Reactivity patterns in the base hydrolysis of coordinated aromatic nitriles

Two new aromatic organonitrile complexes of pentaamminecobalt(III) have been synthesized, the complexes of 2- and 4-nitrobenzonitrile. The kinetics of their base hydrolysis to coordinated carboxamides have been measured and found to follow a simple second order rate law with rate constants of 180 ± 4 M^-1 s^-1 and 510 ± 90 M^-1 s^-1 at 25°C and ionic strength 1.0 for the ortho and para isomers respectively. Rate constants for the hydrolysis of eleven coordinated substituted aromatic nitriles have been found to follow a Hammett type correlation where log k = 1.93σ + 1.30 at 25 °C and ionic strength 1. Carbon-13 NMR studies indicate that the variation in the slope of these Hammett plots for free and coordinated nitrile hydrolysis rates is a transition state effect rather than a ground state phenomenon.