VIBRATIONAL-SPECTRA AND QUANTUM-CHEMICAL INVESTIGATIONS OF STOICHIOMETRIC ANIONS IN THE SERIES TL2P2S6-XSEX AND AG2P2S6-XSEX WITH X=0-6 - EVIDENCE OF THE HEXASELENIDOMETADIPHOSPHATE ANION, P2SE62-

Vibrational spectra are presented for Tl2P2S6-xSex and Ag2P2S6-xSex (x = 0-6) reaction mixtures. Reaction charges with high selenium content s gave glassy products in which the main building units appear to be t he hexaselenidometadiphosphate anion, P2Se62-. For intermediate x valu es mixed thio-seleno-metadiphosphates are formed. Geometry and equilib rium energy are calculated and the vibrational spectrum of the P2Se62- anion is well modelled by ab initio quantum chemical calculations. Fo r the probably microcrystalline Tl2P2S6-xSex series, Se seems to first enter the terminal positions of the anions. According to the calculat ions, bridged selenium and terminal sulfur are favored in mixed P2S6-x Sex2- anions, and the spectra seem to verify this for the Ag2P2S6-xSex series. No evidence for monomeric PSe3- was found.