Success and serendipity during studies aimed at preparing carbenerhodium(I) complexes

The preparation of a series of new square-planar and half-sandwich type carbenerhodium(I) complexes will be described. The key to success is the use of the bis(stibane)rhodium compound trans-[RhCl(C₂H₄)(SbiPr₃)₂] as starting material from which in a stepwise manner the complexes trans-[RhCl(=CRR′)(SbiPr₃)₂] (L = PiPr₃, AsiPr₃, SbEt₃) and [C₅H₅Rh(=CCR′)L] (L = SbiPr₃, PiPr₃, PMe₃, CO, CNtBu) have been obtained. Displacement of the carbene ligand in either trans-[RhCl(=CPH₂)L₂]L = SbiPr₃, PiPr₃ or [C₅H₅Rh(=CPh₂)(PiPr₃)] by CO or CNtBu leads to the formation of the corresponding carbonyl- or isocyanidrhodium compounds and the C---C coupling products Ph₂C=C=O and Ph₂C=C=NtBu, respectively. The carbene ligand is also involved in the selective formation of the isomeric olefins CH₂=CHCPh₂H and Ph₂C=CHCH₃ on treatment of trans-[RhCl(=CPh₂)(SbiPr₃)₂] and trans-[RhCl(=CPh₂)(PiPr₃)₂] with ethene. The most spectacular reaction of the bis(triisopropylstibane) complexes, however, occurs on warming of trans-[RhCl(=CRR′)(SbiPr₃)₂] in the absence of any substrate which yields the first representatives of dinuclear transition-metal compounds containing a tertiary stibane ligand in a bridging position. Some exploratory studies on the reactivity of the Rh₂(μ-SbiPr₃) complexes indicate that the triisopropylstibane can be replaced by SbMe₃, SbEt₃ or CNtBu without destroying the dimetallic core of the molecule.