The electrochemical reactions between CO2 and fac-Re(dpk)(CO)(3)Cl(1) are solvent dependent, controlled by the rate of diffusion of electroactive species from the electrode surface. Fast diffusion due to solvent or scan rate variations inhibits the carboxylation of the carbonylic carbon atom of the radical anion fac-Re{(C5H4N)(2)(CO)radical anion}(CO)(3)Cl (II) and leads to the generation of a coordinatively unsaturated fac-Re[(C5H4N)(2){(CO)radical anion}](CO)(3) (VIII) that binds CO2 at the metal center to form fac-Re[(C5H4N)(2){(CO)radical anion}](CO)(3)(CO2) (IX). Slow diffusion facilitates the carboxylation of the carbonylic carbon atom of II to form fac-Re[(C5H4N)(2){C(O-)(CO2.)](CO)(3) (III) that undergoes a second electronic transfer followed by chemical steps leading to the formation of fac-Re[(C5H4N)(2){C(O)(CO2H)}](CO)(3) (VI).