Experimental results on the electroreduction of CO, on gas diffusion electrodes modified by M-PC. renorted in a nrevious naner M. Furuva and K. Matsui. J. electrounul. Chem. 271, 181 (1989)1, are interpieted qualitatively by-means of Tat&s theory based on the Hiickel model [R. Tat&, Z.. Chek 98 (1966); R. Taube, Pure appl. Chem. 38,429 (1974)]. The reason why some of the M-PC produce CO or CH, while others yield HCOOH is explained by the electron configurations in the metallic atoms.
The reduction of CO, to CO is attributed to the strongly electron-donating HOMO of [M-PC]"-arising from the electron-accepting LUMO of neutral M-PC which spreads over the nitrogen atoms surrounding M. The final production of CO occurs when the created CO is promptly desorbed from the metallic atom M having such a bare d-shell with the doubly occupied ) orbital. If, on the contrary, the CO molecule is bound to the metallic atom by a u-bonding, the reduction may proceed up to CH, production.
In metals having outermost s or p electrons, the electron occupation of the spread LUMO by ionization will not take place and such a strong reduction as to produce CO may not be possible. Then the electron transfer from M-PC to CO, will result in the production of [CO,]-followed by HCOOH creation instead of CO. The anion [CO,]-bents as LOCO = 134", and the C-will probably form a u-bond with dzl or pz of M, making the so-called carbon-dioxide complex, until further reduction to HCOOH.