[(ArPMI)Mo(CO)4] complexes (PMI=pyridine monoimine; Ar=Ph, 2,6-di-iso-propylphenyl) were synthesized and their electrochemical properties were probed with cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC). The complexes undergo a reduction at more positive potentials than the related [(bipyridine)Mo(CO)4] complex, which is ligand based according to IR-SEC and DFT data. To probe the reaction product in more detail, stoichiometric chemical reduction and subsequent treatment with CO2 resulted in the formation of a new product that is assigned as a ligand-bound carboxylate, [(PMI)Mo(CO)3(CO2)]2−, by NMR spectroscopic methods. The CO2 adduct [(PMI)Mo(CO)3(CO2)]2− could not be isolated and fully characterized. However, the CC coupling between the CO2 molecule and the PDI ligand was confirmed by X-ray crystallographic characterization of one of the decomposition products of [(PMI)Mo(CO)3(CO2)]2−.