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|Title:||Thermodynamic Studies of Metal Carboxylates|
|Author(s):||Long, John Reid|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||In an effort to better understand the chemistry and physical properties of metal clusters a quantitative study of the chemical reactivity of numerous metal carboxylates was performed. Dimeric metal carboxylates were chosen for such a study because of the extensive structural information available on them. The thermodynamics of adduct formation for 1:1 and 2:1 adducts formed by Lewis bases with rhodium(II) butyrate were studied in benzene and methylene chloride solutions. Electrochemical studies of these adducts were carried out in methylene chloride solutions. The thermodynamic data clearly demonstrates substantial changes in the acidic and redox properties of the second metal as a result of base coordination to the first. The metal-metal bonding in the system causes this dimer to be a most unusual Lewis acid, as evidenced by deviations of the E and C predicted enthalpies from those observed. The unusual Lewis acid properties are attributed to the enhanced (pi)-backbonding capability of the rhodium(II) center as a result of extensive mixing of orbitals with (pi)-symmetry on the two metal centers. This causes the rhodium(II) center to be very effective in (pi)-backbonding to the ligands. The reduction potentials of Rh(,2)(C(,4)H(,7)O(,2))(,4)('+), Rh(,2)(C(,4)H(,7)O(,2))(,4)B('+), and Rh(,2)(C(,4)H(,7)O(,2))(,4)B(,2)('+) are analyzed, and provide further support for the extensive (pi)-backbonding capabilities of this metal cluster.
Thermodynamic and spectroscopic studies of rhodium(II) perfluorobutyrate were performed to determine the role of the bridging ligands in the Lewis acid properties of metal carboxylates. The thermodynamic data clearly demonstrates substantial changes in the acid properties of the second metal as a result of base coordination to the first, as was seen for rhodium(II) butyrate. Again, deviations of the observed enthalpies from those predicted by the E and C equation indicate the (pi)-backbonding capability of rhodium(II) perfluorobutyrate. The main effect of the fluorination of the bridging ligands is to make the rhodium centers better electron pair-acceptors.
A proposed molecular orbital diagram predicts that dimeric molybdenum carboxylates are incapable of (pi)-backbonding since they do not possess electron density in the (pi)* orbitals, as do the rhodium carboxylates; for this reason a study of molybdenum(II) perfluorobutyrate was performed. Thermodynamic data for six bases clearly demonstrates substantial changes in the acidic properties of the second metal as a result of base coordination to the first. Deviations of the observed enthalpies from those predicted by the E and C equation were .3 kcal mol('-1) or less for all bases studied. As a result, (pi)-backbonding is believed to be insignificantly small or absent in the metal-ligand bonds of molybdenum(II) perfluorobutyrate, hence substantiating the proposed bonding model, and the molecular orbital diagram.
In addition, various models that attempt to quantize the influence that base binding at the first metal site has upon the base binding properties of the second metal site are presented, and recent literature that is in disagreement with the proposed molecular orbital diagram is discussed.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.
|Date Available in IDEALS:||2014-12-13|