A meso-aryl substituted cobalt(II) tetradehydrocorrin complex (Co(II)TDHC) has been synthesized and investigated. The corrin framework, determined by X-ray crystallographic analysis, is found to be relatively planar except at the Cl and C19 positions. Cyclic voltammetry (CV) measurements indicate two positively shifted reversible redox couples at -0.53 and -1.70 V vs Fc/Fc(+) for [Co-II](+)[Co-I] and [Co-I]/([Co-I](center dot-) and/or [Co-II](-)) ([Co-II] = Co(II)TDHC), respectively, compared with the previously reported cobalt porphyrin complex, because the tetradehydrocorrin ligand efficiently promotes the formation of low-valent metal species due to its monoanionic character. Furthermore, it is found that the current in the CV measurement is significantly enhanced upon addition of H2O under a CO, atmosphere, indicating the progression of electroreductive catalysis by Co(II)TDHC. However, controlled-potential electrolysis (CPE) using Co(II)TDHC under the same conditions shows generation of H-2 as a major product and only a small amount of CO as a CO2 reduction product; Faradaic efficiencies are calculated to be 66.8 and 4.5%, respectively. The CPE with a buffer solution under an N-2 atmosphere reveals that the selective H-2 generation is promoted by the moderate acidification of the solution under CO2 saturation conditions. The present study demonstrates that the significantly stabilized Co(I) species with the monoanionic ligand framework preferentially catalyzes the thermodynamically favored H-2 evolution rather than CO2 reduction.