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.

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The work presented in this dissertation focuses on the [FeFe]-hydrogenase active site as inspiration for the design and synthesis of complexes capable of the electrocatalytic generation of molecular hydrogen from protons and electrons. The majority of work discussed uses gas-phase photoelectron spectroscopy (PES) and density functional theory (DFT) to probe and analyze the bonding and electron distribution in potential catalysts. These two techniques are also used to explore the nature of cyanide as a ligand, due to its presence and unknown role in these enzymes. This dissertation begins with the study of (η⁵-C₅H₅)Fe(CO)₂X (FpX) and (η⁵- C₅Me₅)Fe(CO)₂X (Fp*X) complexes where X = H⁻, Cl⁻, and CN⁻ to assess and compare their π-accepting abilities, which is contradicted in the literature. The shifts in ionization energies measured by PES provide a measure of the relative bonding effects. The results indicate cyanide is, overall, a weak π-acceptor, and the σ- and π-donor interactions are important to understanding the chemistry. The molecule [(μ-ortho-C₆H₄S₂)][Fe(CO)₃]₂ was examined, in part due to the delocalized π-orbitals of the C₆H₄S₂ ligand, which could facilitate the redox chemistry necessary for catalysis. Computations show that upon ionization, the complex adopts a semi-bridging carbonyl; termed “rotated structure”. The reorganization energy of this geometry change was determined, which may provide understanding of how the active site in the enzyme enables electron transfer to achieve this catalysis. Next complexes of the form (μ-SCH₂XCH₂S)[Fe(CO)₃]₂, where X=CH₂, O, NH, ᵗBuN, MeN, were explored in order to provide insight to the unknown atom at the central bridging position of the alkyl chain in the [FeFe]-hydrogenase enzyme. The likelihood of a rotated cationic structure is also shown, with reorganization energy values similar to that seen for [(μ-ortho-C₆H₄S₂)][Fe(CO)₃]₂. The final chapter explores the replacement of selenium for sulfur in (μ- X(CH₂)₃X)[Fe(CO)₃]₂ and (μ-X(CH₂)₂CH(CH₃)X)[Fe(CO)₃]₂, where X is either sulfur or selenium. The PES data show destabilization of the selenium complex ionizations compared to the sulfur complexes and a lower reorganization energy was calculated. The computed HOMO-LUMO gap energy for the selenium-based complex is roughly 0.17 eV smaller than for the sulfur analogs, which may indicate a lower reduction potential is needed.

A series of alkyldithiolatodiironhexacarbonyl complexes of the form &mu:-(RS2)Fe2(CO)6, where RS2 is: 1,2-ethanedithiolate (eth-cat), cis-1,2-cyclopentanedithiolate (pent-cat), cis-1,2-cyclohexanedithiolate (hex-cat), and 2-exo,3-exo-bicyclo[2.2.1]heptanedithiolate (norbor-cat), are reported. These complexes display structures and catalytic behavior toward production of molecular hydrogen with similarities to the active site of the diiron hydrogenase enzymes. Hydrogen production is desirable as an alternative fuel source and these catalysts are capable of producing H2 in the presence of weak acid under electrochemical conditions. Through understanding of the factors which control the catalytic activity of these catalysts it may be possible to contribute to the development of a hydrogen fuel economy.Significant scan-rate dependence under electrochemical conditions is observed, resulting in an initial 1-to-2 electron reduction depending on how quickly the singly reduced species can reorganize. The rate of this reorganization directly corresponds to the internal strain within the system and can be ranked in the following order of increasing rate of reorganization: pent-cat < norbor-cat < eth-cat < hex-cat. Additionally, these catalysts all successfully catalyze protons to molecular hydrogen under electrochemical conditions in the presence of acetic acid via an ECEC catalytic mechanism, where, E is an electrochemical step (reduction) and C is a chemical step (protonation).Density functional theory computations support the reported catalytic processes by calculating physically observable quantities, such as: pKa values, reduction potentials, adiabatic ionization energies and carbonyl stretching frequencies in the infrared (IR) region. These quantities were used to suggest reasonable reactive intermediates within the catalytic cycle. The electronic structure of each catalyst was examined using photoelectron spectroscopy and the global minimum cationic structure, in all cases, involves a structure with a bridging carbonyl ligand, akin to that of the enzyme active site.The most significant outcome of this work is the unprecedented diiron center rotation upon reduction. As conformational strain involving the dithiolate ligand increases, the rate of reorganization of the anion increases leading to cleavage of an iron-sulfur bond to provide an alternative protonation site, a key step toward molecular hydrogen formation. This site is less basic than the unrotated form and helps evolve H2 with thermodynamic favorability.

