M = Mo or W) with acids, HX (X = Cl, Br, or HSO4), in tetrahydrofuran, to yield the corresponding hydrazido(2–)-complexes, trans-[M(NNH2)X(diphos)2]+(diphos = bidentate diphosphine), have been investigated. The mechanism proposed involves the initial rapid formation of an adduct between HX and the substrate, and protonation of a co-ordinated dinitrogen by another molecule of acid, prior to the rate-limiting dissociation of dinitrogen. The reaction between HCl and trans-[M(N2)2(Ph2PCH2CH2PPh2)2] yields the hydrido-complexes [WH(N2)2(Ph2-PCH2CH2PPh2)2]+ and [MoH2Cl2(Ph2PCH2CH2PPh2)2]. The kinetics of formation of these hydrides have also been studied and a rationalisation for their formation, rather than the hydrazido(2–)-complexes, is presented.