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)
Å, b = 22.1309(16) Å, c = 7.8392(4) Å, β = 96.591(4)°, V = 2194.6(2) Å3), whereas CsPu3Mo6O24(H2O) crystallizes as semiconducting
black-red plates (space group C2/c, a = 12.633(5) Å, b = 21.770(8)
Å, c = 7.743(7) Å, β = 96.218(2)°, V = 2117(2) Å3). 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).