Rhodium phosphine olefin complexes of the weakly coordinating anions [BArF4]- and [1-closo-CB11H6Br6]-.: Kinetic versus thermodynamic factors in anion coordination and complex reactivity

Solution and solid-state structures for the pair of complexes Rh{P(Cyp(2))(eta(2)-C5H7)}{eta(6)-(C6H3(CF3)(2))BAr3F} and Rh{P(Cyp(2))(eta(2)-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 [BAr4F](-) anion that results in the latter anion being thermodynamically more weakly coordinating. Qualitative kinetic data arising from reaction with H-2 indicates that the carborane anion is displaced more readily, attributable to the ability of the carborane to lift a Rh-Br interaction.