 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 <unk> 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 ) <unk> ) . The more electronegative ( and harder ) oxide and fluoride can stabilize <unk> + and <unk> + derivatives , e.g. caesium <unk> ( <unk> ) and potassium <unk> ( <unk> ) . 
