H. H. WICKMAN AND A. M. TROZZOLO 29 GENERAL DISCUSSION Prof. N. N. Greenwood (Newcastle upon Tyne) said Two of the most fascinating results in Wickman’s paper are (i) that a change from chlorine to bromine in these compounds alters the sign of D the splitting of the lowest quartet level ; and (ii) that in the chloro-derivative the diethyl compound is ferromagnetic whereas the dimethyl and di-isopropyl compounds are paramagnetic at low temperatures. Is there any explanation of these observations and in particular is it possible to give them any intuitive chemical rationalization ? Dr. H. M. Wickman (Bell Telephone Lab. N.J.) said. In reply to Greenwood a possible explanation of the change in sign of D has been suggested by preliminary results of a crystal field calculation for C, symmetry within the 3d5 configuration representing penta-co-ordinate Fe (111).There are many limitations in this approach and the calculation though a straightforward application of results of Tanabe Sugano and Kamimura is tedious. However for a reasonable choice of parameters specifying the ligand field potential Racah parameters etc. I find that a cross-over occurs between two ground quartet terms as the parameter representing halide charge is varied in a systematic fashion. As spin-orbit interaction was neglected in the calculation the actual splitting within these quartet terms has not been directly computed. On the basis of the cross over however it appears likely that the change in sign of D in the S = 3 spin Hamiltonian in the ethyl-chloro and ethyl-bromo derivatives arises from the appearance of a different ground quartet in the two cases.This situation may be contrasted with the high spin Fe3+ 6S where inversion in sign of D must always be due to variable contributions to the zero-field splitting with the (unique) S = 3 ground term. It should be emphasized that it is not yet possible completely to rule out this possibility in the quartet case. To do this explicitly would require additional data such as optical studies. 30 GENERAL DISCUSSION There are two points which should be noted concerning the paramagnetism of most intermediate spin complexes with respect to the ferromagnetism of the ethyl- chloro derivative. First the molecules containing one iron ion each are apparently packed in the crystal at the Van der Waals radii so that direct or super-exchange in the usual sense must be very weak.Secondly the smallest iron-iron separation is slightly less than 7A and as a result dipolar fields are also weak. Because of the easily polarized sulphur atoms and the associated 71 system of the molecule it is conceivable and perhaps in this case probable that the interaction leading to ferro- magnetism should be termed intermolecular exchange. If true and further experi- ments and other investigations are under way to attempt to clarify this point the lack of ordering in most of the complexes would merely be an expression of the sensitivity to orientation and molecular overlap of this intermolecular exchange mechanism. One must note that the above argument is predicated on the assumption that no crystal phase change or other macroscopic change occurs in the ethyl-chloro derivatives between room and helium temperatures.The magnetic and Mossbauer data do not indicate that any such change is occurring. Dr. C. E. Johnson (A.E.R.E. Harwell) said Would Wickman like to say more about his crystal field calculations and especially how he decides what charges to put on the ligand? Dr. H. H. Wickman (Bell Telephone Lab. N.J.) said As mentioned previously these calculations are preliminary and have been carried out only for one case thus far. For the results quoted above the " charge " on the sulphur was taken such that if there were six sulphurs octahedrally coordinated about the iron the cubic field strength would be about 85% of that required for a 6A1 to 2T2 cross-over. (This initial choice was suggested by experimental data in tris-dithiocarbamates where there is in fact such a cross-over in octahedral coordination of six sulphur ligands.) The halide charge was then varied for a range of strengths encompassing the fixed charge of the sulphur.