Second-sphere proton-transfer residues around transition-metal CO2-reduction (CO2RR) catalysts have emerged as powerful design elements: by sustaining a local proton inventory at the active site, they can boost rates and steer selectivity. For example, such motifs deliver ~two orders of magnitude enhancement in CO2-to-CO conversion for iron tetraphenylporphyrin, and they can even switch product selectivity from CO to HCOOH for a manganese bipyridine tricarbonyl catalyst in anhydrous acetonitrile. Notably, manganese systems depend strongly on an external proton source—CO2 does not react with the active Mn species without it. In contrast, analogous rhenium bipyridine tricarbonyl catalysts often suffer when proton-responsive units are introduced, exhibiting lower faradaic yields, higher overpotentials, and rapid degradation. This project targets two remedies: (1) lowering the CO2RR overpotential by tuning the primary coordination sphere to slow degradation, thereby preserving second-sphere functionality during turnover; and (2) shifting selectivity from CO to HCOOH using amine-based additives. Recent reports show up to 11% faradaic yield of HCOOH (with H2 as the major product) for a Re(bpy) dicarbonyl complex in the presence of amines, with a ligand-based—rather than metal-hydride—pathway proposed for H2/HCOOH formation. We will benchmark electrocatalytic performance by electrochemistry and quantify products by GC-TCD (gaseous) and NMR (liquid). To capture transient intermediates and map pathways, we will employ stopped-flow, time-resolved FTIR, and UV–Vis spectroscopy at sub-millisecond resolution. Complementary DFT and TDDFT calculations (Gaussian and ORCA) will provide vibrational and electronic spectra, reaction energetics, transition states, and pKₐ values for key intermediates with and without second-sphere residues. Integrating computation with experiment will elucidate the full mechanistic cycle and the role of the second coordination sphere in CO2RR, enabling the design of Re catalysts that reduce CO2 selectively and efficiently at lower applied potentials. The resulting insights will pave the way to higher efficiency and tunable selectivity toward either CO or HCOOH.