Stereoelectronic Effects Characterizing Nucleophilic Carbene Ligands Bound to the CpRuCl (Cp = η5-C5Me5) Moiety:  A Structural and Thermochemical Investigation

The reaction of [Cp*RuCl]4 (1) with carbene ligands affords unsaturated Cp*Ru(L)Cl [Cp* = η5-C5Me5; L = 1,3-R2-imidazol-2-ylidene [R = cyclohexyl (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 investigations 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-trimethylphenyl)imidazol-2-ylidene] and the widely used PCy3. Structural information from single X-ray studies for complexes 2, 3, 5, 6, Cp*Ru(IMes)Cl (7), Cp*Ru(PCy3)Cl (8), and Cp*Ru(PiPr3)Cl (9) permits a quantitative treatment of steric parameters associated with these ligands.