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

Abstract

Polarised neutron diffraction from two orientations of tetragonal single crystals of [AsPh ₄ ][TcNCl ₄ ] 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 [TcNCl ₄ ] ^- ion. This model revealed substantial anistropic π bonding between Tc and Cl resulting in 29(2)% of the spin being delocalised onto the chlorine in-plane 3p _π 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.