Heteropentadienyl analogues of half-open ruthenocenes: Metal-ligand interactions and electronic structure perturbations

The electronic structures of molecules of the form Cp*Ru(η ⁵-Pdl) (where Cp* = η ⁵-pentamethylcyclopentadienyl and Pdl = 2,4-dimethylpentadienyl and various heteropentadienyl ligands, including the azapentadienyl 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-open metallocenes allow direct insight into the bonding capabilities of pentadienyl ligands and the influences of heteroatom substitution on the electronic structure. The number of separate valence ionization bands that are observed corresponds directly to the number of occupied valence metal d orbitals and highest occupied π orbitals of the Cp* and Pdl ligands plus (in the case of the heteropentadienyl ligands) an ionization that derives from the heteroatom (oxygen or nitrogen) lone pair orbital that is in the plane of the 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 expected periodic 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 in good agreement with those determined by experiment and offer an explanation of the unusual trend in ionization energies with heteroatom substitution. The calculations also show enhanced Ru - cyclopentadienyl bonding accompanying a weakening of Ru - pentadienyl bonding as the pentadienyl ligand becomes more electronegative with heteroatom substitution, which is important for understanding the relative structures and chemical reactivities of heteropentadienyl - metal complexes.