Metaborate

A metaborate is a borate anion consisting of boron and oxygen, with empirical formula BO2−. Metaborate also refers to any salt or ester of such anion (e.g. salts such as sodium metaborate NaBO2 or calcium metaborate Ca(BO2)2, and esters such as methyl metaborate CH3BO2). Metaborate is one of the boron's oxyanions. Metaborates can be monomeric, oligomeric or polymeric.

In aqueous solutions metaborate anion hydrolyzes to tetrahydroxyborate [B(OH)4]−. For this reason, solutions or hydrated salts of the latter are often improperly named "metaborates".

Solid state
In the solid state of their salts, metaborate ions are often oligomeric or polymeric, conceptually resulting from the fusion of two or more BO2− through shared oxygen atoms. In these anions, the boron atom forms covalent bonds with either three or four oxygen atoms. Some of the structures are:
 * A trimer with formula B3O6(3-) or (\sB(\sO−)\sO\s)3, consisting of a six-membered ring of alternating boron and oxygen atoms with one extra oxygen atom attached to each boron atom. This form is found, for example, in some anhydrous alkali metal salts like sodium metaborate or potassium metaborate, in α- and β-barium metaborate Ba3(B3O6)2,   and in the mixed salt potassium cadmium metaborate KCdB3O6. The three B–O distances are nearly equal in the potassium salt (133.1, 139.8, and 139.8 pm) but significantly different in the sodium one (128.0, 143.3, and 143.3 pm).


 * A polymer of BO2− units connected by single shared-oxygen bridges; that is, ***(\sB(\sO−)\sO\s)_{n}***. Occurs in calcium metaborate CaB2O4 or Ca(BO2)2.


 * A tridimensional network of BO4 tetrahedral groups, as in "zinc metaborate", which is actually a mixed salt zinc metaborate oxide, with the formula (Zn(2+))4(O(2-))(BO2−)6.


 * A tridimensional regular array of [B(\sO\s)4]− tetrahedra sharing oxygen vertices, as in the high-pressure and high-temperature γ form of lithium metaborate LiBO2.

Aqueous solution
The cyclic trimer anions dissociate almost completely in aqueous solution giving mainly tetrahydroxyborate anions:
 * B3O6(3−) + 6 H2O 3 [B(OH)4]−

Other molecules and anions, such as B(OH)3, [B3O3(OH)4]−, [B3O3(OH)5](2−), and B4O5(OH)4(2−) are less than 5% at 26 °C.

In 1937, Nielsen and Ward claimed that the metaborate anion in solution has a linear symmetric structure −O\sB+\sO− with negative charges on the oxygens and a positive charge on the boron, or O\dB−\dO with negative charge on the boron. However, this claim has been disproved.

Gas phase
The vapor of cesium metaborate has neutral monomers CsBO2 and dimers Cs2[B2O4] as well as ionized versions thereof. The same situation holds for thallium metaborate TlBO2.

Solid solutions
In 1964 Hisatsune and Surez investigated the infrared spectrum of metaborate anions in dilute solid solutions of potassium salt in alkali halides such as potassium chloride KCl.