π-donor or σ-donor ligands with small interactions with the metal orbitals lead to a weak ligand field which increases the energies of t orbitals. These molecular orbitals become non-bonding or weakly anti-bonding orbitals (small Δ). Therefore, addition or removal of electron has little effect on complex stability. In this case, there is no restriction on the number of d-electrons and complexes with 12–22 electrons are possible. Small Δ makes filling e* possible (>18 e) and π-donor ligands can make t antibonding (). These types of ligand are located in the low-to-medium part of the spectrochemical series. For example: [TiF] (Ti(IV), d, 12 e), [Co(NH)] (Co(III), d, 18 e), [Cu(OH)] (Cu(II), d, 21 e).
In terms of metal ions, Δ increases down a group as well as with increasing oxidation number. Strong ligand fields lead to low-spin complexes which cause some exceptions to the 18-electron rule.