Multihydrido-complexes of osmium and related complexes

Abstract

A new route to tetrachloro-complexes of the type trans-[OsCl₄L₂] from mer-[OsCl₃L₃](L = tertiary phosphine or tertiary arsine) is described, which involves treatment with chlorine in the presence of light. The tetrachloro-osmium(III) complex trans-[OsCl₄(PMe₂Ph)₂] reacts with tertiary phosphines, tertiary arsines, alkyl phosphites, or alkyl phosphonites (L′) to give [OsCl₃(PMe₂Ph)₂L′]. Reduction of mer-[OsCl₃L₃] or [OsCl₃(PMe₂Ph)₂L′] with either sodium borohydride or lithium aluminium hydride gives tetrahydrido-osmium complexes of types [OsH₄L₃] or [OsH₄(PMe₂Ph)₂L′]. Similar reduction of trans-[OsCl₄(PMe₂Ph)₂] gives a mixture of [OsH₆(PMe₂Ph)₂] and [OsCl₄(PMe₂Ph)₂]^–. The tetrahydrides of type [OsH₄L₃] react with hydrogen chloride to give fac-[OsCl₃L₃]. The tetrahydrides also react slowly with EtOD in benzene to give the corresponding tetradeuterides. The hydrogen–deuterium exchange is catalysed by strong acids and is completely inhibited by a strong base such as tetrabutylammonium hydroxide. In benzene solution the high-field resonance (hydride) splittings due to coupling with phosphorus nuclei collapse immediately to a singlet resonance on adding CF₃·CO₂H, because of rapid exchange. Conductimetric titration shows that in the presence of a strong acid [OsH₄L₃] is converted into [OsH₅L₃]⁺. I.r. and n.m.r. data are given and discussed.