Bonding and antibonding states of hybridized molecules

[Benoit]

Hey there,

With covalent bonds, we have bonding and antibonding states. If we now have, let's say sp or sp² states, doesn't matter, is there an equivalent bonding or antibonding state related to this sp bond ? I mean, why sp states wouldn't have antibonding states like every normal covalent bond ?

 

[DrDu]

Hybrid orbitals are neither bonding nor anti-bonding. Thats rather a concept from molecular orbital theory which refers to molecular orbitals, not atomic one.s

 

[Benoit]

Ok, but why can't wheuse molecular orbital theory with hybrid orbitals, I don't get why the concept is not the same. Does the molecular orbital theory fails to explain hybrization ?

 

[DrDu]

You can construct both bonding and anti-bonding molecular orbitals also from hybrid orbitals. In MO theory, the energy of the molecule will not depend on whether you start from unhybridized atomic or hybridized atomic orbitals. In Valence Bond theory, in contrast to MO theory, the bond energy depends on the localisation of the electrons, and sometimes you can achieve more localisation using hybrid orbitals.
Btw. hybridisation is not some observable property of the atoms in a molecule which we have to explain, but a concept we use to explain the bonding in molecules and often, alternative assumptions about hybridisation lead to equivalent results.