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On the Paucity of Molecular Actinide Complexes with Unsupported Metal−Metal Bonds: A Comparative Investigation of the Electronic Structure and Metal−Metal Bonding in U2X6 (X = Cl, F, OH, NH2, CH3) Complexes and d-Block Analogues
journal contribution
posted on 2006-08-21, 00:00 authored by Germán Cavigliasso, Nikolas KaltsoyannisDensity functional calculations have been performed on M2X6 complexes (where M = U, W, and Mo and X = Cl,
F, OH, NH2, and CH3) to investigate general aspects of their electronic structures and explore the similarities and
differences in metal−metal bonding between f-block and d-block elements. A detailed analysis of the metal−metal
interactions has been conducted using molecular orbital theory and energy decomposition methods. Multiple (σ
and π) bonding is predicted for all species investigated, with predominant f−f and d−d metal orbital character,
respectively, for U and W or Mo complexes. The energy decomposition analysis involves contributions from orbital
interactions (mixing of occupied and unoccupied orbitals), electrostatic effects (Coulombic attraction and repulsion),
and Pauli repulsion (associated with four-electron two-orbital interactions). The general results suggest that the
overall metal−metal interaction is stronger in the Mo and W species, relative to the U analogues, as a consequence
of a significantly less destabilizing contribution from the combined Pauli and electrostatic (“pre-relaxation”) effects.
Although the orbital-mixing (“post-relaxation”) contribution to the total bonding energy is predicted to have a larger
magnitude in the U complexes, this is not sufficiently strong to compensate for the comparatively greater destabilization
that originates from the Pauli-plus-electrostatic effects. Of the pre-relaxation terms, the Pauli repulsion is comparable
in analogous U and d-block compounds, contrary to the electrostatic term, which is (much) less favorable in the
U systems than in the W and Mo systems. This generally weak electrostatic stabilization accounts for the large
pre-relaxation destabilization in the U complexes and, ultimately, for the relative weakness of the U−U bonds. The
origin of the small electrostatic term in the U compounds is traced primarily to MX3 fragment overlap effects.