Reaction of trans-[FeH(MeCN)(DPPE)2]+ with PhCN to form trans-[FeH(PhCN)(DPPE)2]+ occurred with a single measurable kinetic step in THF, acetone or MeOH solutions (DPPE = Ph2PCH2CH2PPh2). The observed rate constants are independent of the concentration of the nucleophile and do not change very much with the solvent nature. However, the activation parameters are strongly solvent-dependent, especially ΔV‡ that changes from 20 cm3 mol–1 in THF to 35 cm3 mol–1 in neat benzonitrile. Kinetic results for the reactions of trans-[FeH(MeCN)(DPPE)2]+ with substituted benzonitriles indicate that kinetic parameters for the substitution of co-ordinated MeCN also change with the nature of the entering ligand. The reactions of trans-[FeH(L)(DPPE)2]+ complexes (L = MeCN or H2) with the bidentate ligands C6H4(CN)2 (1,2-dicyanobenzene) and Me2PCH2CH2PMe2 (DMPE) have been also monitored by NMR at variable temperature. While for the acetonitrile complex and dicyanobenzene, trans-[FeH{C6H4(CN)2}(DPPE)2]+ is the only product detected with no evidence of the formation of reaction intermediates or side-products, small amounts of free DPPE are detected for the same reaction with the dihydrogen complex to form the same monosubstituted complex. Moreover, whereas the acetonitrile complex does not react with DMPE, free DPPE is the only DPPE-containing product after reaction of trans-[FeH(H2)(DPPE)2]+ with DMPE excess. The whole of the kinetic data and NMR observations are, on the one hand, consistent with a chelate ring-opening mechanism for the reactions of the dihydrogen complex and, on the other hand with a simple dissociative mechanism for the reactions of the acetonitrile complex. In all cases, the absolute values of ΔV‡ for these reactions are larger than usual and also show an unusual dependence on the nature of the solvent and the entering ligand.