The electronic structure of methyl-substituted ferrocenes and early transition metal bent metallocenes by gas phase ultraviolet and X-ray photoelectron spectroscopies.

Abstract

The details of the electronic structure and bonding in ferrocenes and early transition metal bent metallocenes are probed by photoelectron spectroscopy. The fundamental electronic interaction of the methyl group substituted for a hydrogen on a metal-coordinated cyclopentadienyl ring is shown by a combined core and valence pe spectroscopic study of a series of methyl-substituted ferrocenes. Shifts of core and valence ionization energies upon methyl substitution are equivalent and additive for the iron atom. Knowledge of the core ionization energy shifts for both carbon and iron allow the relative changes in atomic charges upon methyl substitution to be found. In these molecules, the methyl group is found not to be an inductive electron donor as is commonly assumed. The primary electronic effect of methyl substitution is on the valence orbitals of the cyclopentadienyl ring. Methylation of the cyclopentadienyl rings of ferrocene causes a dramatic redistribution of valence electron density and greatly increases the covalent nature of metal-ring bonding. An understanding of the electronic effect of methylation of metal-coordinated cyclopentadienyl rings is used to establish the relative locations of ring π and Cl ionizations in the pe spectra of group IV and V early transition metal bent metallocene dichlorides with both unsubstituted cyclopentadienyl and pentamethylcyclopentadienyl ligands. The differences in chloride and methyl ligand bonding to an early transition metal center are reflected in the photoelectron data of the dichlorides and related dimethyls. The relative differences in metal-chlorine and metal-carbon bond strengths are also shown in the pe data. The relationship between bond strengths and ionization energies is detailed for early transition metal bent metallocenes of niobium and tantalum with a variety of ligands. The relative bond strength/ionization energy information for metal-hydrogen and metal-carbon bonds is shown to help in understanding the stability of niobocene and tantalocene ethylene-hydride complexes and their resistance to intramolecular olefin insertion. Evidence from the pe data concerning the electron distribution as well as the relative bond strengths in these ethylene-hydride complexes is found supporting the consideration of these complexes more properly as metallacyclopropane-hydrides.