  Boron in MgB_2 forms layers of honeycomb lattices with magnesium as a space
filler. Band structure calculations indicate that Mg is substantially ionized,
and the bands at the Fermi level derive mainly from B orbitals. Strong bonding
with an ionic component and considerable metallic density of states yield a
sizeable electron-phonon coupling. Using the rigid atomic sphere approximation
and an analogy to Al, we estimate the coupling constant lambda to be of order
1. Together with high phonon frequencies, which we estimate via zone-center
frozen phonon calculations to be between 300 and 700 cm^-1, this produces a
high critical temperature, consistent with recent experiments reporting Tc=39 K
(J. Akimitsu et al., to be published). Thus MgB_2 can be viewed as an analog of
the long sought, but still hypothetical, superconducting metallic hydrogen.
