Transition metal–Schiff's base complexes. Part V. Spin equilibria in some iron mononitrosyls

Abstract

Some apparently penta-co-ordinate mononitrosyls, prepared by the reaction of nitric oxide with the high-spin compounds, NN′-ethylenebis(salicylideneiminato)iron(II) and its 5-Me-, 3-NO₂-, 5-Cl-, 4-Cl-, and 5-NO₂-substituted derivatives, have three unpaired electrons at room temperature. The magnetic moment of the unsubstituted nitrosyl decreases sharply with temperature at ca. 180°K to a value corresponding to one unpaired electron. Co-ordination of the NO molecule brings the ligand field close to the crossover point, but some additional factor, possibly contraction of the lattice, is apparently responsible for the sharpness of the magnetic transition. The magnetic transition to the low-spin isomer is accompanied by a change in NO stretching frequency from 1712 to ca. 1630 cm.^–1. The 5-Me-derivative has a magnetic moment which does not vary with temperature down to 80°K, but from the 5-Me- to the 5-NO₂-derivative in the order given above the magnetic moments show greater diminution with temperature although there is no sharp variation. The magnetic data, and the appearance of an additional NO stretching frequency at low temperature, suggest that considerable quantities of the low-spin isomer of the 5-NO₂-nitrosyl would form at temperatures not far below 80°K. The reaction of NO with the iron(II) complexes is considered an oxidation so that the nitrosyls formally contain iron(III) and NO^–.