I used to dislike MO theory a lot because I thought it was some abstract, arbitary pile of purely theoretical nonsense but from encountering situations in which valence bond theory doesn't work, I'm coming around. One situation that I like to use MO theory to think about is backbonding in coordination complexes. If I'm not mistaken, when carbon monoxide bonds to an electron rich metal as a ligand, the C=O bond length increases indicating that the bond is weakened whereas if it bonds to a metal that is incapable of back bonding, the C=O bond length doesn't change. The only explanation I know of for this is that the electrons delocalised by the C=O ligands antibonding orbital weaken the bond. Is MO theory the only contemporary model that can explain this bond weakening caused by back bonding?