Editing Hydroxide (section)

==Structural chemistry==
The hydroxide ion appears to rotate freely in crystals of the heavier alkali metal hydroxides at higher temperatures so as to present itself as a spherical ion, with an effective [[ionic radius]] of about 153&nbsp;pm.<ref name=wells548/> Thus, the high-temperature forms of KOH and NaOH have the [[sodium chloride#Crystal structure|sodium chloride]] structure,<ref>Victoria M. Nield, David A. Keen [https://books.google.com/books?id=QDT6VQ4GRL8C&pg=PA276 Diffuse neutron scattering from crystalline materials], Oxford University Press, 2001 {{ISBN|0-19-851790-4}}, p.&nbsp;276</ref> which gradually freezes in a monoclinically distorted sodium chloride structure at temperatures below about 300&nbsp;°C. The OH groups still rotate even at room temperature around their symmetry axes and, therefore, cannot be detected by [[X-ray diffraction]].<ref>{{cite journal|last1=Jacobs|first1=H.|last2=Kockelkorn|first2=J.|last3=Tacke|first3=Th.|title=Hydroxide des Natriums, Kaliums und Rubidiums: Einkristallzüchtung und röntgenographische Strukturbestimmung an der bei Raumtemperatur stabilen Modifikation|journal=Zeitschrift für Anorganische und Allgemeine Chemie|volume=531|pages=119|year=1985|doi=10.1002/zaac.19855311217|issue=12}}</ref> The room-temperature form of NaOH has the [[thallium(I) iodide|thallium iodide]] structure. LiOH, however, has a layered structure, made up of tetrahedral Li(OH)<sub>4</sub> and (OH)Li<sub>4</sub> units.<ref name=wells548>Wells, p.&nbsp;548</ref> This is consistent with the weakly basic character of LiOH in solution, indicating that the Li–OH bond has much covalent character.

The hydroxide ion displays cylindrical symmetry in hydroxides of divalent metals Ca, Cd, Mn, Fe, and Co. For example, magnesium hydroxide Mg(OH)<sub>2</sub> ([[brucite]]) crystallizes with the [[cadmium iodide]] layer structure, with a kind of close-packing of magnesium and hydroxide ions.<ref name=wells548/><ref>{{cite journal|last1=Enoki|first1=Toshiaki|last2=Tsujikawa|first2=Ikuji|title=Magnetic Behaviours of a Random Magnet, Ni<sub>''p''</sub>Mg<sub>1−''p''</sub>(OH)<sub>2</sub>|journal=Journal of the Physical Society of Japan|volume=39|pages=317|year=1975|doi=10.1143/JPSJ.39.317|issue=2|bibcode= 1975JPSJ...39..317E}}</ref>

The [[amphoterism|amphoteric]] hydroxide Al(OH)<sub>3</sub> has four major crystalline forms: [[gibbsite]] (most stable), [[bayerite]], [[nordstrandite]], and [[doyleite]].<ref group=note>Crystal structures are illustrated at Web mineral: [http://webmineral.com/data/Gibbsite.shtml Gibbsite], [http://webmineral.com/data/Bayerite.shtml Bayerite], [http://webmineral.com/data/Nordstrandite.shtml Norstrandite] and [http://webmineral.com/data/Doyleite.shtml Doyleite]</ref>
All these [[Polymorphism (materials science)|polymorphs]] are built up of double layers of hydroxide ions—the aluminium atoms on two-thirds of the octahedral holes between the two layers—and differ only in the stacking sequence of the layers.<ref>Athanasios K. Karamalidis, David A. Dzombak [https://books.google.com/books?id=XULsOFSipsgC&pg=PA15 Surface Complexation Modeling: Gibbsite], John Wiley and Sons, 2010 {{ISBN|0-470-58768-7}} pp. 15 ff</ref> The structures are similar to the brucite structure. However, whereas the brucite structure can be described as a close-packed structure, in gibbsite the OH groups on the underside of one layer rest on the groups of the layer below. This arrangement led to the suggestion that there are directional bonds between OH groups in adjacent layers.<ref>{{cite journal|last=Bernal|first=J.D.|author2=Megaw, H.D.|year=1935|title=The Function of Hydrogen in Intermolecular Forces|journal=Proc. R. Soc. A|volume=151|pages=384–420|doi=10.1098/rspa.1935.0157|issue=873|bibcode= 1935RSPSA.151..384B|doi-access=free}}</ref> This is an unusual form of [[hydrogen bond]]ing since the two hydroxide ions involved would be expected to point away from each other. The hydrogen atoms have been located by [[neutron diffraction]] experiments on α-AlO(OH) ([[diaspore]]). The O–H–O distance is very short, at 265&nbsp;pm; the hydrogen is not equidistant between the oxygen atoms and the short OH bond makes an angle of 12° with the O–O line.<ref>Wells, p.&nbsp;557</ref> A similar type of hydrogen bond has been proposed for other amphoteric hydroxides, including Be(OH)<sub>2</sub>, Zn(OH)<sub>2</sub>, and Fe(OH)<sub>3</sub>.<ref name=wells548/>

A number of mixed hydroxides are known with stoichiometry A<sub>3</sub>M<sup>III</sup>(OH)<sub>6</sub>, A<sub>2</sub>M<sup>IV</sup>(OH)<sub>6</sub>, and AM<sup>V</sup>(OH)<sub>6</sub>. As the formula suggests these substances contain M(OH)<sub>6</sub> octahedral structural units.<ref>Wells, p.&nbsp;555</ref> [[Layered double hydroxides]] may be represented by the formula {{chem|[M|1−''x''|''z''+|M|''x''|3+|(OH)|2|]<sup>''q''+</sup>(X<sup>''n''−</sup>)|{{frac|''q''|''n''}}|·''y''H|2|O}}. Most commonly, ''z''&nbsp;=&nbsp;2, and M<sup>2+</sup> = Ca<sup>2+</sup>, Mg<sup>2+</sup>, Mn<sup>2+</sup>, Fe<sup>2+</sup>, Co<sup>2+</sup>, Ni<sup>2+</sup>, Cu<sup>2+</sup>, or Zn<sup>2+</sup>; hence ''q''&nbsp;=&nbsp;''x''.