Editing Lanthanum (section)

==Compounds==

[[Lanthanum oxide]] is a white solid that can be prepared by direct reaction of its constituent elements. Due to the large size of the {{chem|La|3+}} ion, {{chem|La|2|O|3}} adopts a hexagonal 7-coordinate structure that changes to the 6-coordinate structure of [[scandium oxide]] ({{chem|Sc|2|O|3}}) and [[yttrium(III) oxide|yttrium oxide]] ({{chem|Y|2|O|3}}) at high temperature. When it reacts with water, [[lanthanum hydroxide]] is formed:<ref name=Shkolnikov2009>{{Cite journal |first=E.V. |last=Shkolnikov |year=2009 |title=Thermodynamic characterization of the amphoterism of hydroxides and oxides of scandium subgroup elements in aqueous media |journal=Russian Journal of Applied Chemistry |volume=82 |issue=2 |pages=2098–2104 |doi=10.1134/S1070427209120040 |s2cid=93220420}}</ref> a lot of heat is evolved in the reaction and a hissing sound is heard. Lanthanum hydroxide will react with atmospheric [[carbon dioxide]] to form the basic carbonate.<ref name=Greenwood1107>{{harvp|Greenwood|Earnshaw|1984|pp=1107–1108}}</ref>

[[Lanthanum fluoride]] is insoluble in water and can be used as a [[qualitative inorganic analysis|qualitative]] test for the presence of {{chem|La|3+}}. The heavier halides are all very soluble [[deliquescent]] compounds. The anhydrous halides are produced by direct reaction of their elements, as heating the hydrates causes hydrolysis: for example, heating hydrated {{chem|La|Cl|3}} produces {{chem|La|O|Cl}}.<ref name=Greenwood1107/>

Lanthanum reacts exothermically with hydrogen to produce the dihydride {{chem|La|H|2}}, a black, [[pyrophoricity|pyrophoric]], brittle, conducting compound with the [[calcium fluoride]] structure.<ref name=Fukai>{{cite book |last=Fukai |first=Y. |year=2005 |title=The Metal-Hydrogen System, Basic Bulk Properties |edition = 2nd |publisher=Springer |isbn=978-3-540-00494-3}}</ref> This is a non-stoichiometric compound, and further absorption of hydrogen is possible, with a concomitant loss of electrical conductivity, until the more salt-like {{chem|La|H|3}} is reached. Like {{chem|La|I|2}} and {{chem|La|I}}, {{chem|La|H|2}} is probably an [[electride]] compound.<ref name=Greenwood1107/>

Due to the large ionic radius and great electropositivity of {{chem|La|3+}}, there is not much covalent contribution to its bonding and hence it has a limited [[coordination chemistry]], like yttrium and the other lanthanides.<ref name=Greenwood1108>{{harvp|Greenwood|Earnshaw|1984|pp=1108–1109}}</ref> [[Lanthanum oxalate]] does not dissolve very much in alkali-metal oxalate solutions, and {{chem|[La(acac)|3|(H|2|O)|2|]}} decomposes around 500&nbsp;°C. Oxygen is the most common [[donor atom]] in lanthanum complexes, which are mostly ionic and often have high coordination numbers over {{nobr| 6 : 8 }} is the most characteristic, forming [[square antiprism]]atic and [[snub disphenoid|dodecadeltahedral]] structures. These high-coordinate species, reaching up to coordination number&nbsp;12 with the use of [[chelating ligand]]s such as in {{chem|La|2|(S|O|4|)|3| · 9(H|2|O)}}, often have a low degree of symmetry because of stereo-chemical factors.<ref name=Greenwood1108/>

Lanthanum chemistry tends not to involve {{nobr|{{mvar|π}}-bonding}} due to the electron configuration of the element: thus its organometallic chemistry is quite limited. The best characterized organolanthanum compounds are the [[cyclopentadienyl complex]] {{chem|La|(C|5|H|5|)|3}}, which is produced by reacting anhydrous {{chem|La|Cl|3}} with {{chem|Na|C|5|H|5}} in [[tetrahydrofuran]], and its methyl-substituted derivatives.<ref name=Greenwood1110>{{harvp|Greenwood|Earnshaw|1984|p=1110}}</ref>