Transalkynylation and catalytic demercuration of bis(alkynyl)mercurials: two alternative mechanisms

The complexes [RhCl(CO)(PPh3)2] and [MHCl(CO)(PPh3)3](M = Ru, Os) catalyse the demercuration of [Hg(CCR)2](R = Ph, C6H4Me-4, Bun) to provide buta-1,3-diynes, however the reactions proceed by quite different mechanisms as indicated by (i) the isolation of [Ru{C(CCR)CHR}Cl(CO)(PPh3)2] and its reintroduction into the catalytic cycle and (ii) the isolation of the iridium complex [IrCl(CCR)(HgCCR)(CO)(PPh3)2].