Square planar molecular geometry

Numerous compounds adopt this geometry, examples being especially numerous for transition metal complexes. The noble gas compound xenon tetrafluoride adopts this structure as predicted by VSEPR theory. The geometry is prevalent for transition metal complexes with d⁸ configuration, which includes Rh(I), Ir(I), Pd(II), Pt(II), and Au(III). Notable examples include the anticancer drugs cisplatin, [PtCl₂(NH₃)₂], and carboplatin. Many homogeneous catalysts are square planar in their resting state, such as Wilkinson's catalyst and Crabtree's catalyst. Other examples include Vaska's complex and Zeise's salt. Certain ligands (such as porphyrins) stabilize this geometry.

Representative d-orbital splitting diagrams for square planar complexes featuring σ-donor (left) and σ+π-donor (right) ligands.

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_z² and the d_x²−y² 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_z² 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_z² 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_z² 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_z² 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.[1]

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