  We present 13C NMR spin-lattice relaxation measurements (1/T1) in Na2CsC60
and Rb3C60 from 10 to 700K. The large temperature range of this measurement
allow to define unambiguously an increase of 1/T1T with increasing temperature,
which is anomalous in a simple metallic picture, where the Korringa law
predicts 1/T1T = cst. We attribute this increase to the existence of an
additional relaxation channel related to singlet-triplet (ST) excitations of
Jahn-Teller distorted C60^{2-} and C60^{4-}. These units are formed within the
metal on very short time scales (10^{-14} sec) that do not imply static charge
segregation. We show that the amplitude of the ST component depends directly on
the density of states, which indicates an interplay between metallic and
molecular excitations. Such an interaction is also revealed by the high
temperature behavior of Na2CsC60 and CsC60, that we then discuss. A divergence
between the behaviors of 1/T1, the NMR shift and the ESR susceptibility is
interpreted as the result of a rapid increase of the lifetime of the charge
carriers, signaling a tendency to charge localization. In our analysis, the
particular stability of C60^{2n-} is then a common feature of all known
metallic fullerides and allow to reconcile apparently contradicting properties
of these systems.

Strongly Correlated Electrons