| Abstract
| - The electronic structures of molecules of the form Cp*Ru(η5-Pdl) (where Cp* = η5-pentamethylcyclopentadienyl and Pdl = 2,4-dimethylpentadienyl and various heteropentadienyl ligands, including theazapentadienyl ligand 1-tert-butyl-3,5-dimethyl-1-azapentadienyl and the oxopentadienyl ligands 2,4-dimethyl-1-oxopentadienyl and 2,4-di-tert-butyl-1-oxopentadienyl) have been investigated using photoelectron spectroscopy and computational methods. The photoelectron spectra of these half-openmetallocenes allow direct insight into the bonding capabilities of pentadienyl ligands and the influencesof heteroatom substitution on the electronic structure. The number of separate valence ionization bandsthat are observed corresponds directly to the number of occupied valence metal d orbitals and highestoccupied π orbitals of the Cp* and Pdl ligands plus (in the case of the heteropentadienyl ligands) anionization that derives from the heteroatom (oxygen or nitrogen) lone pair orbital that is in the plane ofthe ligand. The heteropentadienyl ligand substitution has strong effects on both the ligand- and the metal-based ionizations. Interestingly, the ease of oxidation of the molecules does not follow the expectedperiodic trend of increasing ionization energy with increasing electronegativity of heteroatom substitution.Density functional calculations give orbital energies and characters for the Cp*Ru(Pdl) molecules ingood agreement with those determined by experiment and offer an explanation of the unusual trend inionization energies with heteroatom substitution. The calculations also show enhanced Ru−cyclopentadienyl bonding accompanying a weakening of Ru−pentadienyl bonding as the pentadienyl ligand becomesmore electronegative with heteroatom substitution, which is important for understanding the relativestructures and chemical reactivities of heteropentadienyl−metal complexes.
- The photoelectron spectra and computational results for a series of half-open ruthenocenes show that nitrogen and oxygen heteroatom substitutions have strong effects on all the valence ligand-based and metal-based ionizations due to extensive orbital mixing and delocalization. Because of these orbital interactions and different bonding strengths, the properties of these molecules do not necessarily follow consistent trends with heteroatom electronegativity.
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