The single crystal of cis-β-Λ-S,S-[Co(CO3)(trien)](ClO4)·H2O (trien = triethylenetetramine) has spontaneously resolved from the aqueous solution of cis-α-[CoCl(NH3)(trien)](ClO4)2 containing hydrogencarbonate anion, whose crystal structure has been determined by X-ray diffraction method. The complex crystallizes in the monoclinic space group P21 with a = 7.410(1), b = 12.354(3), c = 8.676(1) Å, β = 108.88(1)°, and Z = 2. The final R and Rw values for the 3056 observed reflections with Io > 3σ(Io) are 0.045 and 0.055, respectively. The tetradentate ligand, trien, is linked to the cobalt atom in cis-β-configuration, and the carbonato anion is coordinated to the metal atom as a didentate ligand. The Co–N bond lengths are 1.904(6), 1.928(4), 1.941(4), and 1.955(5) Å, and the two Co–O bond lengths are both 1.909(4). The absolute configuration of the complex cation can be designated as Λ(λλδ) and those about two secondary nitrogen atoms are both S.

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.