EXTREME COVALENCE IN THE TC-CL BOND FROM POLARIZED NEUTRON-DIFFRACTION

A polarized neutron diffraction experiment on a salt containing the 4d 1 TcNCl4- ion (S=1/2) yielded the flipping ratios of six reflections c onfined to the hk0 plane. Using nuclear structure factors, F(N), extra polated from the 123 and 295 K x-ray structures, magnetic structure fa ctors, F(M), were obtained from the flipping ratios. The values of F(M ) were fitted to a model of spin populations in the Tc-4d and the Cl-3 (sp) orbitals. It was found that 46(5)% of the spin density is located on the Cl atoms, corresponding to exceptionally high covalence in the Tc-Cl bonding. The amount is distinctly higher than found indirectly by ESR measurements, but is to some extent supported by a local densit y functional DV-Xalpha calculation. Because the data is limited to the one projection, it is not possible to separate the Tc and N spin dens ity contributions.