Borate

Borates are the name for a large number of boron-containing oxyanions. The term "borates" may also refer to tetrahedral boron anions, or more loosely to chemical compounds which contain borate anions of either description. Larger borates are composed of trigonal planar BO₃ or tetrahedral BO₄ structural units, joined together via shared oxygen atoms and may be cyclic or linear in structure. Boron most often occurs in nature as borates, such as borate minerals and borosilicates.

The simplest borate anion, the orthoborate ion, BO3−
3 is known in the solid state, for example in Ca₃(BO₃)₂. In this it adopts a near trigonal planar structure. It is a structural analogue of the carbonate anion CO2−
3, 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 sp²hybrid orbitals on boron. Some compounds termed orthoborates do not necessarily contain the trigonal planar ion, for example gadolinium orthoborate, GdBO₃ contains the polyborate B
3O9−
9 ion, whereas the high temperature form contains planar BO3−
3.

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