Electron Paramagnetic Resonance Spectroscopy of Some Complexes of Manganese(VI) and Rhenium(VI)

This thesis is subdivided into six main parts, which are summarised separately below. Part I The principles of electron paramagnetic resonance spectroscopy are discussed in this introductory section, with particular reference to transition metal complexes of the type studied in this work. Part II This section describes a study of rhenium oxychloride ReOCl4, and some of its adducts ReOCl4O2C4H8, ReOCl4OPCl3, ReOCl4NCCH3 and [(C6H5)4As] [ReOCl5] . The X-band e. p. r. spectra of ReOCl4 and its adducts have been recorded in solution at 290K and in magnetically dilute glasses at 77K and these spectra are analysed in detail. The spin Hamiltonian parameters Ao, go, A, B, Q', g11 and g1 have been extracted from these spectra. These parameters, and data obtained from visible-u.v. spectra have been used to obtain quantitative descriptions of the bonding in these compounds in terms of molecular orbital models. From extended Huckel L. C. A. O. molecular orbital calculations, carried out on the compound ReOCl4, the metal ion spin- orbit coupling constant and the parameter P are estimated to be about 2400 cm-1 and 0.033 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 molecules, and a good agreement is obtained between the values obtained in this way and those derived from the molecular orbital calculations. The unpaired electron lies in a molecular orbital which involves the metal ion 5dxy orbital and it is strongly delocalised (33%) on to the chlorine ligands. g11 is greater than g1 and this is shown to be essentially due to charge-transfer mixing by spin-orbit coupling at the chlorine atoms. The changes in the spin Hamiltonian parameters when a sixth ligand is added to ReCCl4 are accounted for. Part III In this section a detailed study has been made of electron paramagnetic relaxation phenomena in solutions of ReOCl4 in chloroform. This has enabled the separate contributions from spin-rotational interactions, from g-tensor anisotropy, from hyperfine coupling anisotropy, and from unresolved chlorine hyperfine coupling, to the observed e. p. r. linewidths to be evaluated. These results have been used to estimate the size of the chlorine isotropic hyperfine coupling constant and then the extent of delocalisation of the unpaired electron available in the complex on to the chlorine ligands. In this way it has been shown that the extent of delocalisation of the unpaired electron on to chlorine groups in ReOCl4 is about 21%. This result is in reasonable agreement with the deduction made from the analysis of the spin Hamiltonian parameters obtained in Part II. Part IV This section describes a study of the six-coordinated The e. p. r. spectra of these complexes have been recorded in solution at 298K and in magnetically dilute glasses at 77K. These e. p. r. spectra are quite different from those that have been obtained from the other rheniun complexes that have been studied in part II. Each trigonal pyramidal rhenium complex shows a single electron resonance signal in both magnetically concentrated solid and in solution at room temperature. The spectra of magnetically dilute glasses at 77K show small g-tensor anisotropy and vanishingly small rhenium nuclear hyperfine coupling. The small g-tensor anisotropy is very characteristic of sulphur-containing organic radicals and the vanishing small rhenium nuclear hyperfine coupling, almost certainly a direct dipolar coupling, sets an upper limit of the order of 0.1% to the rhenium contribution to the molecular orbital containing the unpaired electron in these complexes. The e. p. r. spectra show that the unpaired electron in these complexes is in a non-bonding molecular orbital trigonal-prismatic complexes, Re(S2C2Ph2)3 and Re(S2C6H3CH3)3 derived from the ligand pi-orbitals. E. P. R. properties, electronic absorption spectra, voltammetric properties, the magnetic properties of ions derived from Re(S2C2Ph2)3, and the unusually high electrical conductivities exhibited by the solids, are all consistent with an electronic ground state configuration (3a1)2(4e )4(2a2)1 for these complexes. Part V This section describes a study of the octacyano-rhenium complexes. E. p. r. spectra of magnetically undiluted polycrystalline [Ph4As]2 [Re(CN)8] have been recorded at room temperature and at 77K. The spectra are characteristic of an unpaired electron moving in an axially symmetric orbital in which the hyperfine coupling A is greater than B, and the g-tensor components are almost identical. (Abstract shortened by ProQuest.)