Electronic and resonance Raman spectra of mixed-valence, linear-chain complexes of platinum and palladium with 1,2-diaminocycloalkanes (N–N), [ MII(N–N)2][ MIV(N–N)2X2]X4(X = halogen)

Abstract

The electronic and resonance Raman spectra of a series of halogen-bridged, linear-chain, mixed-valence complexes, [Pt^II(N–N)₂][Pt^IV(N–N)₂X₂]X₄[N–N = 1,2-diaminocyclohexane (dach); X = Cl, Br, or I], as well as of the complexes [Pd^II(dach)₂][Pd^IV(dach)₂Cl₂]Cl₄ and [Pt^II(N–N)₂][Pt^IV(N–N)₂Br₂]Br₄[N–N = 1,2-diaminocyclopentane (dacp)], have been recorded at ca. 295, 80, and 10 K. Excitation within the contours of the axially polarized M^IVâ†� M^II intervalence band of each complex leads to the appearance of long overtone progressions, v₁ν₁, in the resonance Raman spectrum, where ν₁ is the totally symmetric axial metal–halogen stretching mode. The excitation profile of the ν₁ band maximizes in each case on the low-energy side of the intervalence band maximum. The wavenumbers of the ν₁ band, intervalence band maximum, and excitation profile maximum of the complexes decrease in the order Cl > Br > I, Pt > Pd, and dach > dacp. Although the mixed-valence complexes are chemically pure, they form as mixtures of conformational isomers unless the resolved ligand is used in their preparations. Such conformers have different intervalence band maxima and different ν₁ values and, in consequence, as the exciting-line wavenumber (₀) is changed, different conformers have their ν₁ bands resonance-enhanced, and the apparent value of ν₁ and its overtones change. These observations are discussed with reference to the steric hindrance between the cycloalkane rings in mixed-valence linear-chain complexes.