F-19 NMR-SPECTRA AND X(A)-EXCHANGE REACTIONS OF THE CLUSTER ANIONS [MO6CL8IF(N)AX6-N(A)]2-, X(A) = CL, BR, I - N = 1 - 6

By intermolecular ligand exchange reactions of [(Mo6Cl8i)F6a]2- with [(Mo6Cl8i)X6a]2-, X(a) = Cl, Br, I, in acetonitrile, the outersphere mixed cluster ions [(Mo6Cl8i)F(n)aX6-n(a)]2- are available, which for n = 1 - 6 are characterized by their distinct F-19 nmr chemical shifts. The quantitative and time dependent examination reveals equilibration at room temperature within 2-3 days with exact statistical distribution of all components, inclusive the ratios for the geometric isomers with n = 2,4, cis:trans = 4:1 and n = 3 fac:mer = 2:3, indicating the equivalence of all X(a) positions with respect to exchange reactions. For [(Mo6Cl8i)X6a]2- the reaction rate increases in the series X(a) = Cl < Br < I < F. Systematic F-19 nmr downfield shifts are observed in the mixed fluoro-halogeno clusters with increasing number of X(a) and within the series Cl(a) < Br(a) < I(a). Furthermore an antipodal influence for F(a), trans-positioned to X(a) effects an additional downfield shift. The Mo-95 chemical shifts are linearly depending on the electronegativity of X(a).