Synthesis of the acidic dihydrogen complexes trans-[M(H-2)(CN)L-2](+) and trans-[M(H-2)(CNH)L-2](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)L-2] or trans-[Ru(OTf)(CNH)L-2]OTf

Very acidic complexes trans-[M(eta(2)-H-2)(CN)L-2](+) and trans-[M(eta(2)-H -2)(CNH)L-2](2+), with the dihydrogen ligand trans to the cyanide or to the hydrogen isocyanide ligand, are generated by reaction of trifluoromethanes ulfonic acid (HOTf) with hydrido(cyano) complexes of Fe(II), Ru(II) and Os( II). The use of the different metals and phosphines (dppm= [bis(diphenylpho sphino)methane], dppe=[1,2-bis(diphenylphosphino)ethane], dppp=[1,3-bis(dip henylphosphino)propane], and depe=[1,2-bis(diethylphosphino)ethane]) as anc illary ligands influences the stability and the reactivity of these complex es. The iron and osmium complexes are more stable than the ruthenium comple xes that lose the dihydrogen ligand and coordinate the trifluoromethanesulf onate anion. The crystal structure of trans-[Ru(OTf)(CN)(dppe)(2)] is repor ted. The Ru-OTf bond is weak and so the triflate ligand can be displaced by H-2(g) to give trans-[Ru(eta(2)-H-2)(CN)L-2]OTf. There is a delicate balan ce of stability between the complexes trans-[M(eta(2)-H-2)(CN)L-2](+) and t rans-[M(H)(CNH)L-2](+), M=Fe, Ru, determined by electronics and hydrogen bo nding, both classical (CNH ... OTf-, TfOH ... OTf-) and non-classical (MH2. .. OTf-). Therefore isomerisation reactions between these forms are observe d for the first time. In order to determine where the protonation occurs it is useful to use a cyanide group labeled as (CN)-N-15 or (CN)-C-13. It is significant that the very acidic dihydrogen complex trans-[Ru(eta(2)-H-2)(C NH)L-2]OTf is observed to form from the reaction of the weak Bronsted acids H-2 and trans-[Ru(OTf)(CNH)L-2]OTf in CH2Cl2; the dihydrogen complex relea ses HOTf. The chemistry is of possible relevance to the action of iron-cont aining hydrogenases.