Atoms that have 3 bonds, 2 bonds and 1 lone pair, or 1 bond and 2 lone pairs need 3 orbitals that are 120 degrees apart. Consider the plane of these three orbitals to be the xy plane. Combining an s orbital with a p_{x} orbital and a p_{y} orbital makes three sp^{2} hybrid orbitals.

Borane Molecular Orbitals

The boron atom make three bonds to hydrogen and has no lone pairs of electrons. The molecule is trigonal planar.

Molecules that have 3 bonds, 2 bonds and 1 lone pair, or 1 bond and 2 lone pairs need 3 orbitals that are 120 degrees apart. Consider the plane of these three orbitals to be the xy plane. Combining an s orbital with a p_{x} orbital and a p_{y} orbital makes three sp^{2} hybrid orbitals. Hydrogen uses its 1s orbital to make bonds.

Combine each of the H(1s) orbitals with one of the B(2sp^{2}) orbitals to make a sigma bonding and a sigma antibinding molecular orbital.

There are no remaining hybrid orbitals.

There is one p orbital on boron but there is no adjacent atom with another p orbital.
Add it to the molecular orbital diagram as a non-bonding molecular orbital.

There are a total of 6 electrons to add to the molecular orbital diagram, 3 from boron and 1 from each hydrogen atom.

sp Hybrid Orbitals in BeH_{2}

The Lewis structure shows that the beryllium in BeH_{2} makes 2 bonds and has no lone pairs. It is a linear molecule.

The beryllium atom needs 2 orbitals to form sigma bonds that can overlap with the two hydrogen 1s orbitals. If we consider the bond to be along the z axis, the beryllium atom will combine its 2s and 2p_{z} orbitals to make 2 hybrid orbitals. Each one points towards one of the hydrogen orbitals along the z axis.

Combine each of the H(1s) orbitals with one of the Be(2sp) orbitals to make a sigma bonding and a sigma antibonding molecular orbital.

There are no remaining hybrid orbitals.

There are 2 remaining p orbitals on beryllium but there are no adjacent atoms with another p orbital. Add them to the molecular orbital diagram as non-bonding molecular orbitals.

There are a total of 4 electrons to add to the molecular orbital diagram, 2 from beryllium and 1 from each hydrogen atom.

Professor Patricia Shapley, University of Illinois, 2011