Synthesis of the acidic dihydrogen complexes trans-[M(H2)(CN)L2]+ and trans-[M(H2)(CNH)L2]2+ where M = Fe, Ru, Os and L = dppm, dppe, dppp, depe, and dihydrogen substitution by the trifluoromethanesulfonate anion to give trans-[Ru(OTf)(CN)L2] or trans-[Ru(OTf)(CNH)L2]OTf†

Very acidic complexes trans-[M(η2-H2)(CN)L2]+ and trans-[M(η2-H2)(CNH)L2]2+, with the dihydrogen ligand trans to the cyanide or to the hydrogen isocyanide ligand, are generated by reaction of trifluoromethanesulfonic acid (HOTf) with hydrido(cyano) complexes of Fe(II), Ru(II) and Os(II). The use of the different metals and phosphines (dppm = [bis(diphenylphosphino)methane], dppe = [1,2-bis(diphenylphosphino)ethane], dppp = [1,3-bis(diphenylphosphino)propane], and depe = [1,2-bis(diethylphosphino)ethane]) as ancillary ligands influences the stability and the reactivity of these complexes. The iron and osmium complexes are more stable than the ruthenium complexes that lose the dihydrogen ligand and coordinate the trifluoromethanesulfonate anion. The crystal structure of trans-[Ru(OTf)(CN)(dppe)2] is reported. The Ru–OTf bond is weak and so the triflate ligand can be displaced by H2(g) to give trans-[Ru(η2-H2)(CN)L2]OTf. There is a delicate balance of stability between the complexes trans-[M(η2-H2)(CN)L2]+ and trans-[M(H)(CNH)L2]+, M = Fe, Ru, determined by electronics and hydrogen bonding, both classical (CNH· · ·OTf–, TfOH· · ·OTf–) and non-classical (MH2· · ·OTf–). Therefore isomerisation reactions between these forms are observed for the first time. In order to determine where the protonation occurs it is useful to use a cyanide group labeled as C15N or 13CN. It is significant that the very acidic dihydrogen complex trans-[Ru(η2-H2)(CNH)L2]OTf is observed to form from the reaction of the weak Bronsted acids H2 and trans-[Ru(OTf)(CNH)L2]OTf in CH2Cl2; the dihydrogen complex releases HOTf. The chemistry is of possible relevance to the action of iron-containing hydrogenases.