The interplay of d and f orbitals in the chemistry of the early actinides: High spin versus low spin

Abstract

Early actinides lie at the borderline between d- and f-block elements. Here we describe the coordination capabilities of thorium featuring characteristics from both blocks. Thus analysis of the recently synthesized monoanion [Cp3Th]− indicates a diamagnetic singlet d2f0 ground state for the central thorium(II) atom. In this species the trigonal (Cp3)3− ligand system provides a strong ligand field for the thorium d-orbitals accounting for the low spin of this system. This differs from the high spin unambiguous f-element analogue, [Cp3U]−. For the d0f0 thorium(IV) complex (η8-C8H8)2Th both π→d and π→f forward bonding is observed, where a comparison with (η8-C8H8)2U is given. This indicates that a borderline element such as thorium can exhibit either d- or f-block character according to the surrounding ligand field.