How are antibonding orbitals formed?
An anti-bonding orbital is formed when two atoms approach each other and the overlap between atomic orbitals results in destructive interference.
The destructive interference causes a node to form, which decreases the electron density between the two atoms. This raises both the kinetic and potential energy of any electrons in the resulting anti-bonding orbital. Because the energy of the system is higher when the atoms are near each other than when they are pulled apart, no chemical bond is formed.