Comprehensive analysis of pseudo-octahedral lanthanide(III) and uranium(IV and VI) tri-(3,5-dimethoxyphenyl)phosphine oxide complexes

By utilizing tri-(3,5-dimethoxyphenyl)phosphine oxide (OP(Ph 3,5OMe ) 3 ) as a bulky ligand for complexing f -block elements, a series of pseudo-octahedral complexes of the form mer -LnBr 3 (OP(Ph 3,5OMe ) 3 ) 3 (Ln 3+ = La 3+ –Lu 3+ excluding Pm 3+ ) were synthesized and characterized. This synthetic procedure was then applied to U(IV) and U(VI) where the trans -UX 4 (OP(Ph 3,5OMe ) 3 ) 2 and trans -U VI O 2 X 2 (OP(Ph 3,5OMe ) 3 ) 2 (X − = Cl − , Br − , I − ) complexes were obtained. The pseudo-octahedral local geometry about the metal centers was retained in all 20 compounds. These complexes were characterized by sc-XRD, p-XRD, solid-state Raman spectroscopy, solution and solid-state UV–vis-NIR spectroscopy, and 1 H, 13 C, and 31 P NMR spectroscopies. UV–vis-NIR spectra show only a few, weak 4 f → 4 f transitions for the lanthanide complexes as well as a reduction in the number and intensity of 5 f → 5 f transitions for U 4+ . This is ascribed to the pseudo-centrosymmetric, local coordination environment enforcing Laporte's selection rules. Bond metrics were used in conjunction with Natural Bonding Orbital (NBO) calculations to obtain the Natural Localized Molecular Orbitals (NLMOs) for investigation of the covalent contributions to bonding as it relates to the Inverse Trans Influence. There is a mild polarization of the Ln–L bonds that causes slightly heightened orbital mixing, but these effects are small in comparison with the U(IV) and U(VI) complexes. The observed orbital mixing is weak, but stronger than expected for a hard donor ligand with the Ln(III) cations. In contrast, the U(IV) and U(VI) complexes yield results similar to what would be expected in the lanthanide complexes based on Fajan's rules.