Cyclooctatetraene forms organometallic complexes with some metals, including yttrium, lanthanides, and actinides. Calixarenes also form complexes with cadmium, lead, lanthanides and actinides. Examples are the lanthanides and actinides. There are two series of rare-earth metals, the lanthanides and the actinides. The trivalent lanthanides mostly form ionic salts. The same rule also applies to "f-f" transitions in centrosymmetric complexes of lanthanides and actinides. Lanthanum is the least volatile of the lanthanides. Boron nitride reacts with nitrides of alkali metals and lanthanides to form nitridoborate compounds. Like most of the lanthanides, lanthanum has a hexagonal crystal structure at room temperature. Historically, mischmetal was prepared from monazite, an anhydrous phosphate of the light lanthanides and thorium. Europium has the second lowest melting point and the lowest density of all lanthanides. Samarium is one of the few lanthanides that exhibit the oxidation state +2. The chemical behavior of ytterbium is similar to that of the rest of the lanthanides. The coordination and organometallic chemistry of terbium is similar to other lanthanides. Steenkampskraal monazite provided a supply of the complete set of lanthanides.