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

Abstract

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 ∑ σ m of the substituted Cp ring. These properties also correlate well with the Cp Θ zz values. The trend in the correlations is the more electron-rich the Cp (negative ∑ σ m and Θ zz 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 Θ zz and ∑ σ m values. However, there is a substantial increase in correlation when the sum of the absolute value of the Hammett σ m (∑| σ m|) is employed. The significantly improved correlation when the ∑| σ m| 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 σ m contains information about substituent polarizabilities, at least in the case of Li +- and Na +-substituted Cp anions.