The electronic and magnetic properties of the oxygen deficient manganite CaMnO2.5 are investigated within local spin density functional theory. A magnetization of 4μB corresponding to the expected value form Hund's first rule is computed; it agrees with experimental evidence of a high spin state of MnIII. Relatively small energy differences are found favouring the hypothetical ferromagnetic state characterized by a half-metallic behaviour over a small gap insulating antiferromagnetic ground state. This leads to suggest that both magnetic states are accessible. Spin resolved chemical bonding is proposed whereby the majority spin states are found to be more covalent than minority spin states. Relationship to colossal magnetoresistive compounds is proposed.