 Some ytterbium halides are used as reagents in organic synthesis . For example , ytterbium ( III ) chloride ( <unk> ) is a Lewis acid and can be used as a catalyst in the <unk> and Diels – Alder reactions . Ytterbium ( II ) iodide ( <unk> ) may be used , like samarium ( II ) iodide , as a reducing agent for coupling reactions . Ytterbium ( III ) fluoride ( <unk> ) is used as an inert and non @-@ toxic tooth filling as it continuously releases fluoride ions , which are good for dental health , and is also a good X @-@ ray contrast agent . 
