  Bulk samples of the pyrite chalcogenide solid solutions Co$_{1-x}$Fe$_x$S$_2$
0 <= x <= 0.5, have been prepared and their crystal structures and magnetic
properties studied by X-ray diffraction and SQUID magnetization measurements.
Across the solution series, the distance between sulfur atoms in the persulfide
(S$_2^{2-}$) unit remains nearly constant. First principles electronic
structure calculations using experimental crystal structures as inputs point to
the importance of this constant S-S distance, in helping antibonding S-S levels
pin the Fermi energy. In contrast hypothetical rock-salt CoS is not a good half
metal, despite being nearly isostructural and isoelectronic. We use our
understanding of the Co$_{1-x}$Fe$_x$S$_2$ system to make some prescriptions
for new ferromagnetic half-metals.
