The geometry of a complex species can be understood from the knowledge of type of hybridization of orbitals of central atom. The hybridization of orbitals of central atom in [B(OH)₄]^–  and the geometry of the complex are respectively :

Boron has di electronic configuration:

1s²2s²2p_x​¹2p_y​⁰2p_z​⁰

In the excited state, 2s-orbital electrons are impaired and one electron is shifted to a p-orbital. Now, hybridization occurs between one s- and three p-orbitals to give sp³ hybridization and tetrahedral geometry.