Kinetics of azide attack on organonitriles coordinated to pentaamminecobalt(III)

Four new 5-substituted tetrazole complexes of pentaamminecobalt(III) have been synthesized. The N-1 bonded complexes of 5-(p-methylphenyl)tetrazole, 5-(p-chlorophenyl)tetrazole, 5-(p-nitrophenyl)tetrazole and the N-2 bonded complex of 5-(m-formylphenyl)tetrazole. Kinetic studies of the complex formation reaction, wherein azide ion attacks coordinated organonitrile ligands, have been shown to follow a second order rate law, first order in nitrile complex and azide. Rate constants and activation parameters have been determined for five organonitrile complexes and a Hammett type correlation has been established for the formation reaction described by the equation log k = 2.42σ – 2.44 at 25 °C and ionic strength 1.0 M. Two mechanisms are proposed for complex formation which are consistent with our observations.