Metaborate
A metaborate is a borate anion consisting of boron and oxygen, with empirical formula. Metaborate also refers to any salt or ester of such anion. Metaborate is one of the boron's oxyanions. Metaborates can be monomeric, oligomeric or polymeric.
In aqueous solutions metaborate anion hydrolyzes to tetrahydroxyborate. For this reason, solutions or hydrated salts of the latter are often improperly named "metaborates".
Structure
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 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 or, 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, and in the mixed salt potassium cadmium metaborate. The three B–O distances are nearly equal in the potassium salt but significantly different in the sodium one.
- A polymer of units connected by single shared-oxygen bridges; that is,. Occurs in calcium metaborate or.
- A tridimensional network of tetrahedral groups, as in "zinc metaborate", which is actually a mixed salt zinc metaborate oxide, with the formula.
- A tridimensional regular array of tetrahedra sharing oxygen vertices, as in the high-pressure and high-temperature γ form of lithium metaborate.
Aqueous solution
Other molecules and anions, such as,,, and are less than 5% at 26 °C.
In 1937, Nielsen and Ward claimed that the metaborate anion in solution has a linear symmetric structure with negative charges on the oxygens and a positive charge on the boron, or with negative charge on the boron. However, this claim has been disproved.