Comment on the anomalous paramagnetism of [Fe(NCS)2 phen2]

Recentlyl*2 anomalous variations of magnetic susceptibility with temperature have been reported for [Fe(NCS)2 phenz] and [Fe(NCSe)2 phen2]. At a temperature of about 174% for the former compound and about 2 3 2 " ~ for the latter, there is a large change in the magnetic susceptibility which a t higher temperatures follows a Curie-Weiss type of variation. This behaviour is explained qualitatively in terms of an equilibrium between a lA1 ground state for the iron atom and a low-lying 5T2 state. I n particular, X-ray powder patterns are used to suggest there is no phase change. We believe that such patterns, even of the similarity of those reproduced,l do not represent unequivocal evidence for the absence of a phase change. For example, the powder patterns for the bis(N,N-diethyldithiocarbamato) complexes of copper(rr) and zinc@) are remarkably similar, but a full analysis3 shows the structures are different. We here report measurements which indicate that something other than an electronic equilibrium occurs.