Aquo ions of divalent metal ions are less acidic than those of trivalent cations. Metal cations that bind RNA can be monovalent, divalent or trivalent. Carnosine can chelate divalent metal ions. Although usually trivalent, europium readily forms divalent compounds. Although usually trivalent, ytterbium readily forms divalent compounds. The extracellular domain is where the ligand binds through the use of divalent cations. The binding between dystroglycan and pikachurin requires divalent cations. This increases the reabsorption of divalent cations by secondary active transport. Lead(II) sulfate is insoluble in water, like the sulfates of other heavy divalent cations. It transports both monovalent and divalent cations. The divalent ligands are more strongly complexing than the monovalent ones. It has a negligible capacity to bind divalent ions. Organomercury compounds are always divalent and usually two-coordinate and linear geometry. The classical example is salcomine, the complex with divalent cobalt , usually denoted as Co(salen). It is believed to exhibit resonance between its tetravalent and divalent carbon forms.