Determination of the electronic structure, the spin density distribution, and approach to the geometric structure of substituted cobaltocenes from NMR spectroscopy in solution

Abstract

Cobaltocene (Cp 2Co), 1, the di- and octamethylated derivatives (MeCp) 2Co, 3, and (Me 4Cp) 2Co, 5, and the hitherto unknown tetramethylcobaltocenes (1,3-MeCp) 2Co, 2, and (1,2-MeCp) 2Co, 4, were investigated by 1H and 13C NMR between 197 and 386 K. In order to explain the magnitude and the anomalous temperature dependence of the signal shifts, the theoretical approach outlined in three previous papers has been applied to the cobaltocenes. To this end, the 3d 7 term scheme was evaluated by a Hamiltonian which involves the Coulomb repulsion of the d electrons and their interaction with D 5d-coordinated ligands. The ground state of this crystal field approximation transforms as the irreducible representation 2E 1g so that the molecular ground state which is modified by covalent interactions must transform as the same irreducible representation. The theory applied in this paper takes advantage of the transformation properties. The fine structure is governed by spin-orbit coupling, rhombic perturbation, and the applied magnetic field so that the NMR shifts could be related to the adjustable interaction parameters. A simultaneous fit of 538 experimental shifts yielded the fine structure parameters including the sign of the rhombic perturbation of 2–5, the spin density at the nuclei, and approach to the geometric structure. The cobaltocenes were found to have a larger spin density at the carbon atoms than vanadocene. The 1H and 13C NMR shifts of 2–5 were found to contain a dipolar contribution of typically 15–25% being mainly due to the ligand-centered contribution. The theory provides a simple explanation for the fact that a signal shift changes sign with temperature.