Light-Induced Metastable States in Oxalatenitrosylruthenium(II) and Terpyridinenitrosylruthenium(II) Complexes

Two extremely long lived metastable states (SI and SII) can be accessed by irradiation with light in the blue-green
spectral range at temperatures below 200 K in Cs2[Ru(ox)(NO)Cl3], [Ni(cyclam)][Ru(ox)(NO)Cl3]·3H2O, and
[Ru(terpy)(NO)(OH)Cl][PF6]. The crystal structures of the ground states of the oxalate-containing compounds are
presented, and the influence of the atomic distances of the cations/anions is discussed with respect to the decay
temperatures. The radiationless thermal decay of the metastable states is detected by differential scanning calorimetry
(DSC) for the three compounds. Both metastable states decay exponentially in time under isothermal conditions.
The excited states are energetically separated from the ground state by potential barriers given by the activation
energy of the Arrhenius law. In [Ni(cyclam)][Ru(ox)(NO)Cl3]·3H2O the enthalpy maximum of the thermal decay of
SII appears at 182 K, which is a relatively high decay temperature for SII. The reason for this strong temperature
shift compared to those of the other compounds could be due to the polarization effect of Ni2+ on the electron
density at the Ru site via the Cl atom.