[en] Two metastable nitrosyl linkage isomers SI and SII are generated by light irradiation in the spectral range 370-500 nm in the two diamagnetic compounds [RuNO(NH₃)₅][Co(CN)₆] and [RuNO(NH₃)₅]₂[ZrF₆]₃ as well as in the paramagnetic compound [RuNO(NH₃)₅][Cr(CN)₆]. The frequencies of the ν(NO) stretching vibrations of SI and SII identify SI as the isonitrosyl Ru-O-N isomer and SII as the side-on η² isomer of NO. The population, i.e., the number of generated linkage isomers, is determined from the decrease of the area of the fundamental ν(NO) and of the higher harmonic 2 . ν(NO) of the ν(NO) stretching vibration of the ground state. Using differential scanning calorimetry (DSC) the heat release during the thermal decay of the metastable linkage isomers is determined. The activation energies, frequency factors, and the energetic position of the metastable linkage isomers are determined from the DSC and infrared spectroscopic experiments. It is found that the exchange of the counter ion significantly influences the energetic positions of the linkage isomers, while the activation energy and frequency factor are much less affected