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Protonation of Metal−Metal Bonds in Cp2Ru2(CO)3(PR3) and Cp2Mo2(CO)4(PR3)2

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posted on 20.05.1998, 00:00 by Chip Nataro, Robert J. Angelici
Despite the much higher basicity expected for the Ru bearing the PR3 ligand in Cp(PR3)Ru(μ-CO)2Ru(CO)Cp, NMR studies demonstrate that protonation of this complex with CF3SO3H occurs at the Ru−Ru bond, rather than at the more basic Ru. As determined by calorimetric titration at 25.0 °C in 1,2-dichloroethane solvent, the enthalpy of protonation (ΔHMHM) of the Ru−Ru bond is higher in Cp2Ru2(CO)3(PMe3) (−30.0(4) kcal/mol) than in its carbonyl analogue Cp2Ru2(CO)4 (−18.4(1) kcal/mol). Enthalpies (ΔHMHM) for protonation of the Mo−Mo bond in the dinuclear Mo complexes Cp2Mo2(CO)4(PR3)2 show that the PMe3 complex (−27.4(2) kcal/mol) is dramatically more basic than its PMe2Ph analogue (−18.9(5) kcal/mol). Considering the ΔHMHM values as measures of the basicities of the complexes, these results show that the basicities of metal−metal bonds are highly sensitive to the nature of their associated ligands. In addition, evidence indicates that Ru−Ru bonds are more basic than Ru in comparable mononuclear complexes. The structures, as determined by X-ray crystallographic studies, of Cp2Ru2(CO)3(PMe3) and its protonated derivative Cp2Ru2(CO)3(PMe3)(μ-H)+CF3SO3- are also discussed.