At ambient temperatures in methanol the complexes trans-[Mo(NH)X(dppe)2]+ [where X = F, Cl, Br, or I and dppe = 1,2-bis (diphenylphosphino)ethane] are inert to acid, but give ammonia in high yield on treatment with methoxide. An initial rapid deprotonation of the substrate renders the trans halide labile and when X = Cl, Br, or I the halide is liberated rapidly (k > 300 s-1) to yield a common species which on spectrophotometric evidence is an equilibrium mixture of the contact and solvent-separated ion pairs of [MoN(dppe)2]+ and X-. This cation reacts with methoxide ion (k = 1.7 ± 0.4 × 106 dm3 mol-1s-1), to yield [MoN(OCH3)(dppe)2] which rapidly abstracts a proton from solvent to give [Mo(NH)(OCH3)(dppe)2]+. When X = F, the same sequence of reactions occurs but dissociation of fluoride from the intermediate [Mo(N)F(dppe)2] is rate-limiting (k = 1.78 s-1). The pKa of trans-[Mo (NH)X(dppe)2]+ is very sensitive to the nature of the trans ligand and π-bonding influences dominate. [Mo(NH)(OCH3) (dppe)2]+ reacts (k = 1.57 × 10-4 s-1 at 25 °C, ΔH‡ = 50 kJ mol-1, ΔS‡ = -143 J K-1 mol-1) in basic methanol with rate-limiting phosphine-chelate ring opening to give a product which gave a positive indophenol test for ammonia (70% yield). [Mo(NH)(OCH3)(dppe)2]+ reacts with HX in methanol to form trans-[Mo(NH)X(dppe)2]+ with rate-limiting dissociation of methanol (k5 > 3.9 × 102 s-1) formed by rapid protonation of the co-ordinated methoxy-group (pKa
