Photoelectron Spectroscopy of f-Element Organometallic Complexes. 8. DV-Xα and Gas-Phase UV Photoelectron Spectroscopic Investigation of the Electronic Structure of Tris(η⁵-cyclopentadienyl)uranium(IV) Complexes

The electronic structures of a series of closely related U(η⁵-C⁵H⁵)₃L (L = -CH₃, -NH₂, -BH₄, -NCS) complexes has been studied using the SCF Hartree-Fock-Slater first-principles discrete variational Xα method in combination with He I and He II UV photoelectron spectroscopy. The theoretical results reproduce the experimental He I and He II photoelectron spectroscopic data, thus providing a reliable description of the metal-ligand bonding. Symmetry considerations render the 5f elements well-suited templates for coordination of the Cp₃ ligand cluster. Interactions not restricted by symmetry appear partially or entirely modulated by the angular properties of π₂-related MO's. Bonding interactions with ancillary L ligands involve either 5f_z³ or 6d_z² metal orbitals, depending upon the energies of the unperturbed ligand orbital counterparts. Metal-ligand interactions cause the energies of 5f orbitals to split into a narrow manifold with a remarkable “ligand field” energy shift associated only with the 5f_y(3x²-y²) orbital. The L→M charge donation results in an electronic configuration of the uranium atom which is almost constant throughout the U(C₅H₅)₃L series and similar to that found in a fully relativistic SCF Dirac-Slater calculation on the simpler uranium atom. The stringent necessity of maintaining the uranium center in a particularly stable electronic configuration causes greater donation (hence larger covalency in the bonding) from ancillary L ligands to be compensated for by an increasing (L = -NH₂ < -CH₃ < -NCS < BH₄⁻) ionic character of the U-Cp bonds. The present results show that nonrelativistic DV-Xα calculations, optimized for basis set and potential representation, reproduce experimental photoelectron spectroscopic data, including He I/He II relative intensity changes.