Unusual structure, bonding and properties in a californium borate



The participation of the valence orbitals of actinides in bonding has been debated for decades. Recent experimental and computational investigations demonstrated the involvement of 6p, 6d and/or 5f orbitals in bonding. However, structural and spectroscopic data, as well as theory, indicate a decrease in covalency across the actinide series, and the evidence points to highly ionic, lanthanide-like bonding for late actinides. Here we show that chemical differentiation between californium and lanthanides can be achieved by using ligands that are both highly polarizable and substantially rearrange on complexation. A ligand that suits both of these desired properties is polyborate. We demonstrate that the 5f, 6d and 7p orbitals are all involved in bonding in a Cf(III) borate, and that large crystal-field effects are present. Synthetic, structural and spectroscopic data are complemented by quantum mechanical calculations to support these observations.

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Figure 1: Graphical representations of Cf[B6O8(OH)5].
Figure 2: The magnetic susceptibility obtained from a polycrystalline sample of Cf[B6O8(OH)5] (open circles) compared with the fit (solid line) obtained using a modified Curie–Weiss law.
Figure 3: Room-temperature solid-state absorption spectrum of Cf[B6O8(OH)5] obtained from a cluster of crystals showing ff transitions that are diagnostic for Cf(III).
Figure 4: Photoluminescence spectra of Cf[B6O8(OH)5] on excitation with 420 nm light as a function of temperature.


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We are grateful for support provided by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, Heavy Elements Chemistry Program, US Department of Energy under Grants DE-SC0002215, DE-FG02-13ER16414, DE-SC002183 (N.P., R.M. and L.G.), and DE-AC02-06CH11357 (D.A.D., G.L. and L.S.), and for support from the National Science Foundation CAREER award DMR-0955353 (M.S.). Collaborative work is supported via the Helmholtz Association, Grant Number VH-NG-815. The 249Cf was provided to Florida State University via the Isotope Development and Production for Research and Applications Program through the Radiochemical Engineering and Development Center at ORNL and was purchased via the Gregory R. Choppin Chair Endowment.

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M.J.P. and T.E.A-S. conceived, designed and carried out the synthetic and crystallographic experiments. R.M., N.P., L.G., E.B.G. and D.A.D. designed and carried out the quantum mechanical study. J.T.S., J.N.C. and T.G.P. carried out low-temperature spectroscopic experiments. E.V.A. and W.D. were involved in the crystallographic analysis. G.L. analysed all the spectroscopic experiments. M.S. designed and carried out the magnetic experiments and, along with S.S. and L.S., analysed the magnetic data. T.D.G. and K.L.K. carried out the photoluminescence lifetime measurements. S.M.V.C. prepared and manipulated the original stock of 249Cf at ORNL. All authors discussed and co-wrote the manuscript.

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Correspondence to Thomas E. Albrecht-Schmitt.

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Crystallographic data for Cf[B6O8(OH)5] (CIF 14 kb)

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Polinski, M., Garner, E., Maurice, R. et al. Unusual structure, bonding and properties in a californium borate. Nature Chem 6, 387–392 (2014). https://doi.org/10.1038/nchem.1896

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