From Yellow to Black: Dramatic Changes between Cerium(IV) and Plutonium(IV) Molybdates

Hydrothermal reactions of CeCl₃ and PuCl₃ with MoO₃ and Cs₂CO₃ yield surprisingly different results. Ce₃Mo₆O₂₄(H₂O)₄ crystallizes as bright yellow plates (space group <i>C</i>2<i>/c, a</i> = 12.7337(7) Å, <i>b</i> = 22.1309(16) Å, <i>c</i> = 7.8392(4) Å, β = 96.591(4)°, <i>V</i> = 2194.6(2) ų), whereas CsPu₃Mo₆O₂₄(H₂O) crystallizes as semiconducting black-red plates (space group <i>C</i>2<i>/c</i>, <i>a</i> = 12.633(5) Å, <i>b</i> = 21.770(8) Å, <i>c</i> = 7.743(7) Å, β = 96.218(2)°, <i>V</i> = 2117(2) ų). The topologies of the two compounds are similar, with channel structures built from disordered Mo­(VI) square pyramids and (RE)­O₈ square antiprisms (RE = Ce­(IV), Pu­(IV)). However, the Pu­(IV) compound contains Cs⁺ in its channels, while the channels in Ce₃Mo₆O₂₄(H₂O)₄ contain water molecules. Disorder and an ambiguous oxidation state of Mo lead to the formula CsPu₃Mo₆O₂₄(H₂O), where one Mo site is Mo­(V) and the rest are Mo­(VI). X-ray absorption near-edge structure (XANES) experiments were performed to investigate the source of the black color of CsPu₃Mo₆O₂₄(H₂O). These experiments revealed Pu to be tetravalent, while the strong pre-edge absorption from the distorted molybdate anions leaves the oxidation state ambiguous between Mo­(V) and Mo­(VI)