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 C2/c, a = 12.7337(7) angstrom, b = 22.1309(16) angstrom, c = 7.8392(4) angstrom, beta = 96.591(4)degrees, V = 2194.6(2) angstrom(3)), whereas CsPu3Mo6O24(H2O) crystallizes as semiconducting black-red plates (space group C2/c, a = 12.633(5) angstrom, b = 21.770(8) angstrom, c = 7.743(7) angstrom, beta = 96.218(2)degrees, V = 2117(2) angstrom(3)). The topologies of the two compounds are similar, with channel structures built from disordered Mo(VI) square pyramids and (RE)O-8 square antiprisms (RE = Ce(IV), Pu(IV)). However, the Pu(IV) compound contains Cs+ in its channels, while the channels in CsPu3Mo6O24(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).