  Chemical bonding and electronic structure of MgB2, a boron-based newly
discovered superconductor, is studied using self-consistent band structure
techniques. Analysis of the transformation of the band structure for the
hypothetical series of graphite - primitive graphite - primitive graphite-like
boron - intercalated boron, shows that the band structure of MgB2 is
graphite-like, with pi-bands falling deeper than in ordinary graphite. These
bands possess a typically delocalized and metallic, as opposed to covalent,
character. The in-plane sigma-bands retain their 2D covalent character, but
exhibit a metallic hole-type conductivity. The coexistence of 2D covalent
in-plane and 3D metallic-type interlayer conducting bands is a peculiar feature
of MgB2. We analyze the 2D and 3D features of the band structure of MgB2 and
related compounds, and their contributions to conductivity.
