THE IONIZATION/STRUCTURAL RELATIONSHIPS IN SOME METAL-MOLECULE AND QUADRUPLY-BONDED METAL-METAL INTERACTIONS.
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The University of Arizona.Rights
Copyright © 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.Abstract
This 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.Type
textDissertation-Reproduction (electronic)
Degree Name
Ph.D.Degree Level
doctoralDegree Program
ChemistryGraduate College
Degree Grantor
University of ArizonaCollections
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Metal, ligand, and symmetry influences on metal-metal bonds: Photoelectron spectroscopy and theory(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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Rotational Spectroscopy of Simple Metal Carbon Clusters: Resolving the Beauty of Fine and Hyperfine Interactions in Metal Monoacetylides and Metal Carbides(The University of Arizona., 2016)Pure rotational spectra of simple metal carbon clusters that relevant to transition metal synthesis and catalysis have been obtained using Fourier transform microwave (FTMW) techniques combined with millimeter-wave direct-absorption methods. Rotational spectra of metal acetylides (CuCCH, ZnCCH, Li/Na/KCCH, MgCCH, AlCCH, CrCCH), diatomic metal monocarbide (CrC) and T-shape metal dicarbides (YC₂ and ScC₂) were recorded in the 4–650 GHz frequency regime. Measurements of weaker isotoplogues including ⁶⁶ZnCCH, ⁶⁸ZnCCH, Zn¹³C¹³CH, ZnCCD, Li/Na/KCCD, CrCCD, Y¹³C¹²C, Y¹³C¹³C, Sc¹³C¹³C, were also studied to aid in structural determinations. This work is the first study of ZnCCH and ScC₂ by any type of spectroscopic technique. Hyperfine splittings in MgCCH and Li/Na/KCCH have also been resolved and the weak isotoplogues of YC₂ have been measured for the first time. Potential interstellar molecules ScO and FeCN were studied using the FTMW techniques in the 4–62 GHz frequency regime. Spectra of the zinc insertion product ClZnCH₃ were additionally recorded in the 10–30 GHz (FTMW) and 260–296 GHz (direct absorption) frequency ranges, along with weaker isotopologues Cl⁶⁶ZnCH₃ and Cl⁶⁸ZnCH₃. This works is the first measurement of zinc insertion products using the FTMW-DALAS techniques. The data were analyzed implementing an effective Hamiltonian, allowing for accurate spectroscopic parameters to be established. From rotational constants, the molecular geometries were accurately determined. Electronic properties were also assessed, including the degree of covalent vs ionic character in a chemical bond, and the molecular orbital composition. The fundamental physical and chemical properties of these benchmark species were obtained in order to gain insight into their role in larger molecular systems, test theoretical calculations, and, in certain cases, provide accurate rest frequencies for astronomical searches.
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Electronic structure investigations of multiple bonding between atoms: From metal-nitrogen triple bonds to metal-metal triple and quadruple bonds(The University of Arizona., 2002)The nature of multiple bonding involving transition metal atoms has been explored via photoelectron spectroscopic and computational studies of molecules containing metal-metal quadruple and triple bonds as well as of molecules containing formal metal-nitrogen triple bonds. The principles governing the nature of the multiple bonding in these systems are similar whether the multiple bonding occurs between two transition metals or between a transition metal and a nitrogen atom. First, the electronic structures of the R₃M≡N molecules, where R = ᵗBuO (Cr, Mo, W); iPrO (Mo); (CH₃)₂CF₃CO (Mo); and Cl (Mo), are examined by photoelectron spectroscopy in conjunction with density functional calculations. To assign the features seen in the photoelectron spectra, close attention is paid to the effects of (1) metal substitution and (2) alkoxide (or Cl) substitution. Examination of the photoelectron spectra of the full series of alkoxide-substituted molecules allows the relative positions of the ionizations from the M≡N σ and π orbitals to be identified. Of great importance to the electronic structure of these molecules are the alkoxide orbital combinations that mix strongly with the M≡N σ and π orbitals. The importance of the ancillary ligand combinations is clearly demonstrated by the photoelectron spectroscopic and computational studies of Cl₃Mo≡N. The replacement of the alkoxide ligand with chlorides greatly simplifies the resultant photoelectron spectrum, allowing all of the valence ionizations to be assigned. Next, the bonding in the M₂X₄(PMe₃)₄ molecules, where M = Mo (X = Cl, Br); W (X = Cl); and Re (X = Cl, Br, I), is explored by photoelectron spectroscopic investigations in conjunction with electronic structure calculations. From these investigations, the ionizations from the metal-based orbitals as well as several ligand-based orbitals have been assigned. The first ionization energies of both the molybdenum (δ) and rhenium (δ*) molecules decrease as the electronegativity of the halide increases. The origin of this inverse halide effect is explored. Finally, the nature of the quadruple metal-metal bond in the M₂(chp)₄ molecules (M = Cr, Mo, W; chp = 2-chloro-6-oxo-pyridinate) is probed. For all three metal systems, an ionization from the M₂ δ orbital can be seen. This is only the second time a distinct ionization feature has been noted for ionization of the delta orbital from a dichromium molecule. Comparisons with the previously studied M₂(mhp)₄ molecules (mhp = 6-methyl-2-oxo-pyridinate) allow for a better understanding of the electronic structure of these molecules.
