  We review some of the relevant physical properties of the transition metal
mono-silicides with the FeSi structure (CrSi, MnSi, FeSi, CoSi, NiSi, etc) and
explore the relation between their structural characteristics and the
electronic properties. We confirm the suggestion orginally made by Pauling,
that the FeSi structure supports two quasi-atomic d-states at the transition
metal atom. This shell contains from 0 to 4 electrons in the sequence CrSi to
NiSi. In FeSi the two quasi-atomic d-electrons are responsible for the high
temperature S=1 state, which is compensated for T=0 by two itinerant electrons
associated with the Fe-Si resonance bonds.
