X-ray absorption spectroscopy measurements in Pr${}_{0.5}$Ca${}_{0.5}$CoO${}_{3}$ and (Pr,Y)${}_{0.55}$Ca${}_{0.45}$CoO${}_{3}$ compositions reveal that the valence of praseodymium ions is stable and essentially $+$3 (Pr [4${f}^{2}$]) in the metallic state, but abruptly changes when carriers localize approaching the oxidation state $+$4 (Pr [4${f}^{1}$]). This mechanism appears to be the driving force of the metal-insulator transition. The ground insulating state of Pr${}_{0.5}$Ca${}_{0.5}$CoO${}_{3}$ is a homogeneous Co${}^{3.5\ensuremath{-}\mathrm{\ensuremath{\delta}}}$ state stabilized by a charge transfer from Pr to Co sites: $\frac{1}{2}{Pr}^{3+}+{\mathrm{Co}}^{3.5}\ensuremath{\rightarrow}\frac{1}{2}{Pr}^{3+2\ensuremath{\delta}}+{\mathrm{Co}}^{3.5\ensuremath{-}\ensuremath{\delta}}$, with 2\ensuremath{\delta} \ensuremath{\approx} 0.26${e}^{\ensuremath{-}}$.