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 )(η 2 -C 5 H 7 )}{η 6 -(C 6 H 3 (CF 3 ) 2 )BAr F 3 } and Rh{P(Cyp 2 )(η 2 -C 5 H 7 )}(1- closo -CB 11 H 6 Br 6 ), which contain bound weakly coordinating anions, are reported. While thermochemical data show that enthalpically [1- closo -CB 11 H 6 Br 6 ] - binds less strongly with the metal fragment and it is the large entropy loss for the overall process of coordination of the [BAr F 4 ] - 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.