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 BO−2 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 B₃O3−6 or (−B(−O⁻)−O−)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^[1] or potassium metaborate,^[2][3] in α- and β-barium metaborate Ba₃(B₃O₆)₂,^[4][5][6] and in the mixed salt potassium cadmium metaborate KCdB₃O₆.^[7] 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 BO−2 units connected by single shared-oxygen bridges; that is, ···(−B(−O⁻)−O−)_n···. Occurs in calcium metaborate CaB₂O₄ or Ca(BO₂)₂.^[8][9]

• A tridimensional network of BO₄ tetrahedral groups, as in "zinc metaborate", which is actually a mixed salt zinc metaborate oxide, with the formula (Zn²⁺)₄(O²⁻)(BO−2)₆.^[10][11]

• A tridimensional regular array of [B(−O−)₄]⁻ tetrahedra sharing oxygen vertices, as in the high-pressure and high-temperature γ form of lithium metaborate LiBO₂.^[12]

Aqueous solution

The cyclic trimer anions dissociate almost completely in aqueous solution giving mainly tetrahydroxyborate anions:^[13][14]

B₃O3−6 + 6 H₂O ${\displaystyle {\ce {<=>>}}}$ 3 [B(OH)₄]⁻

Other molecules and anions, such as B(OH)₃, [B₃O₃(OH)₄]⁻, [B₃O₃(OH)₅]²⁻, and B₄O₅(OH)2−4 are less than 5% at 26 °C.^[15][16]

In 1937, Nielsen and Ward claimed that the metaborate anion in solution has a linear symmetric structure ⁻O−B⁺−O⁻ with negative charges on the oxygens and a positive charge on the boron, or O=B⁻=O with negative charge on the boron.^[17] 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.^[18] The same situation holds for thallium metaborate TlBO₂.^[19]