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]Y4, 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 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 X-ray structural results. Resonance-Raman spectra of the complexes display long progressions in the symmetric chain stretching mode v1, v(sym)X-M(IV)-X), corresponding members occurring at higher wavenumbers than for the analogous en complexes. Progressions in v1 reach at most to v1 = 12 for X = Cl, v1 = 9 for X = Br and v1 = 5 for X = 1. Both the trends in E(g) and also those in v1 (max) vary in the order l < Br < Cl, Pd < Pt and BF4 less-than-or-equal-to ClO4 < PF6, implying increased valence localisation to the right in each series.