Dominik Schaniel, Benoit Cormary, Isabelle Malfant, Lydie Valade, Theo Woike, Bernard Delley, Karl W. Krämer and Hans-Ueli Güdel
Photogeneration of two metastable NO linkage isomers with high populations of up to 76% in trans-[RuCl(py)₄(NO)][PF₆]₂·¹/₂H₂O
Phys. Chem. Chem. Phys. 9, 3717-3724 (2007)                 

Abstract: Two light-induced metastable NO linkage isomers with oxygen-bound (SI) and side-on configuration (SII) of NO are generated in trans-[RuCl(py)₄(NO)][PF₆]₂·¹/₂H₂O. Irradiation by light in the blue–green spectral range (450–530 nm) leads to the population of SI. A further irradiation by near infrared light (920–1100 nm) transfers SI into SII at temperatures below 150 K. The heat release during the thermal decay of the linkage isomers shows that SI and SII are separated from the ground state (GS) by potential barriers of E_A(SI) = 0.70(3) eV and E_A(SII) = 0.38(3) eV, and are energetically situated at 1.42(6) eV and 1.07(7) eV above the ground state, respectively. Maximum populations of 76% for SI and of 56% for SII can be generated, as determined by the decrease of the n(NO) stretching absorption band of the ground state. The n(NO) stretching vibration shifts to lower energies by 143 cm^–1 in SI and by 300 cm^–1 in SII, indicating that the linkage isomers are of the same type as found in other octahedrally coordinated transition-metal nitrosyl complexes. The experimental observations are in agreement with results from calculations by the density functional theory, which predict that the metastable states correspond to a side-on bonded (SII) and an isonitrosyl (SI) configuration of the NO ligand. The calculations provide the energy minima of the ground state and the metastable states SI and SII as well as the saddle points along the reaction coordinate Q. This reaction coordinate corresponds to a rotation of the NO ligand by about 90° (SII) and 180° (SI), and therefore allows the comparison between observed and calculated activation energies.