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

The tetrabutylammonium (TBA), tetraphenylphosphonium (TPP) and tetraphenylarsonium (TPAs) salts of the octa-mu3-bromo-hexahalogeno-octahedro-hexamolybdate(2 -) anions [(Mo6Br8i)X6a]2- (X(a) - F, Cl, Br, 1) are synthesized from solutions of the free acids H2[(MO6Br8i)X6a] . 8 H2O with X(a) Cl, Br, I. The crystal structures show systematic stretchings in the Mo-Mo bond length and a slight compression of the Br8i cube in the 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 fluorine compound shows a sharp F-19 nmr singlet at -184.5 ppm.