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

Abstract

The complexes [PtII(en)2][PtIV(en)2X2][PtIII2(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[PtII2(H2P2O5)4]·2H2O with [PtIV(en)2X2]X2 in aqueous solution at 100 °C. Spectroscopic evidence favours a formulation for the complexes which involves a halogen-bridged linear-chain PtII–PtIV cation in association with a dimeric PtIII anion. The absorption spectra are characterised by three bands, the one of lowest wavenumber being assigned to the PtII(dZ2)→ PtIV(dz2) intervalence transition of the chain cation and the next to the dσ→dσ* 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–PtIV–X mode (ν1′) of the cation chain when the exciting line is at resonance with the PtII→ PtIV intervalence band, and by a progression (to 9ν1, X = Br or I) in the PtIII–PtIII 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 PtIII–Br symmetric stretching mode of the anion. The formulation for the complexes is unique in that it involves platinum in three different oxidation states.