Pentamethylcyclopentadienyl anion, ligand structure

A bis(pentamethylcyclopentadienyl)element cation Cp2 E (Cp = CsMcs E = As, Sb) was prepared from Cp2 EF and BF3 as its BF4" salt. The X-ray structure analysis revealed a non-centrosymmetric E—Cp bond situation. Therefore the E—Cp bond may best be described by a. rj or rj rather than a. coordination. As expected, the Cp ligands are not in a coplanar position, they form an angle of 36.5°. The BF4" anion seems to be important in stabilizing the cation attempts with AlC ", BC , BBr4 and SbClg were not successful This may very well be explained by the exceptionally weak basicity of the BF4" anion and one could predict that AsFg and SbFg" salts should also be stable. 

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