  We present a theoretical study of the transport characteristics of molecular
junctions, where first-row diatomic molecules are attached to (001) gold and
platinum electrodes. We find that the conductance of all of these junctions is
of the order of the conductance quantum unit $G_0$, spelling out that they
belong to the transparent regime. We further find that the transmission
coefficients show wide plateaus as a function of the energy, instead of the
usual sharp resonances that signal the molecular levels in the tunneling
regime. We use Caroli's model to show that this is a rather generic property of
the transparent regime of a junction, which is driven by a strong effective
coupling between the delocalized molecular levels and the conduction channels
at the electrodes. We analyse the transmission coefficients and chemical
bonding of gold/Benzene and gold/Benzene-dithiolate (BDT) junctions to
understand why the later show large resistances, while the former are highly
conductive.
