  We have investigated the pressure-induced spin-state transition in Co$^{2+}$
systems in terms of a competition between the Hund's exchange energy ($J$) and
the crystal-field splitting ($\Delta_{CF}$). First, we show the universal
metastability of the low-spin state in octahedrally coordinated Co$^{2+}$
systems. Then we present the strategy to search for a Co$^{2+}$ system, for
which the mechanism of spin-state and metal-insulator transitions is governed
not by the Mott physics but by $J$ vs. $\Delta_{CF}$ physics. Using CoCl$_{2}$
as a prototypical Co$^{2+}$ system, we have demonstrated the pressure-induced
spin-state transition from high-spin to low-spin, which is accompanied with
insulator-to-metal and antiferromagnetic to half-metallic ferromagnetic
transitions. Combined with metastable character of Co$^{2+}$ and the high
compressibility nature of CoCl$_{2}$, the transition pressure as low as 27 GPa
can be identified on the basis of $J$ vs. $\Delta_{CF}$ physics.
