Michael, David James; (1992) The Raman spectroscopy of some halogen-bridged, linear-chain inorganic complexes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Optical studies have been made of a number of halogen-bridged, mixed-valence complexes of platinum, palladium and nickel. These complexes all have a linear chain structure containing the repeat unit -X…MIIL…X-MIVL,= Pt, Pd or Ni, L = equatorial ligand, usually amine and or halide and X = halogen. Electronic spectra of the complexes show a strong, broad, and polarised band in the visible region, which is assigned to an MII - MIV intervalence charge transfer (IVCT) transition. Raman spectra, when obtained using excitation wavelengths within the IVCT band, demonstrate long and intense progressions in the band assigned to the symmetric (X-MIV-X) stretch vibration. Weaker progressions are also sometimes observed, where is a second vibrational mode, e.g. V(MIV-L) or (N-MIV-N). This resonance enhancement occurs as a result of electron-phonon interaction between the IVCT transition and motion along the V1 normal coordinate, i.e. a Frank-Condon mechanism operates in the scattering. The effects of changing the metal, the bridging halogen, the equatorial ligand and the counterion on the optical properties and, by inference, on the structure of the complexes have been investigated. The wave number of the IVCT transition and that of shown to increase on going from bidentate to quadridentate equatorial ligands, as a result of increased hydrogen bonding. Studies of a variety of NiII/NiIV complexes have revealed two distinct types, which differ markedly in their optical characteristics. In addition, the only known example of the more symmetric linear chain structure, [[Ni (R,R-chxn)2Br]Br2 where R,R-chxn = IR,2R-cyclohexanediamine, has been investigated and shown to have Raman characteristics utterly different to those of the II/IV variety of complex. In a study of [Pt(4Me-dien)I][Pt(4Me-dien)I3]I2 and related complexes it is shown that a terdentate ligand may be stabilised in a high energy conformation, when incorporated in the linear chain structure. Unusual features in the resonance Raman spectroscopy of [Pd(R/R-chxn)2][Pd(R,R-chxn)2Br2]Br4 are reported. Investigations are reported of a mixed-metal complex, [CuxPt1_X(en)2][Pt(en)2Cl2](C104)4, analogous to those described above. Change in x dramatically alters the optical spectra. Finally, a discussion is given of the long observed dispersion in V1 with excitation wavelength. A further interpretation is offered of this phenomenon.

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