Anti-Bonding Orbitals.... help?

angeldelight

Ok well as the title suggests, I'm having real trouble grasping/understanding the concept of anti-bonding orbitals. Taking a compound with a carbonyl group for an example, I get that there's a sigma bond formed, and overlap of the 2 Pz orbitals to give a pi bond. Where does the anti-bonding orbital come into it??

In the lecture notes it says the pi orbital results from maximum positive overlap and the pi* from maximum negative overlap but I'm completely lost!

I'd appreciate any help, I tried google but couldn't come across anything where it was explained easily. Nothing in my chemistry book either

ArthurDent

Cotton and Wilkinson- Basic Inorganic Chem is pretting good for explaining the basics of molecular orbital theory and the diagrams are pretty self explanitary

Its all based around ideas of energy levels and electrons try and occupy the lowest available energy level - this means when two atoms bond - as in the case of the carbonyl bond some of the bonds are of a lower energy state that others - thus you have bonding orbitals and antibonding orbitals.

you have to also remember it is only one theory to explain bonds you also have valence bond theory and hybridization theory and to get a clearer view of all of this it is sometimes best to take one nolecue and look how each of these theories explain the obrserved properties of that mol

Bond length, spectrum, etc

HTH

graphene

In order to reduce energy (the key factor in bonding) two similar orbitals will (at least in theory) combine to give both a bonding and an anti-bonding hybrid orbital, the bonding having energy lower than the level of the two orbitals that will combine, the anti-bonding will be higher in energy than the two combining orbitals


This will apply for all sets of orbitals (sigma, pi, delta)

in the case of a carbonyl pi bond there is one electron from C and one from O in this orbital for C=O bonding, as such in the hybrid energy setup we get two electrons in the bonding and none in the anti-bonding (i.e. a bond)

Generally if bonds are present then the anti bonding orbital is vacant

if however another source of electrons were to be added to the carbonyl (eg a base) these electrons would go into the anti bonding orbital, canceling the lowering of energy by hybridisation and the bond will weaken and break

Zaphod

http://www.chem.queensu.ca/people/faculty/mombourquette/FirstYrChem/Molecular/orbitals/