Spectroscopic studies on the nature of the metal-halogen bond
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The work described in this thesis is concerned with the study by spectroscopic methods of the metal - halogen bond in complexes of a variety of metal ions in various stereochemistries and oxidation states. The complexes chosen for study contained, in addition to the halogeno-ligands Cl, Br and I, tertiary phosphines, and hence spectroscopic studies of the metal - phosphorus bond have also been possible. The spectroscopic techniques used were those of low frequency infra-red; ¹H, ¹³C and ³¹P NMR; and electronic absorption spectroscopy. Three distinct but related systems have been investigated. 1) Four - coordinate complexes of nickel (II), palladium (II) and platinum (II) (all square planar), and of cobalt (II) and zinc (II) (both tetrahedral), of form MX₂L₂ (where X = Cl, Br, I; L = PEt₃ and Et₂PhP), and binuclear halogen-bridged species of form M₂X₄L₂ (M = Pd, Pt; L = PEt₃ ; X is as above) have been prepared and studied. Attempts to prepare four-coordinate rhodium (I) complexes of triethylphosphine and diethylphenylphosphine are also described. The complexes have been studied by the spectroscopic techniques outlined above and the results related, where possible, to the nature of the metal - halogen bond. In particular, chemical shift trends with change in halogen for ¹H, ¹³C and ³¹P NMR spectra have been related to a component of Mδπ→Xδπ metal- halogen π-bonding in the complexes. Other explanations for the observed deshielding trends, most notably the paramagnetic anisotropy of the diamagnetic d⁸ metal ion, have also been examined in the light of the constancy of resonances in the NMR spectra of zinc (II) complexes with change in halogen. In the infra-red spectra of the complexes, revised assignments have been made for the metal - phosphorus stretching vibrations, and an intense absorption near 200 cm -¹ has been assigned to the metal - phosphorus - carbon bending mode. 2) (a) Rhenium (V) halogeno - phosphine complexes containing oxo or nitride ligands have been prepared and studied. These compounds have the form ReOX₃L₂ and ReNX₂L₃ (X = Cl,Br,I ; L = PEt₃, Et₂PhP). The stereochemistries of the oxo complexes have been elucidated from NMR spectra by an analysis of the different methyl resonance multiplicities for cis and trans phosphine ligands. In addition to the NMR studies, the low frequency infra-red and electronic spectra of the complexes have been recorded and principal absorptions assigned. (b) Rhenium halogeno-phosphine complexes containing nitric oxide as a ligand have been prepared and studied by ¹H NMR, infra-red and electronic absorption spectroscopy. The complexes have the form ReX₃(NO)L₂ (X = Cl,Br,I ; L = Et₂PhP), with rhenium in the divalent oxidation state. This was suspected from magnetic moment measurements and confirmed from the communicated results of a single crystal X-ray structure determination of a related chloro complex. The electronic spectra can be interpreted in terms of divalent rhenium with the d⁵ electron configuration. The ReX₃(NO)L₂ complexes can be reduced to the univalent rhenium complexes, ReX₂(NO)L₃, with sulphite ion in the presence of excess ofphosphine. Variations in the preparations of ReX₃(NO)L₂ complexes have been studied. Thus, bubbling nitric oxide through solutions of ReX₃L₃ complexes gives ReX₃(NO)L₂ species. In contrast, ReX₄L₂ compounds undergo no reaction with nitric oxide. Other preparative methods investigated were (i) thermal decomposition of Ag₂ReX₆ (X = Cl,Br) salts in a stream of nitric oxide and treatment of the residues with Et₂PhP to give the univalent rhenium complexes, ReX₂(NO)(Et₂PhP)₃ and, (ii), reaction of the anion [Re₅(No)] ⁻² with Et₂PhP did not lead to identifiable products. (c) The phosphine complex chemistry of technetium has been briefly investigated and shown to be analogous with that of rhenium. 3) Octahedral ruthenium (III) and ruthenium (II) complexes have been prepared and studied by ¹H, ¹³C and ³¹P NMR; low frequency infra-red, and electronic absorption spectroscopy. The complexes have the form RuX₃L₃ (X = Cl, Br; L = PEt₃, Et₂PhP) for ruthenium (III) and RuX₃(NO)L₂ (X = Cl,Br,I ; L = PEt₃ , Et₂PhP) for ruthenium (II). In addition, para-substituted phenyldiethylphosphine ligands (p-QPhEt₂P; Q = Me₂N, MeO, Cl) have been prepared and their complexes with ruthenium (II) of stoichiometry RuX₃(NO)L₂ have been studied. The electronic effects of the para-substituents can be correlated with changes in spectroscopic parameters such as the nitrosyl infra-red stretching frequency and the energy of halogen to metal charge transfer transitions in the ultra-violet region. The overall results for these three systems have been compared and contrasted, and a general conclusion is that although δ bonding is the major component of the covalent contribution to the metal - halogen bond, the existence of a component of metal to halogen Mδπ→Xδπ bonding appears to be the best explanation for a number of trends observed in the spectroscopic data.