In inorganic chemistry, an orthoborate is a polyatomic anion with formula or a salt containing the anion; such as trisodium orthoborate . It is one of several boron oxyanions, or borates.
The name is also used in organic chemistry for the trivalent functional group , or any compound (ester) that contains it, such as triethyl orthoborate .
The orthoborate ion is known in the solid state, for example, in calcium orthoborate , where it adopts a nearly trigonal planar structure. It is a structural analogue of the carbonate anion , with which it is isoelectronic. Simple bonding theories point to the trigonal planar structure. In terms of valence bond theory, the bonds are formed by using sp<sup>2</sup> hybrid orbitals on boron.
Some compounds termed orthoborates do not necessarily contain the trigonal planar ion. For example, gadolinium orthoborate contains the planar ion only a high temperatures; otherwise it contains the polyborate anion .
When orthoborate salts are dissolved in water, the anion converts mostly to boric acid and other hydrogen-containing borate anions, mainly tetrahydroxyborate . The reactions of orthoborate in solution are therefore mostly those of these compounds.
In particular, these reactions include the condensation of tetrahydroxoborate with cis-vicinal diols such as mannitol, sorbitol, glucose and glycerol, to form relatively stable anion esters. This reaction is used in analytic chemistry to determine the concentration of borate anions.