In my organometallic chem book it says:
The HOMO of each fragment, M and L, is usually closest in energy to the LUMO of the partner fragment than to any other vacant orbital of the partner.
This doesn't make sense to me. I'll use W(CO)6
as an example. Carbon and oxygen have atomic numbers of 6 and 8 whereas tungsten has an atomic number of 74. I haven't got my head around the whole concept of MOs yet but the highest occupied atomic orbital of tungsten is a 5d orbital while the lowest unoccupied atomic orbitals of carbon and oxygen are 2s orbitals. I don't understand how the hell the carbonyl ligand could have a LUMO that is closer in energy to tungstens HOMO than one of the higher unoccupied molecular orbitals that CO has.