Theoretical aspects of highly correlated fullerides: metal-insulator transition

We review some theoretical aspects connected with the interplay of strong electron correlations and vibron phenomena in negative fullerene ions and in solid fulleride conductors, superconductors and insulators. The first part covers molecular ions, their intra-molecular Jahn-Teller effect, Coulomb (Hund's rule) exchange interactions, molecular vibrons and multiplet splittings. The second part addresses electron propagation in molecular fullerides, with special emphasis given to trivalent cases such as K3C60 and NH3 K3C60, where metallic, superconducting and Mott insulating phases are at play. Dynamical mean field theory approaches to a simplified Hamiltonian for this system are discussed in the light of some of the observed phenomenology. It is argued in particular that NH3 K3C60 is a Mott-Jahn-Teller insulator, which under pressure turns into a strongly correlated superconductor, thus sharing some important elements with the high-Tc cuprates.