Institution Granting Degree
Texas A&M University
Ab initio Hartree-Fock-Roothaan self-consistent-field (HFR-SCF) and generalized valence bond (GVB) geometry optimizations are used to study problems that are inaccessible to experiment. The theoretically determined molecular geometries both reflect bonding interactions in complexes, and indicate the course of reactions.
Calculations on monohydrates of metal ions show that these complexes are planar because of the classical ion dipole interaction. The orbital interaction that favors a nonplanar complex between a water molecule and a transition metal can only induce nonplanarity in the absence of a charge on the metal. Calculations on a the neutral complex Fe$\sp0$-H$\sb2$O indicate a maximum strength of 2 kcal for the orbital induced nonplanarity of a metal monohydrate.
Theoretically determined geometries are used to find the relative importance of axial and bridging ligands in quadruply bonded dichromium(II) complexes. Complete geometry optimizations are shown to be necessary in examining these complexes. Trends in Cr-Cr bond lengths as a function of bridging and axial ligands are calculated and compared to experimental results. The response of the Cr-Cr bond to axial ligation is shown to depend on the nature of the bridging ligand. By comparing calculated trends in the Cr-Cr bond length to experimentally observed values, we are able to predict a Cr-Cr bond length of 2.05 to 2.10 A in tetracarboxylate complexes that lack axial ligands.
Transition states for carbonyl substitution reactions are calculated for several monosubstituted transition metal carbonyl complexes. These complexes are calculated to lose cis carbonyls more readily than trans, in agreement with experiment. The greater ease of cis carbonyl loss had been attributed to both a stronger ground state trans bond, and greater relaxation energy of the cis loss fragment. The results of calculation show a mixture of these two effects for most complexes. The relaxation effects are shown to be more important when the heteroligand is a $\pi$ donor. The direct interconversion of the cis and trans loss isomers is calculated to be slow at room temperature; however, a scheme for trans incorporation of labelled carbonyl upon multiple substitution is shown to be facile.