Kinetic study of the substitution of pyridine by cyanide in the bis(pyridine)cobalt(III)hematoporphyrin-IX: distinguishing between Isub(d) and D mechanism

''Mass-law (rate) retardation'' effect shows that the reaction between the cyanide ion and bis(pyridine)cobalt(III)hematoporphyrin-IX complex to give (CN)2cobalt(III)hematoporphyrin-IX occurs by a purely dissociative (D but not Isub(d)) mechanism in chloroform. Limiting rate constant at the excess of cyanide ion concentration at 25 deg C was found to be 2.5x10-3S-1 and the competition ratio of pyridine (ksub(-)1) and the cyanide ion (k2) for a five coordinate intermediate (pyridin) cobalt(III)hematoporphyrin-IX complex was obtained as ksub(-)1/k2=0.35. (author)