Rhodium Phosphine Olefin Complexes of the Weakly Coordinating Anions [BAr^F₄]^- and [1-<i>closo</i>-CB₁₁H₆Br₆]^-. Kinetic versus Thermodynamic Factors in Anion Coordination and Complex Reactivity

Solution and solid-state structures for the pair of complexes Rh{P(Cyp2)(η2-C5H7)}{η6-(C6H3(CF3)2)BArF3} and Rh{P(Cyp2)(η2-C5H7)}(1-closo-CB11H6Br6), which contain bound weakly coordinating anions, are reported. While thermochemical data show that enthalpically [1-closo-CB11H6Br6]- binds less strongly with the metal fragment and it is the large entropy loss for the overall process of coordination of the [BArF4]- anion that results in the latter anion being thermodynamically more weakly coordinating. Qualitative kinetic data arising from reaction with H2 indicates that the carborane anion is displaced more readily, attributable to the ability of the carborane to lift a Rh−Br interaction.