Synthesis and spectroscopy of linear-chain palladium(II,IV) and platinum(II,IV) complexes involving quadridentate ligands

The effects of the in-plane ligand, the metal atom, the bridging halogen atom and the counter-anion on the electronic structures of a series of linear-chain, halogen-bridged, mixed-valence complexes of platinum and palladium have been investigated. Complexes of the type [ML][MLX₂]Y ₄, where M = Pd or Pt, X = Cl, Br or I, L = 3, 7-diazanonane-1,9- diamine, 4,7-diazadecane-1,10-diamine, 1,4,8,11-tetraazacyclotetradecane or 1,4,8,12-tetraazacyclopentadecane and Y = BF₄, ClO₄ or PF₆, have been examined by means of electronic, Raman and resonance-Raman spectroscopy. The electronic spectra of the complexes each show an intense intervalence charge-transfer band in the visible region, occurring at higher energy, E_g, than it does for analogous bis(ethylenediamine) complexes; this implies greater localisation of valences for the quadridentate than for the bis(ethylenediamine) complexes, in agreement with conclusions drawn from the X-ray structural results. Resonance-Raman spectra of the complexes display long progressions in the symmetric chain stretching mode ν₁, ν_sym(X-M^IV-X), corresponding members occurring at higher wavenumbers than for the analogous en complexes. Progressions in ν₁ reach at most to ν₁ = 12 for X = Cl, ν₁ = 9 for X = Br and ν₁ = 5 for X = I. Both the trends in E_g and also those in ν₁ (max) vary in the order I < Br < Cl, Pd < Pt and BF₄ ≤ ClO₄ < Pf₆, implying increased valence localisation to the right in each series.