 As for all metals , molecular compounds and polyatomic ions of cobalt are classified as coordination complexes , that is , molecules or ions that contain cobalt linked to several ligands . The principles of electronegativity and hardness – softness of a series of ligands can be used to explain the usual oxidation state of cobalt . For example , Co + 3 complexes tend to have ammine ligands . Because phosphorus is softer than nitrogen , phosphine ligands tend to feature the softer Co2 + and Co + , an example being tris ( triphenylphosphine ) cobalt ( I ) chloride ( ( P ( C6H5 ) 3 ) 3CoCl ) . The more electronegative ( and harder ) oxide and fluoride can stabilize Co4 + and Co5 + derivatives , e.g. caesium hexafluorocobaltate ( Cs2CoF6 ) and potassium percobaltate ( K3CoO4 ) . 
