Density functional study on the electronic structure of trioxorhenium organyls

A linear combination of Gaussian-type orbital density functional (LCGTO-DF) investigation of three rhenium-oxo complexes, RReO₃ (R = CH₃, η¹-C₆H₅, η⁵-C₅H₅), is presented, showing the influence of the three different types of ligand-to-metal bonding σ-alkyl, σ-aryl, and π-aryl, respectively. The calculated ionization potentials of these compounds are compared to their He I photoelectron spectra and found to be in excellent agreement. In particular, the present results for H₃C-ReO₃ are much improved over various other theoretical approaches. Special attention is drawn to the -ReO₃ fragment energy levels and to the nonbonding 2p lone pairs on the oxygen atoms; their relative energetic positioning allows conclusions on the charge distribution and thus on the Lewis acidity of the rhenium atom in the active site -ReO₃. To assist this interpretation, Mulliken fragment charges and calculated dipole moments are discussed. A recently proposed relationship between the oxygen charge within the -ReO₃ fragment and ¹⁷O NMR shifts is supported.