The first highly active, halide-free ruthenium catalyst for olefin metathesis

New ruthenium alkylidyne and alkylidene complexes are prepared, in which aryloxide groups function as pseudohalide ligands. The selectivity for alkylidene or alkylidyne products is controlled by steric matching or mismatching between pseudohalide and ancillary donor ligands. Alkylidene 5 achieves up to 40 000 turnovers in ring-closing metathesis of diethyl dially1malonate.