SYNTHESIS, CRYSTAL-STRUCTURE, AND PHASE-TRANSITION OF SE-4(MOOCL4)(2)

Dark green, very air sensitive crystals of Se-4(MoOCl4)(2) are formed from selenium and MoOCl4 at 190 degrees C in a sealed, evacuated glass ampoule;in quantitative yield. The structure is built of nearly square planar Se-4(2+) ions and centrosymmetric dimeric MoOCl4- ions which are linked by bridging Cl atoms. At -21 degrees C Se-4(MoOCl4)(2) undergoes a reversible solid state phase transition of first order. Structure determinations at -70 degrees C and 23 degrees C show that during the phase transition the structures of the ions remain unchanged, while the orientations of the ions with respect to each other change in such a way that in the low temperature form the Se-4(2+) ions obtain a higher coordination number by Cl and O atoms of neighboring MoOCl4- ions.