[ultrashogun]

In my book(Clayden), it explicitly tells me that B and N can never have more than 4 bonds because they only have the orbitals with n = 2 and n = 1 occupied. The book does give examples in which Al and P have more than 4 bonds.

Im just wondering how these kinds of compounds react in generalized reactions with electrophiles and nucleophiles.

If Al has four bonds will the nucleophilic electrons come from one of those bonds, or will they come from the Al atom itself?

If P has four bonds, will a nucleophile attack the P atom and form another bond, or will it attack one of the ligands by means of the sigma antibonding orbital?

In both cases I would say that extra bonds will be added to Al and P, but Im just a bit confused because I dont know for sure which is the HOMO and which the LUMO.