The light actinide elements (U, Np, Pu, and Am) in their hexavalent oxidation states form a wide variety of complexes based on the linear, dioxo (actinyl) cations, AnO22+.  These ions are remarkably stable, having strong covalently bonded O=An=O ions.  Vibrational spectroscopy shows conclusively that the AnO22+ ion exists as a symmetrical, linear group with a regular decrease in An=O bond strength with increasing atomic number from uranium through americium.  For curium, there is no conclusive evidence for the existence of the actinyl ion.  We report single crystal x-ray diffraction, and solution EXAFS measurements that reveal a remarkable shortening, and then lengthening of the An=O bond along the series, U, Np, Pu, Am.  Theoretical studies reveal that the origin of this trend can be traced to changing contributions from 5f and 6d atomic orbitals in the O=An=O bonding core, providing additional insights on the lack of a Cm analog.

Remarkable variability in 5f and 6d orbital contributions to metal-oxygen bonding in actinyl ions from U through Cm

243rd National Meeting (2012)

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