SYNTHESIS, CRYSTAL-STRUCTURE AND SPECTROS COPIC PROPERTIES OF THE CLUSTER ANIONS [(MO6BR8I)X(6)(A)](2-) WITH X(A)=F, CL, BR, I

The tetrabutylammonium (TBA), tetraphenylphosphonium (TPP) and tetraph enylarsonium (TPAs) salts of the a-mu3-bromo-hexahalogeno-octahedro-he xamolybdate(2 -) anions [(Mo6Br8i)X6a]2- (X(a) - F, Cl, Br, 1) are syn thesized from solutions of the free acids H2[(MO6Br8i)X6a] . 8 H2O wit h X(a) Cl, Br, I. The crystal structures show systematic stretchings i n the Mo-Mo bond length and a slight compression of the Br8i cube in t he F(a) to I(a) series. The cations do not change much. The i. r. and Raman spectra show at 10 K almost constant frequencies of the (Mo6Br8i ) cluster vibrations, whereas all modes with X(a) ligand contribution are characteristically shifted. The most important bands are assigned by polarization measurements and the force constants are derived from normal coordinate analysis. The Mo-95 nmr signals are shifted to lower field with increasing electronegativity of the X(a) ligands. The fluo rine compound shows a sharp F-19 nmr singlet at -184.5 ppm.