Vibrational studies of, and model for, halogen-bridged linear-chain mixed-valence complexes

Abstract
A number of complexes of the type [M II (LL) 2 ] [Pt IV (LL) 2 Cl 2 ] [ClO 4 ] 4 , where M = Ni, Pd or Pt and LL = 1,2-diaminoethane (en) or 1,2-diaminopropane (pn), as well as the monomers [Pt IV (LL) 2 Cl 2 ] Cl 2 and [M II (LL) 2 ] Cl 2 have been studied using Fourier transform infrared (f.t.i.r.) and resonance Raman spectroscopy. Under conditions of low tem perature and moderate resolution ( ca . 0.5 cm -1 ) it has proved possible to observe chlorine isotopic splitting in bands due to both the symmetric and the antisymmetric stretching modes of the linear Pt IV Cl 2 units in the chains. On the basis that the one-dimensional electronic properties of these chains are likely to be related to the in-chain Pt IV -Cl stretching force constants, a simple vibrational model is presented, which is able to reproduce the observed splitting patterns to well within experimental error. A comparative study of the Pt IV -Cl stretching force constants for the series of en complexes shows the trend: trans -[Pt IV (en) 2 Cl 2 ] Cl 2 (monomer) > Pd II - Pt IV > Ni II - Pt IV > Pt II −Pt lV . This trend is in line with that of the M II …Pt IV intervalence band maxima, namely Pd II - Pt IV (25000 cm -1 ) > Ni II - Pt IV (22200 cm -1 ) > Pt II - Pt IV (20800 cm -1 ), indicating an increase in M II … Cl chain interaction (and an expected increase in conductance) along the series.