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 (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 units connected by single shared-oxygen bridges; that is, {{chem2|***(\sB(\sO−)\sO\s)_{
n}***}}. 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 The cyclic trimer anions dissociate almost completely in
aqueous solution giving mainly
tetrahydroxyborate anions: : 3 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.
Gas phase The vapor of
cesium metaborate has neutral monomers and dimers as well as ionized versions thereof. The same situation holds for
thallium metaborate . ==Solid solutions==