The peculiar behaviors of trivalent (3d 6 ) and tetravalent (3d 5 ) cobalt ions coordinated to six oxygen atoms forming a regular or distorted octahedral site are emphasized. Low spin (LS), intermediate spin (IS), and high spin (HS) electronic con5gurations can be considered for Co 3؉ (S ؍ 0, S ؍ 1, and S ؍ 2) and for Co 4؉ (S ؍ 1/2, S ؍ 3/2, and S ؍ 5/2) depending on the relative magnitude of electronic correlations and crystal 5eld parameters. A simple model including these e4ects accounts for spin crossover and has led to the derivation of phase diagrams showing the respective stability range of LS, IS, and HS states. Extended HuK ckel tight-binding and density functional theory+full-potential linearized augmented plane wave calculations were carried out for TlSr 2 CoO 5 and SrCoO 3 . Results from both techniques are compared and used to interpret the speci5c properties of these oxides, which are attributed to the peculiar character of cobalt (III) and (IV). Unlike in molecular compounds where only magnetic and optical properties can be modi5ed, spin transitions (intraatomic electron transfer) can be strongly coupled with metal+insulator transitions in ionocovalent oxides. Spin equilibrium can lead to partly occupied energy bands as nonintegral oxidation state does for mixed valence oxides, and, furthermore, spin state ordering can occur upon cooling in a way similar to charge ordering.