Halostibines SbMeX₂ and SbMe₂X: Lewis Acids or Lewis Bases?

Alkylhalostibines have been shown to behave either as Lewis acids toward appropriate neutral ligands or as Lewis bases to low-valent metal fragments. The boundaries of their Lewis acid and Lewis base behavior have been determined, and the structural and spectroscopic consequences of the different behaviors probed. [SbMeX₂(L–L)] (L–L = 2,2′-bipyridyl, 1,10-phenanthroline) and [SbMeX₂(L)₂] (L = OPMe₃, OPPh₃) are five-coordinate, distorted square-pyramidal monomers with the Me group axial, with the bond distances little affected by coordination. Significant changes in the bonding within the group 6 carbonyl complexes [M­(CO)₅(L′)] are evident, with L′→M σ-donation decreasing across the series L′ = SbMe₃ → SbMe₂Br → SbMeBr₂, and an increase in M→L′ π-acceptance within the same series. Intermolecular interactions are a major feature within these systems, ranging from weak, MCO···Sb interactions in [M­(CO)₅(SbMe₂Br)] and [M­(CO)₅(SbMeBr₂)] to remarkably strong O···Sb hypervalent bonds in [Mn­(CO)₅(SbMe₂Br)]­[CF₃SO₃] and [Mn­(CO)₃(SbMe₂Br)₃]­[CF₃SO₃], the latter involving the triflate anions. These lead to large changes in the geometry at Sb and represent very rare examples in which the antimony exhibits significant Lewis acid and Lewis base behavior simultaneously. The coordinated alkylhalostibines are alkylated cleanly with MeLi or ⁿBuLi