PMR spectra of monosubstituted cobalticinium salts

The authors carried out a comparative study of the homo- and heteroannular effect of primary, secondary, and tertiary alkyl, benzyl, and phenyl substituents on the PMR chemical shifts or the protons of cyclopentadienyl ligands in monosubstituted cobalticinium salts. The heteroannular effect of alkyl groups in the PMR spectra of alkylcobalticinium salts increases with their increasing hyperconjugative effect. As a consequence of the complex nature of the electronic effects of the alkyl substituents, the chemical shifts for the α- and β-protons of the substituted Cp ligand in the PMR spectra of alkylcobalticinium salts approach one another until they finally coalesce in the series Me, Et, i-Pr, t-Bu