Radio and Microwave Spectroscopy of Some Complexes of Vanadium, Niobium and Tantalum

Stewart, Charles P (1973) Radio and Microwave Spectroscopy of Some Complexes of Vanadium, Niobium and Tantalum. PhD thesis, University of Glasgow.

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The thesis is subdivided into five main parts, which are summarised separately below. Part I This first part of the thesis is introductory in nature. The basic magnetic and electrostatic interactions which are of importance in understanding the form of electron paramagnetic resonance and nuclear quadrupole resonance spectra are discussed with particular reference to transition metal complexes of the type studied in this work. The way in which these spectra can be interpreted to obtain information about various aspects of the electronic structures of transition metal complexes is also discussed, and finally the spectrometers used in carrying out this work are described briefly. Part II This section describes a study of some oxovanadium (IV) chelates and their adducts with ethanol and pyridine. The X-band epr spectra of some five- and six-membered ring chelates 2+ of the VO2+ ion have been recorded in solution at room temperature and in magnetically dilute glasses at 77K. This study indicates that the chelates form adducts with pyridine and with ethanol, and that they are readily oxidised, especially in hydroxylic solvents, to oxovanadium (V) complexes. The oxidation may be reversible in some cases. A method is described for deriving very accurate values of the spin Hamiltonian parameters from glassy spectra of these complexes, which have effective C2v symmetry, and the parameters which were derived in this way are listed, together with the band maxima in the visible-u. v. spectra of the complexes. The spin Hamiltonian parameters are equated to the atomic orbital coefficients in some of the molecular orbitals involved in the bonding in the complexes, and it is thereby shown that whereas the in-plane d-bonding in the complexes is fairly covalent, the delocalisation of the unpaired electron onto the ligands via in-plane pi -bonding is very slight. The weak C2v component of the ligand field in these complexes mixes a small amount of 3dz2 and 4s orbital character into the 3d x2-y2 orbital which contains the unpaired electron, and the magnetic resonance data is used to estimate the extent of this mixing. Mixing 3d z2 character into the orbital containing the unpaired electron accounts for the in-plane anisotropy of the g- and hyperfine tensor components. Mixing in 4s character accounts for characteristic differences between the hyperfine tensor components observed for the five- and six-membered ring chelates, and for the differences in the changes which occur in these when additional complexing with solvent molecules takes place. The principal values of the hyperfine tensor components and the isotropic contribution to hyperfine coupling can be used to distinguish between five- and six-membered ring chelates. Part III This section describes a study of the complexes (pi-C5-H5)2 MX2 where M = vanadium (IV), niobium (IV) or tantalum (IV) and X = Cl-, SCN-, OCN-, CN- or -C5H5-. The epr spectra of these compounds have been recorded at room temperature and in magnetically dilute glasses at 77K; and these spectra have been analysed in detail. From Huckel L.C.A.O. molecular orbital calculations carried out on the complex (pi-C5H5)2 VCl2, the metal ion spin-orbit coupling constants Sv and S Nb in these complexes are estimated to be 133 cm-1 and 490 cm-1 respectively. Spin Hamiltonian parameters are listed for each substance, and are equated to the atomic orbital coefficients in some of the molecular orbitals involved in bonding in these complexes, and good agreement is obtained between the values derived in this way and those derived from the molecular orbital calculations. Except for the cyanides, in each case the unpaired electron lies essentially in a non-bonding nd z2 metal ion orbital, mixed with a small amount of the corresponding metal ion nd x2-y orbital, the Z-axis coinciding with the C2 axis of the compound; in the cyanides the unpaired electron is delocalised into px orbitals on the ligands. The bonding of the metal ion to the cyclopentadienide rings and to the other ligands X is almost completely covalent, the bonding to the cyclopentalienide rings being stronger than that to the ligands X. Several redistribution complexes of the type (pi-C5H5)2 MXY have also been detected and characterised by their epr spectre. epr techniques can be used to distinguish compounds of the type (pi-C5H5)2 MX2 from other compounds which contain vanadium (IV) and niobium (IV). The difference between the epr spectra arising from complexes containing tantalum (IV) and those containing vanadium (IV) or niobium (IV) may be due to the presence of a large quadrupolar interaction in the tantalum complexes. Part IV This section deals with a detailed study of the linewidths in the epr spectra of solutions of the complexes (pi-C5H5)2 VCl2, (pi-C5H5)2 NbCl2 and (pi-C5H5)2 Nb(pi-C5H5)2 as a function of temperature. In the case of the complexes (pi-C5H5)2 VCl2 and (pi-C5H5)2 NbCl2 these studies have been used to estimate the size of the proton and chlorine nuclear hyperfine coupling constants in these complexes, and hence to estimate the extent to which the unpaired electron in the complexes is delocalised on to the ligands. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Inorganic chemistry
Date of Award: 1973
Depositing User: Enlighten Team
Unique ID: glathesis:1973-78645
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:06
Last Modified: 30 Jan 2020 15:06

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