Superexchange couples localized moments through virtual hopping via intermediate, often nonmagnetic, ligand orbitals. Half-filled orbitals sharing an effective hopping amplitude give an antiferromagnetic scale in the repulsive one-band limit. Ligand geometry, orbital occupation and exchange within intermediate states can change the balance and allow ferromagnetic coupling. Real charge motion can remain gapped while these virtual processes couple an insulating magnet's spins.
