  Spin-state transitions, observed in many transition metal compounds
containing Co$^{3+}$ and Fe$^{2+}$, may occur with the change of temperature,
pressure, but also with doping, in which case the competition of single-site
effects and kinetic energy of doped carriers can favor a change in the spin
state. We consider this situation in a simple model, formally resembling that
used for manganites in Phys. Rev. Lett. 95, 267210 (2005). Based on such a
model, we predict the possibility of a jump-like change in the number of
Co$^{3+}$ ions undergoing spin-state transition caused by hole doping. A
tendency to the electronic phase separation within a wide doping range is
demonstrated. Phase diagrams with the regions of phase separation are
constructed at different values of the characteristic parameters of the model.
