Coordination Chemistry of Homoleptic Actinide(IV)–Thiocyanate Complexes

The synthesis, X‐ray crystal structure, vibrational and optical spectroscopy for the eight‐coordinate thiocyanate compounds, Et₄N]₄Pu^IV(NCS)₈], Et₄N]₄Th^IV(NCS)₈], and Et₄N]₄Ce^III(NCS)₇(H₂O)] are reported. Thiocyanate was found to rapidly reduce plutonium to Pu^III in acidic solutions (pH<1) in the presence of NCS^?. The optical spectrum of Et₄N]SCN] containing Pu^III solution was indistinguishable from that of aquated Pu^III suggesting that inner‐sphere complexation with Et₄N]SCN] does not occur in water. However, upon concentration, the homoleptic thiocyanate complex Et₄N]₄Pu^IV(NCS)₈] was crystallized when a large excess of Et₄N]NCS] was present. This compound, along with its U^IV analogue, maintains inner‐sphere thiocyanate coordination in acetonitrile based on the observation of intense ligand‐to‐metal charge‐transfer bands. Spectroscopic and crystallographic data do not support the interaction of the metal orbitals with the ligand π system, but support an enhanced An^IV–NCS interaction, as the Lewis acidity of the metal ion increases from Th to Pu.