PREPARATION OF F-19 NMR SPECTROSCOPIC EVI DENCE AND STUDY OF THE FORMATION OF METAL-MIXED CLUSTER ANIONS [(MO6-NWNCL8(1))F6(A)]2-, N=0-6

The complete system of metal-mixed octahedral cluster ions [(Mo6-nWnCl 8i)F6a]2-, n = 0 - 6, is prepared by tempering Mo powder with WCl6 at 600-degrees-C. A mixture containing inclusively the geometric isomers (n = 2, 3, 4) all ten possible species is transferred into the tetra-n -butylammonium salts (TBA)2[(Mo6-nWnCl8i)F6a]. In the F-19 nmr spectru m the 24 expected signals are observed, assigned on the basis of their chemical shifts, multiplicities and intensities, and confirmed by a 2 D-F-19-F-19 COSY spectrum. From the integrated intensities the distrib ution of the different components is derived revealing a non-statistic al formation, in that isomers with Mo ... Mo or W ... W atoms in trans -positions in comparison to those with mixed Mo ... W axes are favoure d, and that especially the homoleptic compounds Mo6 and W6 are present to an over-average extent. Evaluation of F-19 chemical shifts reveals that F bound to W which is in antipodal position to Mo resonates at h igher field compared to F bound to W in a W ... W arrangement, caused by an increased shielding, which is synonymous to a positive antipodal -effect by Mo. Vice versa F bound to Mo with an antipodal W resonates at lower field compared with F bound to Mo in an Mo ... Mo arrangement caused by an increased deshielding and synonymous a negative antipoda l-effect by W. The chemical shifts, resulting from antipodal-effects, are different for the compounds within the [(Mo6-nWnCl8i)F6a]2--system . The difference of the antipodal effects of successive substitution p roducts results in characteristic values designated as antipodal shift constants, depending on the kind of substituents, which is valid for other cluster systems, too.