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Theoretical studies on the unusual bonding features of transition metal clusters and reactivities on ruthenium/osmium dihydrogen complexes

Authors Xu, Zhitao
Issue Date 2000
Summary Modern theoretical calculation methods have been used to study the electronic structures and properties (including reactivity) of some transition metal clusters and transition metal dihydrogen complexes. The objectives of our studies are to explore the structure, bonding and reactivity of those compounds with unusual or non-classical bonds. In order to understand the intriguing bonding feature of a newly synthesized RhBi7Br8 compound, ab initio and DFT calculations have been carried out for this molecular complex. Based on detailed orbital interaction analysis, an unusual five-center-four-electron bond is discovered in this complex. Such kind of five-center-four-electron bond has also been found in other transition metal complexes with a similar {MBi7} core, such as [{MBi7}Br8] (M = Co, Rh and Ir) and [{MBi7}Br10 (M = Cu, Ag and Au). The electronic structures of many electron-rich Ni(Co)-S(Se) transition metal clusters do not obey any existing electron counting rules and have long been believed to be difficult to understand. A simple orbital approach has been developed to appreciate their electron counts with the aid of extended Hukel molecular orbital calculations and their magnetic properties have also been discussed based on our orbital interaction analyses. A new emerging class of transition metal clusters consists of two entirely different types of metal complex fragments in terms of ligands accepting and donating properties. Ab initio and DFT quantum chemical calculations have been used to study the metal-metal interactions in these "xenophilic" clusters. Bonding models, based on a "local metal frontier orbital" approach, are proposed to account for their electronic and magnetic behaviors. Through our detailed analyses, we found that the paramagnetic properties of these clusters are determined from the coupling among those metal centers bonded to π-donating ligands. Transition metal dihydrogen complexes are another new class of transition metal complexes characterized by their non-classical M-(η2-H2) bonds. These complexes are believed to be involved in many catalytic reactions and have attracted much interest. In the dissertation, the acidities of transition metal dihydrogen complexes of the general type trans- [LM(H2PCH2CH2PH2)2(η2-H2)]n+ (M = Ru or Os; L = H-, CH3-, F-, CF3-, CN-, Cl-, Br-, CO, NCH, NH3 and PH3 ; n = 1 or 2) have been studied. We calculated the deprotonation energies of these compounds using the B3LYP density functional method and found that the deprotonation energies correlate well with experimental pKa values. The influence of L on the acidities of these transition metal dihydrogen complexes has been used discussed in terms of the electronic properties of L. The reaction mechanism of the hydrogenation of norbornadiene (NBD) through the trans dihydrogen complex RuH(η2-H2(NBD)(PPh3)2 has also been explored using density functional theory calculation at BLYP level. It has been found that the first hydrogen transferred to the olefin ligand is more likely from the hydride ligand rather than from the dihydrogen ligand. The higher reaction barrier of the later pathway is due to the trans arrangement of the two hydride ligands in the trans dihydride transition states and intermediate involved.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2000
Language English
Format Thesis
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