For 25 years the two metastable electronic states (SI and SII), accessible in the nitroprusside anion [Fe(CN)5NO]2− (NP) by irradiation with blue polarized light, have been the subject of intense investigations from several research groups. This review article summarizes the most important experimental results in the field of photochromatic effects investigated by nuclear resonant methods. The experimentally determined data have been compared with the results derived from density functional theory calculations. The majority of results convincingly indicates: (i) the SI state is represented by the isonitrosyl conformation (Fe-O-N) of the (NO+)-group and (ii) the SII state is represented by a dynamic conformation, in which a bending vibration of Fe-O-N as well as a rotation of the (NO+)-group around the main molecular axis simultaneously occur. A possible formation of a new third metastable state observed in rare-earth nitroprussides, which is stable at room temperature, will be discussed.
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