The ethylene and acetylene molecules

The valence electronic structure of the ethylene molecule, C2H4, can be described in terms of a set of a bonding molecular orbitals approximately localized in the CC and CH bonds 

The geometry of the molecule points to the involvement of approximately Isp hybrid orbitals of the C atoms in the description of the localized [ orbitals . The double bond 

The higher energy of the tt m.o. relative to the a framework means U.at the bond energy for a C=C is less than twice the C-C bond energy. 

One of the unitary transformations of the m.o.s which leaves the ectronic wavefunction unaltered is the combination of the tt m.o. and the a orbital localized in the CC bond, xcc- 

This leads to two equivalent molecular orbitals of mixed a, tt character, which are sometimes called banana bonds  

---

The ethylene and acetylene molecules I and the corresponding anti-bonding one, vacant,... 

---