LASER DESORPTION IONIZATION FOURIER-TRANSFORM ION-CYCLOTRON RESONANCEMASS-SPECTROMETRY OF [(NB(6)X(12))X(2)(H2O)(4)]CENTER-DOT-4H(2)O, X=CL OR BR/

Tetraaquadodeca-mu-halodihalohexaniobium tetrahydrates [(Nb(6)X(12))X( 2)(H2O)(4)] . 4H(2)O, X = Cl or Br, were studied using laser desorptio n/ionization (LDI) Fourier transform ion cyclotron resonance mass spec trometry. Only negative ions were found to form in the gas phase under LDI conditions. The quasimolecular ion, M(-) = [(Nb(6)X(12))X(2)](-) and the fragment ions, [M-X(n)](-), n = 1-3, are the highest abundance ions in the LDI mass spectra, indicating the stability of the core cl uster, (Nb(6)X(12)). Clusters with chlorine atoms as ligands show rela tively high affinity for an additional chlorine atom, while clusters w ith bromine atoms as ligands prefer to lose a bromine atom. The result s are compared with those of mass spectra of analogous tantalum cluste rs.