Vibrational Studies of, and Model for, Halogen-Bridged Linear-Chain Mixed-Valence Complexes

A number of complexes of the type [M^II(LL)₂] [Pt^IV(LL)₂Cl₂] [ClO₄]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)₂Cl₂] Cl₂ and [M^II(LL)₂] Cl₂ have been studied using Fourier transform infrared (f.t.i.r.) and resonance Raman spectroscopy. Under conditions of low temperature 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^IVCl₂ 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 Cl₂] Cl₂ (monomer) > Pd^II-Pt^IV > Ni^II-Pt^IV > Pt^II-Pt^IV. This trend is in line with that of the M^II-> Pt^IV intervalence band maxima, namely Pd^II-Pt^IV (25 000 cm^-1) > Ni^II-Pt^IV (22 200 cm^-1) > Pt^II-Pt^IV (20 800 cm^-1), indicating an increase in M^IIldots Cl chain interaction (and an expected increase in conductance) along the series.