STRUCTURAL STUDY OF THE CONFORMATIONAL ISOMERS OF (ETA(4)-BENZYLIDENEACETONE)FE(CO)(2)L AND (ETA(4)-DIENE)FE(CO)(2)L COMPLEXES BY INVERSE TRIPLE-RESONANCE FE-57 NMR

For the first time, 2D(P-31, Fe-57){H-1}-triple-resonance NMR spectros copy was used at low temperatures (213 and 179 K) to study the conform ers of (eta(4)-benzylideneacetone)- and (eta(4)-diene)Fe-0(CO)(2)L com plexes (L = phosphanes, phosphites). In some cases not only the Fe-57 chemical shifts and the (1)J(Fe-57, P-31) coupling constants of the ma jor isomer but also those of a less populated isomer were determined, Based on earlier P-31 and C-13 NMR and molecular orbital studies of th e complexes, the trends of the Fe-57 chemical shifts are discussed in terms of the paramagnetic shielding constant, For both series investig ated the apical conformers (compared with the basal) exhibit the more shielded iran nuclei and the larger Fe-57, P-31 coupling constants ind icating stronger coordination of the phosphorus ligands in the apical position.