Polarised neutron diffraction and the spin density in [As(C6H5)4][TcNCl4]

The study of magnetism in large single crystals of paramagnets by polarised neutron diffraction (PND) measures Fourier components of the crystal magnetisation density. Over the last few years we have used this to study magnetic exchange pathways, spin canting, metal ion anisotropies and, particularly, metal-ligand bonding. In complexes of heavy transition metals the bonding is not well understood and the detailed data potentially available from PND would provide a stringent test of theory. Suitable materials are rare, but one such is [As(C6H5)4][TcNCl4], in which magnetically independent, square pyramidal, spin-half TcNCl4- ions exist in a simple crystal structure. Preliminary PND showed extensive covalent delocalisation of the single paired spin from the technetium onto the chlorine ligands. We have now collected an extensive PND data set on this material which shows, inter alia, that the spin-density on the technetium complex is extremely delocalised and highly anisotropic. A simplified interpretation is that there is preferential occupation of the Tc 4dxy orbital with covalent delocalisation into Cl 3p, but only in the Cl4 plane. The Tc-N bond is strongly spin-polarised, a correlation effect. Other detail is not so simply explained, and good quality ab-initio quantum mechanical calculations do not reproduce the observations