  Within the two-band model of superconductivity, we study the dependence of
the critical temperature Tc and of the isotope exponent alpha in the proximity
to an electronic topological transition (ETT). The ETT is associated with a
3D-2D crossover of the Fermi surface of one of the two bands: the sigma subband
of the diborides. Our results agree with the observed dependence of Tc on Mg
content in A_{1-x}Mg_xB_2 (A=Al or Sc), where an enhancement of Tc can be
interpreted as due to the proximity to a "shape resonance". Moreover we have
calculated a possible variation of the isotope effect on the superconducting
critical temperature by tuning the chemical potential.
