Covalency in the f-element–chalcogen bond

In this presentation, Nik Kaltsoyannis of University College, London, offered a computational look at bonding between actinide or lanthanide metals (plutonium, uranium, cerium, or lanthanum) and imidodiphosphinochalcogenides, examining ligands containing the row-16 elements, oxygen, sulfur, selenium, and tellurium.

Kaltsoyannis emphasized the experimental observation that the uranium–chalcogen bond is significantly shorter than the equivalent lanthanum–chalcogen bond, despite the very similar ionic radii of the six-coordinate U ³⁺ and La ³⁺ ionic species; he cited recent Los Alamos work duplicating that finding for trivalent plutonium and cerium imidodiphosphinochalcogenides.

Using both Gaussian and Density Functional codes, Kaltsoyannis showed molecular orbital energy diagrams for both lanthanum and uranium and their bonding to the progressively less-electronegative chalcogens as one progresses from oxygen though sulfur to selenium and tellurium. The significant finding is that the increased degree of bond covalency—or decrease in ionicity—(which, strictly on this electronegativity basis might be expected to increase similarly from oxygen -> tellurium) is not, in fact, equivalent for uranium and lanthanum. In reality, the studies show, the increased covalency is greater when uranium is the metal ion in the complex as compared to complexes containing lanthanum.

A computational analysis of the energy levels of 18 valence molecular orbitals indicates that the lanthanum and uranium d atomic orbitals contribute to these molecular orbitals to about the same extent. This finding implies that the difference between the actinide and the lanthanide must reside in the contribution of f electrons to bonding. This research in progress tentatively concludes that the 5f electrons in the actinide must make a larger contribution to covalency than the 4f electrons of the lanthanide.

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