[Ru(py)4Cl(NO)](PF6)2·0.5H2O: a model system for structural determination and ab initio calculations of photo-induced linkage NO isomers

Structure analysis of ground state (GS) and two light-induced (SI and SII) metastable linkage NO isomers of [Ru(py)₄Cl(NO)](PF₆)₂·0.5H₂O is presented. Illumination of the crystal by a laser with λ = 473 nm at T = 80 K transfers around 92% of the NO ligands from Ru—N—O into the isomeric configuration Ru—O—N (SI). A subsequent irradiation with λ = 980 nm generates about 48% of the side-on configuration \rm Ru \lt ^N_O (SII). Heating to temperatures above 200 K or irradiation with light in the red spectral range transfers both metastable isomers reversibly back to the GS. Photodifference maps clearly show the N—O configurations for both isomers and they could be used to find a proper starting model for subsequent refinements. Both metastable isomers have slightly but significantly different cell parameters with respect to GS. The main structural changes besides the Ru—O—N and \rm Ru \lt ^N_O linkage are shortenings of the trans Ru—Cl bonds and the equatorial Ru—N bonds. The experimental results are compared with solid-state calculations based on density functional theory (DFT), which reproduce the observed structures with high accuracy concerning bond lengths and angles. The problem of how the different occupancies of SI and GS could affect refinement results was solved by a simulation procedure using the DFT data as starting values.