Stereoelectronic effects characterizing nucleophilic carbene ligands boundto the Cp*RuCl (Cp* = eta(5)-C5Me5) moiety: A structural and thermochemical investigation

The reaction of [Cp*RuCl](4) (1) with carbene ligands affords unsaturated C p*Ru(L)Cl [Cp* = eta(5)-C5Me5; L = 1,3-R-2-imidazol-2-ylidene [R = cyclohex yl (ICy, 2); 4-methylphenyl (ITol, 3); 4-chlorophenyl (IpCl, 4); adamantyl (IAd, 5)] and 4,5-dichloro-1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMesCl, 6)] complexes 2-6 in high yield. Solution calorimetric investigat ions of this series provides a measure of the electron donor properties of all ligands, and comparisons are made with IMes [1,3-bis(2,4,6-trimethylphe nyl)imidazol-2-ylidene] and the widely used PCy3. Structural information fr om single X-ray studies for complexes 2, 3, 5, 6, Cp*Ru(IMes)Cl (7), Cp*Ru( PCy3)Cl (8), and Cp*Ru((PPr3)-Pr-i)Cl (9) permits a quantitative treatment of steric parameters associated with these Ligands.