Electronic spectra, resonance–Raman spectra, and excitation profiles of the mixed-valence compound Reihlen's green [PtII(C2H5NH2)4][PtIVBr2(C2H5NH2)4]Br4·4H2O

Abstract

The mixed-valence compound Reihlen's green, [Pt^II(C₂H₅NH₂)₄][Pt^IVBr₂(C₂H₅NH₂)₄]Br₄·4H₂O, displays a resonance-Raman (r.r.) spectrum when irradiated within the contour of the mixed-valence charge-transfer transition in the visible. It is characterised by a progression in ν₁, the Br–Pt^IV–Br symmetric breathing fundamental, which reaches 7ν₁. Additional progressions nν₁+ν₂[where ν₂ is the δ(Br–Pt^IV–N) bending fundamental] to n= 4 and nν₁+ν₃(where ν₃ is probably a lattice mode) to n= 3 are also observed. The observation of such progressions permits the spectroscopic constants ω₁, x₁₁, x₁₂, and x₁₃ to be determined as 179.6, –0.37, 0.3, and 0.3 cm^–1 respectively. The diffuse-reflectance spectrum of the compound indicates that there are two overlapped mixed-valence charge-transfer transitions in the visible (main band at 18 250 cm^–1, shoulder at ca. 16 000 cm^–1) both of which (on the basis of previous single-crystal work) are polarised parallel to the chain axis. The excitation profile of ν₁(Stokes), 2ν₁(Stokes), and ν₁(anti-Stokes) have maxima at 16 100, 16 300, and 15 800 cm^–1 respectively. The observation that axial vibrational modes are resonance-enhanced on irradiation of a compound with an exciting line whose frequency falls within the band contour of an axially polarised electronic transition is considered to be an important result which may open the way to making more precise assignments of the electronic spectra of many compounds.