Lanthanum oxide is a white solid that can be prepared by direct reaction of its constituent elements. Due to the large size of the La3+ ion, La2O3 adopts a hexagonal 7-coordinate structure that changes to the 6-coordinate structure of scandium oxide (Sc2O3) and yttrium oxide (Y2O3) at high temperature. When it reacts with water, lanthanum hydroxide is formed:[1] 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.[2]
Lanthanum fluoride is insoluble in water and can be used as a qualitative test for the presence of La3+. 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 LaCl3 produces LaOCl.[2]
Lanthanum reacts exothermically with hydrogen to produce the dihydride LaH2, a black, pyrophoric, brittle, conducting compound with the calcium fluoride structure.[3] 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 LaH3 is reached.[2] Like LaI2 and LaI, LaH2 is probably an electride compound.[2]
Due to the large ionic radius and great electropositivity of La3+, there is not much covalent contribution to its bonding and hence it has a limited coordination chemistry, like yttrium and the other lanthanides.[4] Lanthanum oxalate does not dissolve very much in alkali-metal oxalate solutions, and [La(acac)3(H2O)2] decomposes around 500 °C. Oxygen is the most common donor atom in lanthanum complexes, which are mostly ionic and often have high coordination numbers over 6: 8 is the most characteristic, forming square antiprismatic and dodecadeltahedral structures. These high-coordinate species, reaching up to coordination number 12 with the use of chelating ligands such as in La2(SO4)3·9H2O, often have a low degree of symmetry because of stereo-chemical factors.[4]
Lanthanum chemistry tends not to involve 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 La(C5H5)3, which is produced by reacting anhydrous LaCl3 with NaC5H5 in tetrahydrofuran, and its methyl-substituted derivatives.[5]
References
edit- ^ E.V. Shkolnikov (2009). "Thermodynamic Characterization of the Amphoterism of Hydroxides and Oxides of Scandium Subgroup Elements in Aqueous Media". Russian Journal of Applied Chemistry. 82 (2): 2098–2104. doi:10.1134/S1070427209120040. S2CID 93220420.
- ^ a b c d Greenwood and Earnshaw, p. 1107–8
- ^ Fukai, Y. (2005). The Metal-Hydrogen System, Basic Bulk Properties, 2d edition. Springer. ISBN 978-3-540-00494-3.
- ^ a b Greenwood and Earnshaw, pp. 1108–9
- ^ Greenwood and Earnshaw, p. 1110