This research has been directed at the study of organosulfur frontier orbital interactions with transition metal d-orbitals. Two novel thioether complexes tricarbonyl(1,4,7-trithiacyclononane)molybdenum(O) and tricarbonyl(2,5,8-trithianonane)molybdenum(O), have been prepared and structurally characterized by single crystal x-ray crystallography. The facial configuration of the carbonyl ligands provides a unique point of reference for describing the two polythioether ligands in terms of the free ligand's frontier orbitals. The relative carbonyl stretching frequencies of the two metal complexes indicate that 1,4,7-trithiacyclononane is a poorer donor than 2,5,8-trithianonane. This result is explained in terms of mechanical constraints placed on the mesocyclic polythioether which are absent in its acyclic analogue. The coordinatively unsaturated species (n⁵-C₅H₅)MO(NO)(SC₆H₅)₂ has been characterized by x-ray crystallography and its electronic structure has been modeled using Fenske-Hall molecular orbital calculations. The monomeric nature and chemical inertness of (n⁵-C₅H₅)MO(NO)(SC₆H₅)₂ are attributed to dπ-pπ bonding between the thiolate ligands and an empty molybdenum dπ orbital. The dπ-pπ interaction simultaneously strengthens the metal thiolate bond and makes the complex less susceptible to nucleophilic attack by raising the energy of the LUMO. The rotational orientations of the thiolate ligands observed in the solid state support this electronic model. For (n⁵-C₅H₅)Fe(CO)₂SR, the dπ-pπI antibonding interaction between the thiolate ligand and the metal has been modeled using Fenske-Hall molecular orbital calculations and experimentally investigated by photoelectron spectroscopy. The calculations predict that the HOMO is metal-sulfurn-antibonding and largely sulfur 3p in character. The observed HOMO ionization energies of (n⁵-C₅H₅)Fe(CO)₂SC₆H₄-p-Z; Z = OMe, H, Cl, CF₃, N0₂; correlate with several chemical properties including the rate of electrophilic attack on the sulfur by alkyl halides to give the thioether complex [(n⁵-C₅H₅)Fe(CO)₂(SR₂)]X and by electron-deficient alkynes to give the heterometallacycle (n⁵-C₅H₅)(CO)FeS(R)-C=C=C=0. The latter reaction is compared to the similar reaction of alkenes and alkynes with (n⁵-C₅H₅)Fe(CO)₂PR₂ to give (n⁵-C₅H₅)(CO)FeP(R)₂-C=C-C=0. X-ray crystal structures of one of the sulfur-containing and one of the phosphorus-containing heterometallacycles have been obtained.

The chemistry and bonding of alkynes and vinylidenes in organometallic complexes have been investigated. A variety of these complexes have been synthesized and characterized by X-ray crystallography, temperature-dependent NMR, molecular orbital calculations, and most importantly, HeI, HeII and MgKα photoelectron spectroscopy (PES). The core and valence ionizations are found to be very informative with regard to the relative bond strengths and stabilities of these complexes. The first step involved preparation of the series of complexes R-CpM(CO)₂(alkyne) (R-Cp = Cp, MeCp and Me₅Cp). When M = Mn, Re (alkyne = 3-hexyne, 2-butyne and hexafluoro-2-butyne), the molecular mirror plane bisects the alkyne (horizontal conformation). PES shows the alkyne (π(⊥)) orbital forms a filled-filled interaction with the frontier metal orbital which is significantly destabilized. The ionizations derived from the two alkyne π orbitals are not split. When M = V, the alkyne (C₂H₂, 3-hexyne, etc.) coincides with the molecular mirror plane (vertical conformation). PES shows the alkyne π(⊥) orbital donates electrons to the electron deficient vanadium and the metal backbonds strongly to the alkyne. Electronic factors controlling the conformations in the d⁶ manganese case has been much discussed in the literature. Another factor not previously identified is necessary for understanding the conformation in the d⁴ vanadium case. The energy of the LUMO reveals that this factor is donation of cyclopentadienyl electrons into an empty d orbital of the electron deficient vanadium. Rearrangement of alkyne complexes to terminal vinylidene and bridging vinylidene complexes, similar to reactions of organic molecules on metal surfaces, were also investigated. The series of [R-CpMn(CO)₂]₂(μC=CHR') (R' = H, Me) (Chapter 6) and CpMn(CO)₂(C=CHBuᵗ) (Chapter 7) complexes were prepared. PES showed that the terminal vinylidene ligand has less filled-filled interaction with the metal and stabilizes the metal more than the alkyne does. The bridging vinylidene accepts more electron density from the metals and stabilizes the metals more than the terminal vinylidene. The removal of antibonding electrons from the HOMO of the metal fragment by the bridging vinylidene leaves net metal-metal bonding interaction and forms a stable dimetallocyclopropane structure.

Principles of transition metal electronic structure are presented to enable an understanding of the activation of C-H and C=C bonds by metals. A multitechnique approach utilizing core and valence photoelectron spectroscopies (p.e.s.) and molecular orbital calculations has been used to gain these insights. In the first half of the dissertation three principles are developed: ligand additivity, core-valence ionization correlation, and ring methylation. In the latter half of the dissertation these principles are seen to be crucial for understanding ionization data for the C-H and C=C activated species. Additive (with respect to ligand substitution) electronic effects, including additive core and valence ionization potentials, are shown in the p.e.s. of phosphine substituted molybdenum carbonyls. These additive effects demonstrate that the electronic effects of ligand substitution are predictable from empirical models. The core-valence ionization correlation enables direct comparison of XPS (core) and UPS (valence) ionization data and allows separation of bonding and overlap induced valence shift effects from Coulombic and relaxation shift effects. In the study of trimethylphosphine substituted cyclopentadienylmanganese tricarbonyl complexes, both the ligand additivity and core-valence ionization correlation principles are less valid than for the molybdenum carbonyl complexes because of loss of the very influential carbonyl backbonding. Methylation of the cyclopentadienyl ring in this system adds another independent variable of electronic structure perturbation and enables separation of the one-center and two-center Coulombic contributions to the core shifts. The above principles are used in the later chapters to show that the initial activation of the C-H bond in alkenylmanganese tricarbonyl complexes is dominated by the interaction of the C-H sigma bonding level with empty metal acceptor levels. The activation stops at the agostic stage rather than proceeding to full β-hydribe abstraction because there is, in these molecules, no gain in the number of pi electrons between the allyl and diene hydride endpoints of the abstraction cycle. Activation of the C=C bond in the cyclopentadienylmetal olefins is similar for Co and Rh complexes despite little similarity in the valence ionization spectra. The spectral differences are largely caused by the relaxation energy differences between Co and Rh. These complexes also provide interesting examples of electron delocalization through the metal. Permethylation of the cyclopentadienyl ring shifts the olefin pi ligand ionizations more than the expected Coulombic shift.