Photoinduced linkage isomers in a model ruthenium nitrosyl complex: Identification and assignment of vibrational modes

Photoinduced NO-linkage isomers were investigated in the solid state of labelled trans-[Ru(^14/15NO)(py₄)F](ClO₄)₂ complex by combined IR-spectroscopy and DFT calculations. Based on the experimental data and the DFT calculations of this isotopically labelled ^14/15NO nitrosyl compound, we present a complete assignment of the vibrational bands of three nitrosyl linkage isomers in a range from 4000 to 200 cm^-1. The calculated IR-spectra match well with the experimental data allowing reliable assignment of the vibrational bands. The structural change from the Ru-NO (GS) to the Ru-ON (MS1) and Ru-η²-(NO) (MS2) linkage configuration leads to the downshift of the ν(NO) and ν(Ru-(NO)) bands, and a corresponding increase of the energy of the ν(Ru-F) band. The shift of the bands corresponds to the change of the Ru-(NO) and Ru-F bond lengths: increase of the Ru-(NO) bond length leads to the decrease of the energy of the ν(Ru-(NO)) band; decrease of the Ru-F bond length leads to the increase of the energy of the ν(Ru-F) band. These observations can be extrapolated to the family of related nitrosyl complexes and therefore be used for the qualitative prediction of the Ru-(NO) and Ru-L_trans-to-NO bond lengths of different linkage isomers in the framework of one complex. While the formation of linkage isomers is a reversible process, long-time irradiation sometimes induces irreversible reactions such as the release of NO. Here, we show that the photolysis of trans-[Ru(^14/15NO)(py₄)F](ClO₄)₂ in KBr pellets may lead to the release of nitrous oxide N₂O, conceivably through the formation of a {Ru-(κ²-ONNO)} intermediate.