Electron spin resonance of some chromium(i) and iron(i) nitrosyl complexes.

The pentacyanonitrosyl chromium (I) anion has been studied in fluid and solid solutions in a magnetically dilute single crystal by electron spin resonance. The single crystal results show that the Cr-N-O group has a linear configuration. The e.s.r. results and those obtained from U.V. spectroscopy are consistent with the unpaired electron being placed in a molecular orbital based on dxy, but the fine structure, which was observed on one of the U.V. absorption bands, could not be easily explained, since the peak to peak separations are much greater than any of the metal-ligand stretching frequencies. Superhyperfine structure from 14N(NO) and 13C (of all the cyanides) has also been observed, the former being interpreted as arising from spin polarisation of a filled IT- bonding molecular orbital. The penta-aquo and penta-ammine complexes have also been studied together with intermediates in the acid hydrolysis of [Cr(CN)5NO]3-, The results, though less detailed, are generally similar to those of the penta-cyano complex. The U.V. spectra for all these species have been interpreted and the derived energy level separations together with some estimated molecular orbital cooefficients have been used to calculate g-values in good agreement with those obtained experimentally. A spin polarisation mechanism involving 6-bonding orbitals has been suggested in order to explain the isotropic hyperfine coupling for the ligands other than the nitrosyl. A kinetic study has been made on the last two stages of the acid hydrolysis of [Cr(CN)5NO]3- by e.s.r. The rate constants for both stages are 1st order. Approximate base strengths for the reacting species and the enthalpies and entropies of activation for the reactions have been determined. The nature of the intermediate species is also discussed. Species formed in the ?-irradiation of sodium nitro-prusside have been studied and their natures discussed. Analysis of the results shows that, for the main species formed at room temperature, there is a loss in axial symmetry of the g-tensor and that the Fe-N-O bond deviates ~10 from linearity. With large radiation dosages it was possible to relate the spectra from radicals trapped in pairs to various sites in the crystal. E.s.r. spectra have also been obtained from by-products in the reactions of sodium nitroprusside with sodium sulphide and sodium dithionite. Possible structures for these species have been discussed. The iron (I) nitrosylbisdiethyldithiocarbamate has been studied in fluid and rigid solutions; the latter at Q-, X-, and S-band frequencies. Hyperfine coupling has been obtained from both and 14N and 57Fe and the spin density in the 14N orbitals related to the angle between the g- and 14N hyperfine-tensors. The reasons for the bending of the Fe-N-O group are discussed. The linear combination of atomic orbitals (LCAO) method for constructing molecular orbitals has been applied to the case of C4v symmetry. The dipolar interaction between an unpaired electron in a dXY orbital and a ligand nucleus has been described and applied to the [Cr(CN)5NO]3- case. Estimates of the isotropic and anisotropic spin resonance parameters have been made for unpaired electrons in various transition metal atomic orbitals.