Diastereomeric pentacoordinate hypervalent stiboranes with an Sb−Fe bond {4a and 4b:
RfRfm*Sb*FeCp(CO)2 {Rf = o-C6H4C(CF3)2O-, Rfm* = o-C6H4C*(CF3)(Me)O-} were synthesized by the reaction of stiboranide anion, RfRfm*Sb*-Li+ (3-Li), with CpFeI(CO)2 in the
presence of AgBF4. The carbonyl group of 4 was replaced with triphenylphosphine by
irradiation with a tungten lamp to give a mixture of four diastereomers {5a−5d: RfRfm*Sb*Fe*Cp(CO)(PPh3)}. Each of the diastereomers was separated by TLC, and the relative stereochemistry was determined by X-ray crystallographic analysis. The thermal equilibration
from the pure diastereomer of 5 indicated that the isomerization took place through inversion
(pseudorotation) at the central antimony atom. The pseudorotational barriers of 5 were much
higher than those of Rf2Sb*Cl and RfRfm*Sb*(p-CH3C6H4). These results are consistent with
the electron-donating properties of the group 8 transition metal fragment. Hypervalent
stiboranes {6, Rf2Sb*Fe*Cp(CO)(PMe3); 7, Rf2Sb*Fe*Cp(CO)(PEt3)} were also prepared by
similar procedures. The order of pseudorotational barriers [2 {Rf2Sb*Fe*Cp(CO)(PPh3)} (32.8,
33.2 kcal/mol) > 7 (32.5, 32.9 kcal/mol) > 6 (32.2, 32.7 kcal/mol)] suggests that the steric
effect of the iron ligand also played a role. The pseudorotational barriers of the corresponding
ruthenium compounds, RfRfm*Sb*RuCp(CO)2 (12a and 12b), were slightly higher than those
of the corresponding iron compounds (4a and 4b).