 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).
