Vibrational spectra and normal coordinate analysis of Mo-92, Mo-100,Cl-35, and Cl-37 isotopomers of the cluster anions [(Mo(6)X(8)(i))Y-6(a)](2-); X(i)=Cl,Br;Y-a=F,Cl,Br,I

The tetrabutylammonium (TBA) salts of the octa-mu(3)-halogeno-hexahalogeno-octahedro-hexamolybdate (2-) anions [(Mo(6)X(8)(i))Y-6(a)](2-); X(i) = Cl, Br; Y-a = F, Cl, Br, I; have been synthesized using Mo-92, Mo-100, Cl-35, and Cl-37. Th, 10 K IR and Raman spectra reveal significant frequency shifts due to the isotopic labelling of the Mo-6 cage, the inner sphere halides X(8)(i) or the outer sphere ligands YI, respectively. The normal coordinate analysis yields (Mo-Mo) valence force constants of about 1.3 to 1.5 mdyn/A. For the mu(3)-bonded halogenes Cl-i and Br-i valence force constants of 1.1 resp. 1.0 mdyn/Angstrom are calculated. The values for (Mo-Y-a) bonds are found in the usual halide range. The observed isotopic shifts are verified very well by the calculations, allowing detailed assignment of the IR and Raman spectra of these compounds for the first time.