ELECTRONIC STRUCTURE AND BONDING FACTORS OF TRANSITION METAL - PENTADIENYL AND (FLUOROALKYL)PHOSPHINE COMPLEXES: PHOTOELECTRON SPECTROSCOPY AND COMPUTATIONAL METHODS

Abstract

The research detailed in this dissertation focuses primarily on analyzing the electronic structure and bonding interactions of phosphines and pentadienyl ligands with transition metal center using valence photoelectron spectroscopy (PES) and computational methodologies. Valence photoelectron spectroscopy is the most direct experimental probe of electronic structure and bonding. The ionization features provide information on orbital interactions and characters separate from other effects, charge distributions, electron richness, electron configurations and molecular symmetry.The electronic and bonding factors of the pentadienyl ligands have been explored in the first series of high-valent pentadienyl transition metal molecules, Cp(6,6-dmch)ZrX2 (Cp = eta5-cyclopentadienyl, X = Cl, Br, I; 6,6-dmch = eta5-6,6-dimethylcyclohexadienyl). Unlike the well known Cp2ZrX2 analogues, these Cp(6,6-dmch)ZrX2 molecules are intensely colored, and reflect a dramatic reversal in the favorability of the bonding depending on the metal oxidation state. The results indicate that the color of the Cp(6,6-dmch)ZrX2 complexes is due to a 6,6-dmch ligand-to-metal charge transfer band. Compared to the Cp2ZrX2 analogs, the Cp(6,6-dmch)ZrX2 molecules have a considerably less stable HOMO that is pentadienyl-based, and an essentially unchanged metal-based LUMO. The lowest unoccupied orbital of pentadienyl is stabilized relative to cyclopentadienyl and becomes a better potential δ electron acceptor, thus contributing to the differences in structure and reactivity of the low-valent and high-valent metal complexes.The key electronic and bonding differences between simple pentadienyl, heterodienyl and cyclopentadienyl ligands in Cp*Ru(L) systems are also probed. The Cp*Ru(heteropentadienyl) molecules have extensively delocalized electronic structures and show unique electronic features compared to their simple pentadienyl counterparts. The ionization features for these systems are reassigned in this work according to experimental observations.The sigma and pi bonding effects of a fluorinated phosphine chelate ligand, dfepe, in Mo(CO)4(dfepe) and CpMn(CO)(dfepe) molecules are investigated. The PES studies reveal that the dfepe ligand is similar and slightly less effective in stabilizing the metal-based orbitals by backbonding, and the amount of stabilization is dependent on the electron richness at the metal center. The theoretical calculations do not agree well with the experimental results.