Unusual pathways for the reaction between [MCl2(Me2SO)4] (M = Os, Ru) and hydrazine dihydrochloride: deoxygenation of sulfoxides vs. coordination of hydrazinium

Reaction of trans-[OsCl2(Me2SO)4] 1 with excess N2H4·2HCl leads to the osmium(III) compound mer-[OsCl3(NH3)2(Me2S)] 2 in the course of concerted reactions, i.e. oxidation of the metal, deoxygenation of the sulfoxide, disproportionation of hydrazine and substitution by NH3. In contrast, interaction of cis-[RuCl2(Me2SO)4] cis-3 with hydrazine dihydrochloride brings about easy substitution (without redox conversion) leading to mer-[RuCl3(N2H5)(Me2SO)2] 4 which is a rare example of a hydrazinium complex. X-Ray single-crystal diffraction analyses were performed on 1, 2 and 4. In 4, one Ru–S bond is unusually short due to the enhanced π bonding contribution as a result of an intramolecular H-bond between the Me2SO and the cis N2H5+ group. The complexes show anodic oxidations and 4, that in aqueous medium undergoes spontaneous dehydrochlorination, exhibits by controlled potential electrolysis a multi-electron oxidation process with anodically-induced H+ loss, oxidation of the hydrazine ligand to N2 and N-oxides, and of Me2SO to SO2. The anodic waves of Me2SO or H2O solutions of cis-3 were assigned to trans-[RuCl2(Me2SO-S )4] trans-3 and [RuCl(H2O)2(Me2SO-S )3]+, respectively. The oxidation potential values were interpreted on the basis of redox potential–structure relationships and the Lever electrochemical parameter EL was tentatively estimated for both S- and O-coordinated Me2SO and for the hydrazinium ligand, showing that Me2SO-S in our complexes behaves as a significant π-electron acceptor and N2H5+ as a rather weak net electron donor, and applied to predict the oxidation potential of some Ru and Os complexes.