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

The complexes [RhCl(CO)(PPh(3))(2)] and [MHCl(CO)(PPh(3))(3)] (M = Ru, Os) catalyse the demercuration of [Hg(C=CR)(2)] (R = Ph, C(6)H(4)Me-4, Bu(n)) to provide buta-1,3-diynes, however the reactions proceed by quite different mechanisms as indicated by (i) the isolation of [Ru{C(C=CR)= CHR}Cl(CO)(PPh(3))(2)] and its reintroduction into the catalytic cycle and (ii) the isolation of the iridium complex [IrCl(C=CR)(HgC=CR)-(CO)(PPh(3))(2)].