Hydroxide - Structural Chemistry

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 pm. Thus, the high-temperature forms of KOH and NaOH have the sodium chloride structure, which gradually freezes in a monocinically distorted sodium chloride structure at temperatures below about 300 °C. The OH groups still rotate even at room temperature around their symmetry axes and, therefore, cannot be detected by X-ray diffraction. The room-temperature form of NaOH has the thallium iodide structure. LiOH, however, has a layered structure, made up of tetrahedral Li(OH)₄ and (OH)Li₄ units. 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)₂ (brucite) crystallizes with the cadmium iodide layer structure, with a kind of close-packing of magnesium and hydroxide ions.

The amphoteric hydroxide Al(OH)₃ has four major crystalline forms: gibbsite (most stable), bayerite, nordstrandite and doyleite. All these 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. 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. This is an unusual form of hydrogen bonding since the two hydroxide ion involved would be expected to point away from each other. The hydrogen atoms have been located by neutron diffraction experiments on αAlO(OH) (diaspore). The O-H-O distance is very short, at 265 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. A similar type of hydrogen bond has been proposed for other amphoteric hydroxides, including Be(OH)₂, Zn(OH)₂ and Fe(OH)₃

A number of mixed hydroxides are known with stoichiometry A₃MIII(OH)₆, A₂MIV(OH)₆ and AMV(OH)₆. As the formula suggests these substances contain M(OH)₆ octahedral structural units. Layered double hydroxides may be represented by the formula q+(Xn–)_q/n·yH₂O. Most commonly, z = 2, and M2+ = Ca2+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+ or Zn2+; hence q = x.