The Curium Aqua Ion

The coordination environment of the hydrated Cm3+ ion is probed both in the solid state and in solution. The
analysis of single-crystal X-ray diffraction data from [Cm(H2O)9](CF3SO3)3 determines that the Cm species is
surrounded by nine coordinating waters with a tricapped-trigonal-prismatic geometry involving six short Cm−O
distances at 2.453(1) Å and three longer Cm−O distances at 2.545(1) Å. The Cm nona-aqua triflate is isostructural
with the series of lanthanide and actinide [R(H2O)9](CF3SO3)3 (R = La−Lu, Pu) compounds. A similar nona-aqua
geometry is seen for the coordination environment of Cm in aqueous solution, as probed by high-energy X-ray
scattering and extended X-ray absorption fine structure spectroscopy, although the splitting in the first coordination
shell is increased from 0.092(2) in the solid to 0.16(2) Å in solution. This increase in splitting of the Cm−water
distances in the first coordination sphere is discussed in terms of its potential relevance to the previously observed
decrease in coordinating waters with decreasing ionic radius about the f-ion in solution.