THE ANOMALOUS EPR-SPECTRA OF THE IRON(III) DICHALCOGENOCARBAMATES

Recently, a new limiting resonance structure was proposed to more comp letely explain the bonding in the spin-crossover tris(diorganodichalco genocarbamato)iron(III) complexes, Fe(XYCNRR')3 (where XY = SS, SSe or SeSe; R and R' are organic substituents). This new limiting resonance structure involved a low-spin Fe(II) and an unpaired electron on the N of the radical ligand. This proposal was the result of the observati on of structure in the narrow X-band EPR line at g almost-equal-to 2 a nd a computer fit of this structured line which supported an assignmen t of electron density to an I = 1 nucleus. The X-band EPR spectra (120 K) of 20 of these complexes have been re-examined, as pure powders an d diluted in the corresponding Co(III) or In(III) complex matrix; 14 o f these exhibit this structure in the region g almost-equal-to 2 which can be fit to an electron on an I = 1 nucleus. An alternative explana tion of the structured line resulting from an impurity of Fe(XYNCRR')2 NO is possible, but, if this is correct, the source of the NO is not l igand decomposition, since the X-band EPR spectrum (120 K) of Fe(S2CNE t2)3 prepared using N-15 (I = 1/2) is identical to the natural abundan ce complex. The EPR signal is believed to be of a common impurity in e ach of the Fe(XYCNRR'), complexes.