Thermodynamic and Spectroscopic Studies of Some Metal Dimers
Cosmano, Richard Joseph
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https://hdl.handle.net/2142/70369
Description
Title
Thermodynamic and Spectroscopic Studies of Some Metal Dimers
Author(s)
Cosmano, Richard Joseph
Issue Date
1987
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Inorganic
Abstract
The structure-reactivity relationships have been investigated in this study for three metal dimers including tetra-n-butyratodirhodium (II), tetra-n-butyratodiruthenium (II,III) chloride, and tetra-n-heptanatodicopper (II). These complexes were studied using a combination of calorimetry, epr spectroscopy, and uv-visible absorption spectroscopy.
The epr results on the rhodium (II) butyrate dimer, taken in conjunction with the calorimetric and spectroscopic data, indicate that the metal complex can indeed undergo a pi-stabilization interaction in addition to sigma bonding with bases (for example, triphenylphosphine) capable of accepting some of the metals' pi$\sp*$ electron density. The molecular orbital scheme can be represented for a 1:1 adduct in terms of a three center bond and for a 2:1 adduct as a four center bond. The cations of 1:1 adducts are epr silent due to the double degeneracy of the highest occupied orbitals, while the 2:1 adduct cations will show an epr signal only if the axial donor interacts strongly enough to raise a singly degenerate orbital above the pi$\sp*$ orbitals. This happens when triphenylphosphine, N methylimidazole, or pyridine is the axial ligand. The presence of an epr for the 2:1 cations does not depend on the type of bonding (pi versus sigma) as reported in the literature, but rather on the strength of the interaction.
The ruthenium (II,III) butyrate dimer also exhibits pi-stabilization phenomena analogously to the rhodium complex, but the relative strength of the pi interaction is lower due to the lower amount of pi$\sp*$ electron density and the greater the positive charge on the metal core. The calorimetric results indicate that this bonding effect is nonetheless present with pyridine of acetonitrile as donor ligands since the measured enthalpies of reaction are greater than those predicted by the E and C equation. Attempts to similarly characterize the bonding interactions of the copper (II) heptanoate dimer were foiled as the dimer structure was readily broken down by the solvent and/or axial base ligands. Based on the related epr results, however, it was possible to make some conclusions as to the type of bonding present in the metal core and compare them to those presented in the literature. (Abstract shortened with permission of author.)
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