Square planar

Splitting of d-orbitals

A general d-orbital splitting diagram for square planar (D_4h) transition metal complexes can be derived from the general octahedral (O_h) splitting diagram, in which the d_z and the d_x−y orbitals are degenerate and higher in energy than the degenerate set of d_xy, d_xz and d_yz orbitals. When the two axial ligands are removed to generate a square planar geometry, the d_z orbital is driven lower in energy as electron-electron repulsion with ligands on the z-axis is no longer present. However, for purely σ-donating ligands the d_z orbital is still higher in energy than the d_xy, d_xz and d_yz orbitals because of the torus shaped lobe of the d_z orbital. It bears electron density on the x- and y-axes and therefore interacts with the filled ligand orbitals. The d_xy, d_xz and d_yz orbitals are generally presented as degenerate but they have to split into two different energy levels with respect to the irreducible representations of the point group D_4h. Their relative ordering depends on the nature of the particular complex. Furthermore, the splitting of d-orbitals is perturbed by π-donating ligands in contrast to octahedral complexes. In the square planar case strongly π-donating ligands can cause the d_xz and d_yz orbitals to be higher in energy than the d_z orbital, whereas in the octahedral case π-donating ligands only affect the magnitude of the d-orbital splitting and the relative ordering of the orbitals is conserved.