Infrared Absorption Spectra of Sodium Pentacyanonitrosylosmate(II) in Two Excited Electronic Metastable States

J. A. GÜIDA; O. E. PIRO; P. J. AYMONINO

INORGANIC CHEMISTRY

American Chemical Society

Año: 1995 vol. 34 p. 4113 - 4116

0020-1669

Abstract. Pentacyanonitrosylosmate(2-) ion ( [Os(CN)5NO]2-), as the sodium salt, can be excited selectively to two longlived 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 Na2[Fe(CN)5NO].2H2O, and in RuNO complexes. IR bands due to CN, NO, and OsN stretching and OsNO bending modes show frequency downshifts up011 excitation of [Os(CN)5N0]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 < TII, in inverse order to that reported for other complexes. Furthermore, we report a thermally driven MSI ---> MSII excitation transfer process taking place at temperatures in the range TII > T > TI. This proves thta, 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.