THE IONIZATION/STRUCTURAL RELATIONSHIPS IN SOME METAL-MOLECULE AND QUADRUPLY-BONDED METAL-METAL INTERACTIONS.
AuthorBLEVINS, CHARLES HENRY, II.
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PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractThis dissertation describes the experimental study of the electronic-structural relationships of selected mononuclear transition-metal sulfur dioxide, cyclopentadienyl and carbonyl complexes and the application of the information gained from these to the study of quadruply-bonded dimetallic complexes. These pertinent observations result from the application of photoelectron spectroscopy (p.e.s.) as a probe into the bonding, charge-distribution and excited state effects which contribute to the specifics of the ground and excited state molecular structures. The first part of this discussion centers around a specific study of the exemplary bonding probe, SO₂, with the well characterized ArM(CO)₂ metal fragment, where Ar = Bz and Cp and M = Cr, Mn and Re. A comparison of the ionization information with the structural details and molecular orbital calculations reveals not only the surprising coordinating similarity of SO₂ and CO in these complexes, but also the electronic origin for the counter-intuitive SO₂ bonding configuration. This work then moves to a more dramatic example of electronic control of ground state molecular structure; the crystallographically determined distortion of the coordinated Cp ring in Cp*Rh(CO)₂. The electronic origin of this distortion is graphically shown with the aid of two and three dimensional experimental and theoretical electron density maps. The structural effects of removing bonding electrons from quadruply-bonded dimetallic complexes is then investigated. This study incorporates the use of high-resolution p.e.s. for the novel observations of metal-metal vibrational structure in the predominantly metal ionizations providing direct information of the bonding influence of specific metal electrons. Particular attention is focused on the delta-ionization process of MO₂(O₂CCH₃)₄. The final chapter presents a comprehensive study of the valence and core ionizations of the series of quadruply-bonded M₂(X₂CR)₄ complexes, where M₂ = Cr₂, Mo₂, MoW, and W₂, X = O and S, and R = H, CH₃, CD₃, CF₃, CH₂CH₃, CH₂CH₂CH₃ and C(CH₃)₃. The changes in the electronic structure in both the ground and excited states of these molecules is presented and, where appropriate, compared to structural changes. The study of this series not only demonstrates how p.e.s. can be used to monitor the electronic effects of specific chemical modifications, but also reveals surprising excited state features related to facile charge-reorganization processes.
Degree GrantorUniversity of Arizona
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Metal, ligand, and symmetry influences on metal-metal bonds: Photoelectron spectroscopy and theoryLynn, Matthew Allen (The University of Arizona., 2000)Three sets of metal-metal bonded systems of the form M₂(L ͡ L)₄ have been studied by gas-phase ultraviolet photoelectron spectroscopy and electronic structure calculations to understand the electronic structures of and bonding in these molecules. The ligand sets range from the relatively poor electron donor trifluoroacetate ligand, to hydroxymethylpyridinate (mhp), and finally to the relatively strong electron donor N,N'-diphenylformamidinate (form) ligand. Not only does this study elucidate the methods by which metal and ligand interact throughout a series of differing electron donor ligand sets, but it also presents a cohesive understanding of the electronic structures of these systems in terms of overall molecular symmetry. In particular, the relative stabilities and orbital characters of the metal-metal bonding and antibonding orbitals are probed to understand the ability of a particular ligand set to affect the ability of two metal atoms to bind together. First, compounds of the form M₂(form)₄ (M = Cr, Mo, W, Ru, Rh, Pd) are examined. The spectra of the metal-metal quadruple bond-containing systems (i.e., M₂(form)₄ where M = Cr, Mo, W) are used to identify several metal- and ligand-based ionization features, which can then be used to identify the additional metal-based features in the spectra of the remaining systems. Given the ease with which functional groups can be added to the formamidinate ligand, a series of substituted Mo₂(form)₄ systems have been prepared and their ionization data have been compared with solution-phase electrochemical results. Next, the electronic structures of M₂(O₂CCF₃)₄ (M = Mo, Rh) are studied. Variable energy photon experiments reveal a predominance of ligand character in the M-M σ and π orbitals, despite the relatively poor overall electron donor ability of the ligand. The means by which such a ligand can interact by symmetry with these metal orbitals are studied by computational methods. Finally, the bonding in M₂(mhp)₄ (M = Cr, Mo, W, Ru, Rh) systems is probed. The lower symmetry of these molecules and the intermediate donor properties of this ligand set allow for correlation with the electronic structures of M₂(form)₄ and M₂(O₂CCF₃)₄. Unlike for the higher symmetry systems, ligand involvement in the M-M δ bond is observed and can be understood in terms of molecular symmetry arguments.
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