Dominik Schaniel, Theo Woike, Bernard Delley, Colette Boskovic, Daniel Biner, Karl W. Krämer and Hans-Ueli Güdel
Long-lived light-induced metastable states in trans-[Ru(NH₃)₄(H₂O)NO]Cl₃·H₂O and related compounds
Phys. Chem. Chem. Phys. 7, 1164-1170 (2005)                 

Abstract: The existence of two light-induced long-lived metastable states SI, SII in irradiated trans-[Ru(NH₃)₄(H₂O)NO]Cl₃·H₂O and trans-[Ru(NH₃)₄(OH)NO]Cl₂ is revealed by differential scanning calorimetry measurements and calculations based on density functional theory. Irradiation with light in the blue spectral range leads to the population of SI, while SII can be obtained by transferring SI into SII with irradiation of light in the near infrared spectral range. The population and transfer of the metastable states is described by exponential functions and the thermal decays are evaluated according to Arrhenius' law, yielding activation energies of E_A(SI)= 0.95(3) eV, E_A(SII)= 0.69(3) eV and frequency factors of Z(SI)= 2 × 10¹⁴ s^–1, Z(SII)= 3 × 10¹³ s^–1 for trans-[Ru(NH₃)₄(H₂O)NO]Cl₃·H₂O, while E_A(SI)= 0.91(3) eV, E_A(SII)= 0.60(3) eV, Z(SI)= 6 × 10¹⁴ s^–1, Z(SII)= 1 × 10¹³ s^–1 for trans-[Ru(NH₃)₄(OH)NO]Cl₂. The observations are compared with the ground state potential surface calculated by density functional theory, where the metastable states correspond to a side-on bonded (SII) and isonitrosyl(SI) configuration of the NO ligand. The calculations provide the energetic minima of the ground state and the metastable states SI and SII as well as the saddle points along the reaction coordinate Q, which corresponds roughly to a rotation of the NO ligand by about 90°(SII) and 180°(SI), and therefore allows for the comparison between observed and calculated activation energies.