INFRARED-SPECTRA AND MOLECULAR-STRUCTURE OF EXCITED ELECTRONIC METASTABLE STATES OF THE NITROPRUSSIDE ANION, [FE(CN)5NO]2-

Na2[Fe(CN)5NO].2H2O (SNP) and Ba[Fe(CN)5NO].3H2O (BNP) irradiated at low temperature with light in the green-blue region exhibit two new sets of infrared (IR) bands. These can be assigned to two, long-lived, electronically excited metastable states of the [Fe(CN)5NO]2- (NP) ion. Upon heating, these states depopulate following decay processes with different-onset temperatures. We considerably extend here previous polarized IR data on irradiated SNP (100) plates to include the other basal planes. An IR-active CN, NO and FeN stretching modes and FeNO bending modes of NP in both metastable states exhibit frequency down shifts. This points to a softening of the corresponding bonds upon excitation. Relative frequency shift values observed for modes associated with the FeNO group are about one order of magnitude larger than the corresponding values for CN stretching modes. This supports the conclusion that the metastable states are reached through an electronic transition involving mainly the metal(nd)-NO bonding. We employ dichroic measurements in SNP to estimate the orientation in the lattice of the transition dipole moment vector corresponding to the NO mode of NP in both metastable states. Results show that the FeNO group is not appreciably bent upon excitation of NP to either of these states.