Lithium and sodium cation binding of cyclopentadienyl anions: Electronic effects of cyclopentadienyl substitution

The Li+ and Na+ binding of substituted cyclopentadienyl (Cp) anions were investigated using computational techniques. The ring centroid-metal distances and the binding energies of the Cp-metal complexes correlate very well with the ∑σm of the substituted Cp ring. These properties also correlate well with the Cp Θzz values. The trend in the correlations is the more electron-rich the Cp (negative ∑σm and Θzz values values), the shorter the Cp-metal bond and the stronger the binding energy. The NBO metal charges correlate, though not very well in either case, with the Cp Θzz and ∑σm values. However, there is a substantial increase in correlation when the sum of the absolute value of the Hammett σm (∑|σm|) is employed. The significantly improved correlation when the ∑|σm| values are employed leads us to propose a model for substituted Cp charge transfer upon Li+ or Na+ complexation, and it also informs us that the Hammett substituent constant σm contains information about substituent polarizabilities, at least in the case of Li+- and Na+-substituted Cp anions.

Cp–Li and Cp–Na complexes involving substituted Cp rings were investigated using high-level ab initio molecular orbital theory. The Cp-metal distances and binding energies correlate very well with the Cp ∑σm values. Furthermore, the metal charge correlates very well with the Cp ∑|σm| values.Figure optionsDownload as PowerPoint slide