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 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 .