The way into the bridge: a new bonding mode of tertiary phosphanes, arsanes, and stibanes.

Until recently, tertiary phosphanes, arsanes, and stibanes were considered to bind to transition-metal centers only in a terminal coordination mode. Investigations on the reactivity of square-planar trans-[RhCl(=CRR')(L)(2)] compounds revealed that compounds in which L=SbiPr(3) can be converted upon heating into dinuclear complexes [Rh(2)Cl(2)(micro-CRR')(2)(micro-SbiPr(3))] with the carbene and stibane ligands in bridging positions. Although attempts to replace the stibane in these complexes with a tertiary arsane or phosphane failed, substitution of the chloro ligands for acetylacetonates followed by bridge-ligand exchange allowed the preparation of the phosphane- and arsane-bridged compounds [Rh(2)(acac)(2)(micro-CRR')(2)(micro-PR(3))] and [Rh(2)(acac)(2)(micro-CRR')(2)(micro-AsMe(3))]. The acac ligands can be replaced by anionic Lewis bases to give either monomeric [Rh(2)X(2)(micro-CRR')(2)(micro-ER(3))] or dimeric chain-like [XRh(micro-CRR')(2)(micro-ER(3))Rh(micro-X)(2)Rh(micro-CRR')(2)(micro-ER(3))RhX] molecules.