LIGAND-SUBSTITUTION REACTION OF COBALT(III) COMPLEXES OF [COCL(N)(5)](2-TYPE WITH CARBONATE - ACTIVATION OF KINETIC PROCESSES BY STERIC CHELATE EFFECT())

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 [C o(CO3)(N)(5)](+)-type have been investigated in a weakly alkaline aque ous 0.10 mol dm(-3) sodium perchlorate solution at ambient temperature s, in order to elucidate any possibility for the fixation of atmospher ic CO2 on the coordination sphere of the transition metal center in th e form of CO32-. The HCO3- in the aqueous solution easily and substitu tionally coordinates to the cobalt(III) complexes in place of coordina ted chloro ligand via two consecutive rate-determining steps, the fast er one with two parallel pathways of aquation and hydroxide-anation an d the slower one with a pathway of carbonate-anation. Of these reactio n pathways, the hydroxide-anation was explained by a dissociative inte rchange mechanism via conjugate base (IdCB), whereas the carbonate-ana tion was explained by a nucleophilic attack of coordinated oxygen atom on the central cobalt(III) atom to the carbon atom of HCO3- while mai ntaining 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 c obalt(III) center as well as that of preceding aquation and hydroxide- anation steps chiefly by its entropy effect. The pentadentate tetren b ackbone structure was recommended for the rapid fixation of CO2 in the form of CO32- on the cobalt(III) complexes of [CoCl(N)(5)](2+)-type.