Ligand‐Substitution Reaction of Cobalt(III) Complexes of [CoCl(N)₅]²⁺‐type with Carbonate: Activation of Kinetic Processes by Steric Chelate Effect

Abstract

Kinetics and mechanisms of coordination by ligand‐substitution of HCO3− for Cl− on the cobalt(III) complexes of [CoCl(N)5]2+‐type having different (N)5 backbone chelate ring structures to form those of [Co(CO3)(N)5]+‐type have been investigated in a weakly alkaline aqueous 0.10 mol dm−3 sodium perchlorate solution at ambient temperatures, in order to elucidate any possibility for the fixation of atmospheric CO2 on the coordination sphere of the transition metal center in the form of CO2−3. The HCO3− in the aqueous solution easily and substitutionally coordinates to the cobalt(III) complexes in place of coordinated chloro ligand via two consecutive rate‐determining steps, the faster one with two parallel pathways of aquation and hydroxide‐anation and the slower one with a pathway of carbonate‐anation. Of these reaction pathways, the hydroxide‐anation was explained by a dissociative interchange mechanism via conjugate base (IdCB), whereas the carbonate‐anation was explained by a nucleophilic attack of coordinated oxygen atom on the central cobalt(III) atom to the carbon atom of HCO3− while maintaining the Co – O bond throughout the step by taking the result of an EHMO calculation into consideration. The (N)5 backbone chelate ring structure served to enhance the rate of coordination of HCO3− on the cobalt(III) center as well as that of preceding aquation and hydroxide‐anation steps chiefly by its entropy effect. The pentadentate tetren backbone structure was recommended for the rapid fixation of CO2 in the form of CO2−3 on the cobalt(III) complexes of [CoCl(N)5]2+‐type.