CHARGE AND SPIN DENSITY IN SOME HYDRATES OF NICKEL SALTS: BONDING AND ARTIFICIAL EFFECTS. I. Olovsson, G. J. McIntyre^*, H. Ptasiewicz-Bak^**, Institute of Chemistry, University of Uppsala, Box 531, S-751 21 Uppsala, Sweden, ^*Institut Laue-Langevin, B. P. 156, F-380 42 Grenoble Cedex 9, France, ^**Nuclear Chemistry & Technology, Dorodna 16, 03-195 Warsaw, Poland

There appears to be conflicting experimental evidence on the redistribution of the charge density in the lone-pair and other regions of a molecule due to the interaction with its nearest neighbours. In some experimental as well as theoretical deformation density maps a decrease in the lone-pair density has been reported, whereas in other cases an increase has been found. It appears that two major, counteracting factors are responsible for these differences: an increase in the lone-pair density is expected due to the polarizing influence of the neighbours, whereas simple superposition of the isolated monomer deformation densities may lead to an apparent decrease due to the overlap with the negative contours of the neighbouring atom. Depending on which of these factors is the dominant one, an increase or decrease in the lone-pair density may be observed.

These facts will be illustrated by recent results on some hydrates of nickel salts. The charge densities have been determined at RT and 25K by multipole refinement against single-crystal X-ray intensity data. The charge densities based on the fitted deformation functions of all atoms in the structure are compared with the individual densities calculated from deformation functions of only Ni or the separate water molecules. In this way the effect of simple superposition of the individual densities has been studied. Polarization of the lone-pair densities, reflecting the different coordination of the water molecules, is clearly evident, an effect which is normally considered too small to be detected experimentally.

The spin densities have been determined by polarized neutron diffraction at 1.5 K in a magnetic field of 4.6 T. The charge and spin densities observed are in good agreement with those expected from ligand field theory for a d⁸ electron configuration of Ni²⁺ in both weak and strong ligand fields.