VIBRATIONAL-SPECTRA OF THE CLUSTER COMPOUNDS (M6XI12)XA2.8H2O,M=NB,TA XI=CL,BR XA=CL,BR,I

IR and, for the first time, Raman spectra at 80 K of the cluster compounds (M6X12i)X2a.8 H2O; M = Nb, Ta; X(i) = Cl, Br; X(a) = Cl, Br, I, have been recorded, characterized by typical frequencies of the (M6X12i) unit, which are only slightly influenced by the terminal X(a) ligands. The most intense line with the depolarisation almost-equal-to 0.2 in all Raman spectra is caused by in-phase movement of all atoms and assigned to the symmetric metal-metal vibratin nu-1, observed for the clusters (Nb6Cl12i) at 233-234, for (Nb6Br12i) at 186-187, for (Ta6Cl12i) at 199-203, and for (Ta6Br12i) at 176-179 cm-1. The iR spectra exhibit in the same series intense bands at 233, 204, 207, and 179 cm-1, assigned to the antisymmetric metal-metal vibration. The metal-metal frequencies are significantly higher than discussed before. The tantalum clusters show on excitation with the krypton line 647.1 nm in the region of a d-d transition at 645 nm a resonance Raman effect with series of overtones and combination bands. In case of (Ta6Br12i) another polarisized band is observed at 229 cm-1 and assigned to the Ta-Br(i) vibration nu-2. From the progressions of nu-1 and nu-2 anharmonicity constants of about -3 cm-1 are calculated indicating a strong distortion of the potential curves.