A characterization of complexes in terms of the pairing of electron spins. If the energy splitting owing to the crystal field/ligand field is small, then, in accord with Hund’s rule, a high-spin complex results, as all the available orbitals are singly filled by electrons with parallel spins before the electrons pair up. If, however, the energy splitting is large, then a low-spin complex arises, as it is energetically favourable for the electrons to pair off in the orbitals of the lower-energy states before they start to occupy the higher-energy orbitals. High-spin complexes are more common than low-spin complexes. See also crystal-field theory; ligand-field theory.