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, [PtII(N–N)2][PtIV(N–N)2X2]X4[N–N = 1,2-diaminocyclohexane (dach); X = Cl, Br, or I], as well as of the complexes [PdII(dach)2][PdIV(dach)2Cl2]Cl4 and [PtII(N–N)2][PtIV(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 MIVâ†� MII 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.