  The low-temperature transport properties of a molecule are studied in the
field-effect transitor geometry. The molecule has an internal mechanical mode
that modulates its electronic levels and renormalizes both the interactions and
the coupling to the electrodes. For a soft mechanical mode the spin
fluctuations in the molecule are dominated by the bare couplings while the
valence changes are determined by the dressed energies. In this case, the
transport properties present an anomalous behavior and the Kondo temperature
has a weak gate voltage dependence. These observations are in agreement with
recent experimental data.
