Extreme covalence in the Tc-Cl bond from polarized neutron diffraction

A polarized neutron diffraction experiment on a salt containing the 4d1 TcNCl4− ion (S=1/2) yielded the flipping ratios of six reflections confined to the hk0 plane. Using nuclear structure factors, FN, extrapolated from the 123 and 295 K x-ray structures, magnetic structure factors, FM, were obtained from the flipping ratios. The values of FM 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 density functional DV–Xα calculation. Because the data is limited to the one projection, it is not possible to separate the Tc and N spin density contributions.