Structure and reactivity of transition-metal complexes with polyatomic ligands. Part IV. Electron spin resonance spectra of γ-irradiated sodium nitroprusside

Electron spin resonance spectra have been recorded for γ-irradiated hydrated and anhydrous sodium nitroprusside. The paramagnetic species in the anhydrous salt, and in polarographically reduced sodium nitroprusside in dimethylformamide is identified as the ion [Fe(CN)5NO]3– having linear Fe–N–O bonds, whereas the species in the hydrated salt is possibly [Fe(CN)4(CNOH)(NO)]2– having an Fe–N–O bond angle of about 170°. The directions and the magnitudes of the g and 14N hyperfine tensors are in accord with an electron configuration e4, b22, a11, the a1 orbital being primarily dz2. Some radicals were trapped in pairs giving a weakly interacting triplet state. Single-crystal measurements of the spectra from such pairs showed that the unpaired electron was localised mainly on the iron nucleus. None of these results is compatible with the configuration 1e4, b22, 2e1 in which the 2e orbital is largely π*(NO).