Crystal field theory can be used to model tetrahedral and square planar transition metal complexes in an analogous manner to the application of this theory in octahedral complexes. For example, to model the tetrahedral tetrachloronickelate(II) ion, each chloride ligand is replaced by a negative point charge, resulting in a tetrahedral crystal field. Due to the influence of this field, the dxy, dyz, and dxz orbitals are higher in energy than the dx2−y2 and dz2 orbitals. This is attributed to the stronger interaction between the tetrahedral crystal field and the dxy, dyz, and dxz orbitals. The higher-energy orbitals possess t2 symmetry and are referred to as the t2 set, while the lower-energy orbitals have e symmetry and comprise the e set. In comparison to the splitting of the d orbitals in octahedral complexes, the relative energies of the orbitals in tetrahedral complexes are inverted and the crystal field splitting energy, or Δtet, is lower. In square planar complexes such as the tetracyanonickelate(II) ion, all the ligands lie in the xy plane. Here, a square planar crystal field is obtained by replacing the cyanide ligands with negative point charges. Under the influence of this field, the d orbitals of the metal ion are split into four different energy levels. Here, the dx2−y2 orbital is the highest-energy orbital and has lobes pointing directly at the ligand charges. The dxy orbital is next in energy with lobes lying in the same plane as the ligand charges. The dz2 orbital is yet lower in energy, attributed to a small overlap between the dz2 orbital and the crystal field in the xy plane. The lowest energy set of orbitals, dxz and dyz, have relatively minimal interaction with the crystal field. The crystal field splitting energy in square planar complexes, or Δsp, is defined as the energy difference between the highest-energy orbital, dx2−y2, and the lowest-energy orbitals, dyz and dxz. Assuming the same metal ion and ligand molecules for all complexes, the ratio of Δtet, Δsp, and Δoct is 0.44:1.7:1.