Water-soluble, half-sandwich metallocarbonyl complexes of different metals, i.e., Fe, Ru, W, and Mo, bearing imidato ligands were shown to be photoactive, CO releasing molecules. The (η5-C5H5)M(CO)n(η1-N-imidato) complexes (M = Fe, Mo, W; n = 2, 3) released CO under illumination by visible light, while the (η5-C5H5)Ru(CO)2(η1-N-maleimidato) complex was activated by UV light for CO release. Thus, the mechanism of CO loss upon illumination, was investigated by several methods, including myoglobin assay, LC/QTOF, IR and 1H NMR. Our investigations highlighted that CO release was due to the total decomposition of the complexes. The photoinduced degradation of maleimide complexes led to the Diels-Alder reaction products due to cycloaddition of cyclopentadiene (released from the complex) to the maleimidato ligand of the starting complex.