The electron domain geometry for BH3, or boron trihydride, is trigonal planar.
In BH3, the boron atom is surrounded by three hydrogen atoms and has no lone pairs of electrons. This configuration leads to three regions of electron density around the boron atom. To minimize repulsion between these electron regions, they arrange themselves as far apart as possible. The most efficient formation for three regions of electron density is a trigonal planar shape, where the bond angles between the hydrogen atoms are approximately 120 degrees.
This trigonal planar geometry is significant because it dictates the molecular shape and has implications for the chemical reactivity of the molecule. The absence of lone pairs on boron also means that there is a planar structure, making BH3 a useful compound in various chemical reactions, especially in organic synthesis.