INFRARED-ABSORPTION SPECTRA OF SODIUM PENTACYANONITROSYLOSMATE(II) DIHYDRATE IN 2 EXCITED ELECTRONIC METASTABLE STATES

Pentacyanonitrosylosmate(2-) ion ([Os(CN)(5)NO](2-)), as the sodium sa lt, 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 stret ching and OsNO bending modes show frequency downshifts upon excitation of [Os(CN)(5)NO](2-) to MSI and MSII states. This implies softening o f the corresponding bonds. Frequency shifts of vibrational modes of th e OsNO group are much larger than the corresponding shifts of CN modes . This further supports the conclusion that metastable states are reac hed 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 proces s taking place at temperatures in the range T-II > T greater than or e qual to T-I. This proves that, at the equilibrium molecular conformati on, the MSII energy level lies below the MSI level and that there must be a crossing of the corresponding electronic energy curves at therma l energies above the MSI vibrational ground level.