Effect of Substituents on the Formation of Vinylideneruthenium(II) Complexes. X-ray Structures of RuCl<sub>2</sub>{CC(Z)Ph}(dcpmp) (Z = H, SiMe<sub>3</sub>; dcpmp = C<sub>5</sub>H<sub>3</sub>N(CH<sub>2</sub>PCy<sub>2</sub>)<sub>2</sub>)

Kinetic studies have been carried out for the formation of vinylideneruthenium(II) complexes RuCl<sub>2</sub>{CC(Z)R}(dcpmp) [Z = H, R = <i>p</i>-MeOC<sub>6</sub>H<sub>4</sub> (<b>2a</b>), <i>p</i>-MeC<sub>6</sub>H<sub>4</sub> (<b>2b</b>), Ph (<b>2c</b>), <i>p</i>-BrC<sub>6</sub>H<sub>4</sub> (<b>2d</b>), <i>t</i>-Bu (<b>2e</b>), ferrocenyl (<b>2f</b>); Z = SiMe<sub>3</sub>, R = Ph (<b>5a</b>), SiMe<sub>3</sub> (<b>5b</b>); dcpmp = 2,6-bis{(dicyclohexylphosphino)methyl}pyridine] from RuCl<sub>2</sub>(NCMe)(dcpmp) (<b>1</b>) and alkynes (RC⋮CZ). The reactions with terminal alkynes proceed via preliminary dissociation of the MeCN ligand from <b>1</b>, giving vinylidene complexes <b>2a</b>−<b>2f</b> in quantitative yields. More electron-rich and less sterically demanding alkynes tend to provide higher reaction rates. On the other hand, the reactions with silylalkynes afford equilibrium mixtures of <b>1</b> and β-silylvinylidene complexes <b>5</b>. The X-ray structures of <b>2c</b> and <b>5a</b> are reported.