  X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 and
(Pr,Y)0.55Ca0.45CoO3 compositions reveal that the valence of praseodymium ions
is stable and essentially +3 (Pr [4f 2]) in the metallic state, but abruptly
changes when carriers localize approaching the oxidation state +4 (Pr [4f 1]).
This mechanism appears to be the driving force of the metal-insulator
transition. The ground insulating state of Pr0.5Ca0.5CoO3 is an homogeneous
Co3.5-d state stabilized by a charge transfer from Pr to Co sites: 1/2Pr3+ +
Co3.5 \to 1/2Pr3+2d + Co3.5-d, with 2d \approx 0.26 e-.
