Covalence and spin polarisation in tetraphenylarsonium tetrachloronitridotechnetate(VI) studied by polarised neutron diffraction

Polarised neutron diffraction from two orientations of tetragonal single crystals of [AsPh4][TcNCl4] at 1.5 K and a magnetic field of 4.6 T gave a combined data set of 116 magnetic structure factors. These were fitted by a 26-parameter model which includes proton nuclear-spin orientation to give a spin-density model for the formally 4d(1) [TcNCl4](-) ion. This model revealed substantial anisotropic pi bonding between Tc and Cl resulting in 29(2)% of the spin being delocalised onto the chlorine in-plane 3p(pi) orbitals, - 15(1)% onto the nitrogen as a result of spin-polarising the short Tc-N bond, and substantial spin polarisation on the technetium site of 4d and more diffuse density. Three unconstrained Hartree-Fock and density functional calculations with good basis sets failed to reproduce these observations adequately, but an unconstrained density functional calculation with gradient corrections and a relativistic treatment of the core gave encouraging agreement with experiment.