Realizing an effective catalytic reaction with Co-1 was complicated by the formation of [(R,R)-( iPr DuPhos)Co(CO)2]2 in the presence of syngas (CO/H2). Formation of the carbonyl dimer was observed as the product of photodriven H2 release from Co-1 and by thermal hydrogen atom abstraction (HAA) by addition of TEMPO•. Suppressing dimer formation under visible light irradiation in the presence of synthesis gas was identified as a catalyst design principle to improve catalytic reductive hydroformylation performance. A selection of bis(phosphine)cobalt carbonyl hydrides including: