For asymmetric hydrocyanation, popular chiral ligands are chelating aryl diphosphite complexes. Hyponitrite can act as a bridging or chelating bidentate ligand. These ligands are called chelants, chelators, chelating agents, or sequestering agents. Protein samples usually contain salts, solvents, buffers, preservatives, reducing agents and metal chelating agents. Polonium also forms soluble compounds with some chelating agents, such as 2,3-butanediol and thiourea. They are used as ligands for chelating metals. The bond is either an ester, chelating, or bidentate bridging linkage. Polydentate ligands are chelating agents and classified by their denticity. Builders (also called chelating or sequestering agents) are water softeners. Modern catalysts rely increasingly on chelating ligands, especially diphosphites. In addition to chelating agents, humic acids can be added to increase nutrient uptake. It also dissolves well in solutions of chelating agents such as EDTA and oxalic acid. Sodium polyacrylate is commonly used in detergents as a chelating agent. Many microbial species produce water-soluble pigments that serve as chelating agents, termed siderophores. It is a complex of 4 Mg ions chelating six anthocyanin molecules.