I've recently been taught that the 4f orbitals in lanthanides are "core-like", supposedly meaning they have radius smaller than the 4d orbitals, therefore they are not available on the outside of the atom to form covalent bonds with ligands. The lecturer then went on to say that, therefore, the bonding of lanthanides to ligands is entirely ionic.
It is my understanding that a bond is deemed ionic when there is a large electronegativity difference between the two atoms, i.e. the bonding molecular orbital is near-identical in energy to the atomic orbital of the more electronegative atom, as shown below for sodium fluoride:
However, if this is the case, how can lanthanides form any bonds if their valence orbitals are not available? Surely orbital overlap with the ligand is still required to form a molecular orbital, even if the resulting orbital is ionic in nature?