Complexes of (diphosphine)halogenonickel(II) with the anions of pentaborane(9) and 1-bromopentaborane(9)

Abstract

Diphosphine complexes of nickel(II) halides react readily at low temperature with K[B₅H₈] and K[1-BrB₅H₇] to give the corresponding complexes [Ni(µ-B₅H₈)X(dppe)] and [Ni(µ-1-BrB₅H₇)X(dppe)][X = Cl, Br, or I, dppe = 1,2-bis(diphenylphosphino)ethane]. The complex [Ni(µ-B₅H₈)Br(dppp)] has also been prepared [dppp = 1,3-bis(diphenylphosphino)propane]. The complexes are monomeric and non-conducting in solution and their n.m.r. spectra (¹H, ¹¹B, and ³¹P) are best interpreted in terms of a square-planar NiX(dppe) group inserted via a three-centre two-electron B–Ni–B bond into a bridging position between two basal boron atoms of the pentaborane cluster. The four basal boron atoms and three bridging hydrogen atoms are equivalent on the n.m.r. time scale, implying fluxional behaviour. In this, the nickel complexes differ from ‘isoelectronic’(16-electron) copper(I) and cadmium(II) complexes of the type [Cu(µ-B₅H₈)(PPh₃)₂] and [Cd(µ-B₅H₈)Cl(PPh₃)] which are non-fluxional.