orbital on the B atom has a2’’ symmetry and no symmetry
match can be found with an LGO of the H3 fragment.
Thus, the 2pz orbital is non-bonding in BH3. The MO
approach describes the bonding in BH3 in terms of three
MOs of a1’ and e’ symmetries. The a1 orbital possesses -
bonding character which is delocalized over all four atoms.
The e’ orbitals also exhibit delocalized character, and the
bonding in BH3 is described by considering a combination
of all three bonding MOs.