Synthesis, spectroscopy, and structure of the mixed-valence complexes [PtII(en)2][PtIV(en)2X2][PtIII2(H2P2O5)4X2](en = 1,2-diaminoethane, X = Br or I)

Abstract

The complexes [Pt^II(en)₂][Pt^IV(en)₂X₂][Pt^III₂(H₂P₂O₅)₄X₂](en = 1,2-diaminoethane) have been obtained as copper coloured needles (X = Br) or metallic green dichroic microcrystals (X = I) by the reaction of K₄[Pt^II₂(H₂P₂O₅)₄]·2H₂O with [Pt^IV(en)₂X₂]X₂ in aqueous solution at 100 °C. Spectroscopic evidence favours a formulation for the complexes which involves a halogen-bridged linear-chain Pt^II–Pt^IV cation in association with a dimeric Pt^III anion. The absorption spectra are characterised by three bands, the one of lowest wavenumber being assigned to the Pt^II(d_Z²)→ Pt^IV(d_z²) intervalence transition of the chain cation and the next to the d_σ→d_σ* transition of the dimeric anion. The complexes are unusual in that they contain class II (partly-localised valence) cation chains while at the same time the interaction between the cation and anion is of the class I (highly-localised valence) type. The infrared, Raman, and resonance-Raman spectra of the two complexes have been studied in detail, the Raman spectra at ca. 80 K being characterised (among other things) by a progression [to 10ν₁′(X = Br) and 5ν₁′(X = I)] in the symmetric X–Pt^IV–X mode (ν₁′) of the cation chain when the exciting line is at resonance with the Pt^II→ Pt^IV intervalence band, and by a progression (to 9ν₁, X = Br or I) in the Pt^III–Pt^III stretching mode (ν₁) of the anion at resonance with the σ→σ* transition. Single-crystal resonance-Raman spectra of the bromide at ca. 15 K show progressions in ν₁′and ν₁ as follows: v₁′ν₁′ to v₁′= 11, v₁v₁ to v₁= 12, v₁ν₁+ν₂ to v₁= 9, and v₁ν₁+ 2ν₂ to v₁= 8, where ν₂ is the Pt^III–Br symmetric stretching mode of the anion. The formulation for the complexes is unique in that it involves platinum in three different oxidation states.