RESUMO
The molecular structures of gas-phase strontium hydroxide complexes are quantum chemically calculated using density functional theory, and the effects of hydroxyl groups on strontium coordination are studied. It is found that the presence of a single hydroxyl group results in the near-degeneracy of complexes with a coordination number (CN) of 5, 6 and 7. The presence of a second hydroxyl group destabilises the heptacoordinated complexes, and marks the onset of a weakening of the Sr-O(H(2)O) bonds, as evidenced by analysis via the quantum theory of atoms in molecules (QTAIM) and measurements of the average angle between the Sr-O(H(2)O) bond and the H(2)O dipole moment. A third hydroxyl group strongly destabilises both CN = 6 and 7 complexes through significant weakening of the Sr-O(H(2)O) interaction; here, hydrogen bonding interactions between hydroxyl groups and water molecules begin to dominate. The tetrahydroxide complex is found to be electronically unstable in the gas phase, but can be stabilised by coordination of explicit water molecules. Replacement of the explicit water molecules by a continuum solvation model poorly reproduces the polarisation of the wavefunction by the explicit solvent, suggesting that a combined approach incorporating both explicit solvation and a continuum model is required for the accurate modelling of this dianionic complex.
RESUMO
The electronic and geometric structures of the title species have been studied computationally using quasi-relativistic gradient-corrected density functional theory. The valence molecular orbital ordering of UO2(2+) is found to be pi g < pi u < sigma g << sigma u (highest occupied orbital), in agreement with previous experimental conclusions. The significant energy gap between the sigma g and sigma u orbitals is traced to the "pushing from below" mechanism: a filled-filled interaction between the semi-core uranium 6p atomic orbitals and the sigma u valence level. The U-N bonding in UON+ and UN2 is significantly more covalent than the U-O bonding in UON+ and UO2(2+). UO(NPH3)3+ and U(NPH3)2(4+) are similar to UO2(2+), UON+, and UN2 in having two valence molecular orbitals of metal-ligand sigma character and two of pi character, although they have additional orbitals not present in the triatomic systems, and the U-N sigma levels are more stable than the U-N pi orbitals. The inversion of U-N sigma/pi orbital ordering is traced to significant N-P (and P-H) sigma character in the U-N sigma levels. The pushing from below mechanism is found to destabilize the U-N f sigma molecular orbital with respect to the U-N d sigma level in U(NPH3)2(4+). The uranium f atomic orbitals play a greater role in metal-ligand bonding in UO2(2+), UN2, and U(NPH3)2(4+) than do the d atomic orbitals, although, while the relative roles of the uranium d and f atomic orbitals are similar in UO2(2+) and U(NPH3)2(4+), the metal d atomic orbitals have a more important role in the bonding in UN2. The preferred UNP angle in [UCl4(NPR3)2] (R = H, Me) and [UOCl4(NP(C6H5)3)]- is found to be close to 180 degrees in all cases. This preference for linearity decreases in the order R = Ph > R = Me > R = H and is traced to steric effects which in all cases overcome an electronic preference for bending at the nitrogen atom. Comparison of the present iminato (UNPR3) calculations with previous extended Hückel work on d block imido (MNR) systems reveals that in all cases there is little or no preference for linearity over bending at the nitrogen when R is (a) only sigma-bound to the nitrogen and (b) sterically unhindered. The U/N bond order in iminato complexes is best described as 3.
