Pentacyanonitrosylosmate(2-) ion ([Os(CN)(5)NO](2-)), as the sodium salt, can be excited selectively to two long-lived electronic metastable states (MSI and MSII) by irradiation at low temperature with light in the blue-near-UV region. The MSI and MSII states are similar to those found in nitroprusside salts, including isomorphous Na-2[Fe(CN)(5)NO]. 2H(2)O, and in RuNO complexes. IR bands due to CN, NO, and OsN stretching and OsNO bending modes show frequency downshifts upon excitation of [Os(CN)(5)NO](2-) to MSI and MSII states. This implies softening of the corresponding bonds. Frequency shifts of vibrational modes of the OsNO group are much larger than the corresponding shifts of CN modes. This further supports the conclusion that metastable states are reached through an electronic transition involving mainly the metal(nd)-NO grouping (n = 3, 4, 5). Upon heating, the MSI and MSII states of the osmium compound decay at two different onset temperatures, TI < T-II, in inverse order to that reported for the other complexes. Furthermore, we report a thermally driven MSI --> MSII excitation transfer process taking place at temperatures in the range T-II > T greater than or equal to T-I. This proves that, at the equilibrium molecular conformation, the MSII energy level lies below the MSI level and that there must be a crossing of the corresponding electronic energy curves at thermal energies above the MSI vibrational ground level.