The electronic structure and reaction dynamics of molecular and cluster anions in the gas phase has been investigated using negative ion velocity-map imaging photoelectron spectrometer. Photoelectron images provide important information about both energies and symmetries of the parent anion orbitals from which photoelectron originates. The symmetry and the ordering of several low-lying electronic states of neutral nitromethane (X¹A′, a³A″, b³A″, and A¹A″) are assigned based on a group theoretical analysis of the transitions angular distributions and the results of DFT calculations. The through-bond electronic coherence in meta- and para-dinitrobenzene anions is explored by recording a series of photoelectron images in 532-266 nm wavelength range. Photoelectron angular distributions for both isomers exhibit oscillatory behavior characteristic of the quantum interference effect, suggesting that dinitrobenzene anions retain their high symmetry electronic structures in the gas phase. Photoelectron imaging experiments on [O(N₂O)(n)]⁻, n =0–9 at 266 and 355 nm provide clear evidence of a switch from the cova)lent NNO₂⁻ cluster core to the atomic O⁻ core occurring between n = 3 and 4. The experimental results and theoretical modeling indicate that despite the greater stability of NNO₂⁻ relative to the O⁻ + N₂O⁻ dissociation limit, an O⁻ cluster core becomes energetically favored over NNO₂⁻ for n > 3, due to the more efficient solvation of the atomic anion. The photodissociation dynamics of I₂⁻ and IBr⁻ anions on the respective A' excited-state anion potentials is effectively unraveled in 780 nm pump - 390 nm probe time-resolve experiments. The time-dependent photoelectron spectra and classical trajectory calculations of the IBr⁻ dissociation provide the first rigorous dynamical test of the recently calculated A′ potential for this system. The photoelectron anisotropy cyclic variation observed in photodissociation of I₂⁻ is interpreted in the context of dual-center quantum interference model. The 390 nm pump – 390 nm probe experimental data reveal fast (≤100 fs) and delayed (~ 700 fs) appearance of the I⁻ channel in the photodissociation of I₂Cl⁻ and BrICl⁻ anions respectively. The difference in the reaction time-scales is attributed to the distinct dissociation pathways available for the anions to form I⁻ product.

Mass-selected cluster anions are employed as model micro-solutions to study solvent effects on the structural motifs and electronic structure of anionic solutes, including the roles of the solvent in controlling the outcomes of photochemical processes. Interaction of light with cluster anions can potentially lead to cluster photodissociation in addition to photodetachment. We investigate these competing processes by means of photoelectron imaging spectroscopy combined with tandem time-of-flight (TOF) mass spectrometry. Photoelectron images are reported for members of the [(CO2)n(H2O)m]- cluster series. For homogeneous solvation, the photodetachment bands show evidence of cluster core switching between a CO2- monomer anion and a covalent (CO2)2- dimer anionic core, confirming previous observations. The Photoelectron Angular Distributions (PADs) of the monomer- and dimer-based clusters reveal an interference effect that result in similar PADs. Stabilization of the metastable CO2- anion by water solvent molecules is highlighted because its ability to "trap" the excess electron on CO2. Most surprising is the effect of the water solvent in quenching the autodetachment channel in excited states normally embedded in the electron detachment continuum, allowing excited CO2-(H2O)m clusters to follow reaction paths that lead to cluster fragmentation. Observed O- based photoproducts are attributed to photodissociation of the CO2- cluster core and are dominant for small parent clusters, whereas a water evaporation channel dominates for larger clusters. Addition of a second CO2 to these clusters is shown to preferentially form monomer based clusters, whose photodissociation exhibit an additional CO3- based channel, characteristic of a photoinitiated intracluster ion-molecule reaction between nascent O- and the additional CO2 solvent molecule. Changes in the PADs of NO- are monitored as a function of electron kinetic energy for the NO-(N2O)n and NO-(H2O)n cluster anions. In contrast with hydration, angular distributions become progressively more isotropic for the N2O case, particularly when the photoelectron kinetic energies are in the vicinity of the 2Pi shape resonance of the N2O solvent molecules. First time observation of the CH3SOCH- anion of dimethylsulfoxide is reported along with the photoelectron images of this organic anion and of the monohydrated cluster. Observed photodissociation products are HCSO- and SO-.

Typescript. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1990. / Includes bibliographical references (leaves 118-122) / Microfiche. / xv, 122 leaves, bound ill. 29 cm

Thesis (PhD)--University of South Australia, 2003.

Thesis (Ph.D.)--University of Delaware, 2008. / Principal faculty advisor: Robert L. Opila, Dept. of Materials Science & Engineering. Includes bibliographical references.

Diss. (sammanfattning) Göteborg : Univ. , 2010. / Härtill 5 uppsatser.