Electrocatalytic CO2 reduction is of continued interest to sustainable energy research. Mononuclear transition metal complexes from Group 6 to Group 10 with a select subset of ligand frameworks have been demonstrated to be efficient electrochemical CO2 reduction catalysts. Here, we review the known mononuclear complexes from Group 6 to Group 10, examining trends in activity, electronic structure of catalytic intermediates, and product selectivity. The correlation between differences in electronic structure and CO2 reduction activity between these metal centers are discussed.