Modification of the Jahn-Teller theorem: Temperature-dependent rhombic distortion of the 2E1g ground state of cobaltocene inferred from susceptibility and NMR measurements.

Abstract

The Jahn-Teller theorem has been modified on the basis of statistical physics: The ensemble average of the molecular energy is supposed to be minimal with respect to distortions of the nuclear framework. The consequences of this formulation are discussed for the ${D}_{5d}$-coordinated cobaltocene having the degenerate electronic ground state $^{2}\mathrm{E}_{1\mathrm{g}}$. At low temperatures T155 K, the symmetrical structure is unstable, and the strength of the rhombic distortion depends on temperature. At moderate temperatures T\ensuremath{\ge}155 K, however, the symmetrical configuration ${D}_{5d}$ is stable. The three adjustable parameters of the theory were successfully fitted to susceptibility data reported in the literature.