NO photoswitches: a single photoinduced linkage isomer in diamagnetic {MNO}⁸ complexes

Representative for the {MNO}⁸ series of metal-nitrosyl compounds, the neutral {CoNO}⁸ cobalt nitrosyl complex [Co(fpin)NO(phen)] (1) with fpin = perfluoropinacolate(^2-) and phen = phenanthroline(⁰) exhibits a single photoinduced linkage isomer (PLI) that can be generated by irradiation with red light. The high population of 80% PLI enables the unambiguous determination of its structure as a bent κO isonitrosyl Co-O-N with a Co-O-N angle of 123.1(3)° compared to the bent κN nitrosyl Co-N-O ground state with an angle of 121.199(14)°. While the photoswitching in this diamagnetic {CoNO}⁸ complex is triggered by a metal-to-ligand charge transfer transition like in the well-known {MNO}⁶ complexes, the vibrational response is opposite, yielding a blue-shift of the N-O stretching vibration upon isonitrosyl PLI generation in {MNO}⁸ and a red-shift in {MNO}⁶. First-principles DFT calculations, performed on the photocrystallography models, provide results consistent with the observed structures and vibrational spectra. Additionally, the analysis of the Bader charges shows that the charge on the NO is more positive in the PLI compared to the GS for both the {MNO}⁸ and {MNO}⁶ complexes. Hence, the experimental vibrational spectra should not be directly used as sole evidence to derive underlying structures or electron density changes.