The Curium Aqua Ion S. Skanthakumar,* Mark R. Antonio, Richard E. Wilson, and L. Soderholm* Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439 Received September 21, 2006 The coordination environment of the hydrated Cm 3+ ion is probed both in the solid state and in solution. The analysis of single-crystal X-ray diffraction data from [Cm(H 2 O) 9 ](CF 3 SO 3 ) 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(H 2 O) 9 ](CF 3 SO 3 ) 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. Introduction In its most stable trivalent oxidation state, curium (Z ) 96) has a spherically symmetric, half-filled 5f 7 shell and a 8 S 7/2 electronic ground state, assuming Russell-Saunders coupling. As a result, Cm 3+ is often used as a probe of heavier actinide speciation. 1 For example, Cm 3+ was recently used to assess the impact of including a second coordination sphere on the calculated aqua-coordination complex. 2 With the use of density functional theory (DFT), it was determined that Cm 3+ is surrounded by nine water molecules, in a trigonal prismatic arrangement of six prismatic waters with a Cm-O distance of 2.47 Å and three equatorial waters with a Cm-O distance of 2.48 Å. Studies of this type are significantly simplified by a spherically symmetric valence shell, which eliminates the need to assign unpaired valence electrons to specific orbitals. Another important feature of a half-filled shell is the resultant large energy gap to the first excited J ) 7 / 2 state, which results in a long luminescence lifetime that is sensitive to coordinating ligands through hybridization effects. The Cm luminescent lifetime has been shown to correlate with water coordination 3 and is considered a sensitive probe of complexation in solution. 4,5 The absolute number of coordinating waters is predicted assuming that in an aqueous solution of noncoordinating ligands there are nine waters in the first coordination shell, an assumption based largely on lanthanide work. 6-8 Although luminescence measurements are now considered a useful tool to probe Cm 3+ speciation, a detailed structural study of the coordination environment in aqueous solution has yet to be undertaken. The results from such a study will underpin the assumptions behind the optical studies and provide a metrical understanding of Cm 3+ aqua coordination that will support the basis upon which a predictive capability for the solution behavior of Cm under more complex and diverse conditions can be built. A predictive knowledge of actinide solution coordination preferences and stabilities is important for several reasons, most notably those associated with the difficulties that are encountered in working with many of the transuranic isotopes, difficulties that result from both their limited availability and their chemical and radiological health risks. In addition, these man-made elements have no geochemical history available to guide the understanding of their behavior * To whom correspondence should be addressed. E-mail: Skantha@ anl.gov (S.S.), LS@anl.gov (L.S.). (1) Edelstein, N. M.; Klenze, R.; Fanghanel, T.; Hubert, S. Coord. Chem. ReV. 2006, 250, 948-973. (2) Yang, T.; Bursten, B. E. Inorg. Chem. 2006, 45, 5291-5301. (3) Yusov, A. B.; Perminov, V. P.; Krot, N. N.; Kazakov, V. P. SoV. Radiochem. 1986, 28, 403-407. (4) Beitz, J. V. Radiochim. Acta 1991, 52/53, 35-39. (5) Kimura, T.; Choppin, G. R.; Kato, Y.; Zenko, Y. Radiochim. Acta 1996, 72, 61-64. (6) Habenschuss, A.; Spedding, F. H. J. Chem. Phys. 1979, 70, 3758- 3763. (7) Habenschuss, A.; Spedding, F. H. J. Chem. Phys. 1979, 70, 2797- 2806. (8) Habenschuss, A.; Spedding, F. H. J. Chem. Phys. 1980, 73, 442- 450. Inorg. Chem. 2007, 46, 3485-3491 10.1021/ic061798b CCC: $37.00 © 2007 American Chemical Society Inorganic Chemistry, Vol. 46, No. 9, 2007 3485 Published on Web 04/04/2007