Attila Kovács and Rudy J.M. Konings

A theoretical study of the structure and bonding of UOX₄ (X=F, Cl, Br, I) molecules.

The importance of inverse trans influence.

ChemPhysChem, 7 (2006) 455-462

The structural and bonding properties of the uranium(VI) oxyhalides UOX₄ (X = F, Cl, Br, I) have been investigated by quantum chemical calculations at three different levels of theory: quasi-relativistic density functional theory (DFT) in conjunction with a triple-zeta all electron basis set as well as MP2 and the Becke3-Perdew-Wang91 exchange-correlation functional in conjunction with relativistic effective core potentials (ECP). The computations located four stationary points on the potential energy surface: a tetragonal pyramid with O in the apical position (C_4v), a trigonal bipyramid with O in the equatorial position (C_2v), a trigonal bipyramid with O in the axial position (C_3v), and a C_s structure derived from C_3v by opening the X_eq–U–X_eq angle to near 180º. The C_s minimum, however, seems to be an artifact of the Becke-Perdew functional on the flat potential energy surface. In the C_3v structures the linear X_ax–U=O moieties show clearly the geometrical consequences of the inverse trans influence (ITI) effect. This interaction can stabilise these sterically less favoured geometries. Two important trends are revealed by our computations: (i) UOX₄ with the small X = F prefers the C_3v structure, whereas with increasing halogen size the sterically less crowded C_2v structure gets more importance. (ii) MP2 theory accounts to a less extent for the ITI effect with respect to DFT, which results in different structural preferences (MP2 for C_2v, DFT for C_3v) in the heavier halides. In addition, an important bonding property of the UOX₄ molecules is the clear triple-bond character of the formally double U=O bonds.

The relevant structures.

The A₁ molecular orbital representing sU=O and sU-X contributions in the C_3v structures.

The two p orbitals of the UºO triple bond.