  We have performed systematic first-principles calculations on di-carbide,
-nitride, -oxide and -boride of platinum and osmium with the fluorite
structure. It is found that only PtN$_{2}$, OsN$_{2}$ and OsO$_{2}$ are
mechanically stable. In particular OsN$_{2}$ has the highest bulk modulus of
360.7 GPa. Both the band structure and density of states show that the new
phase of OsN$_{2}$ is metallic. The high bulk modulus is owing to the strong
covalent bonding between Os 5\textit{d} and N 2\textit{p} states and the dense
packed fluorite structure.
