The electrochemical reduction of CO₂ by derivatives of rhenium carbonyl bipyridyl at carbon cathodes in acetonitrile solution is shown to take place by different routes depending on the concentration of water in the system. In the absence of water and CO₂, reduction of fac-[Re(dmbipy)(CO)₃Cl](dmbipy = 4,4′-dimethyl-2,2′-bipyridyl) leads first to fac-[Re(dmbipy˙^–)(CO)₃Cl] followed by elimination of Cl^– and partial dimerisation of the resultant five-co-ordinate species to give both fac-[Re(dmbipy)(CO)₃] and [{Re(dmbipy)(CO)₃}₂]. Further reduction gives rise to fac-[Re(dmbipy˙^–)(CO)₃], which is the final product. Cathodic reduction of [Re(dmbipy)(CO)₃Cl] in the presence of CO₂ and absence of water leads to direct attack of CO₂ on fac-[Re(dmbipy˙^–)(CO)₃Cl] to give [Re(dmbipy)(CO)₃(CO₂H)]E, which can undergo further reduction to give [Re(dmbipy˙^–)(CO)₂(CO₂H)]. By contrast, in the presence of H₂O, the complex E can be protonated to yield [Re(dmbipy)(CO)₃(CO₂H₂)]⁺ which is attacked by acetonitrile to yield [Re(dmbipy)(CO)₃(MeCN)]⁺ and CO, this latter complex being identified unambiguously by comparison with the species formed in the oxidation of the original rhenium complex.

The mechanism of anionic electropolymerisation of [(vbpy)Re(CO)₃Cl], where vbpy is 4-vinyl-4′-methyl-2,2′-bipyridyl, has been studied with ellipsometry, and the nature of the film formed established. The chemical transformations of this film on reduction to – 1.9 V have been studied by in situ FTIR spectroscopy in the presence and absence of CO₂, and the spectra assigned using Timney's ligand effect parameters. This analysis shows that the reduction chemistry is somewhat simplified as compared to the homogeneous case studied earlier, with dimerisation of the reduction products much less prevalent. The films are highly active for CO₂ reduction but show long-term degradation, and one possible mechanism for this is explored with a combination of ellipsometry and FTIR.