Square planar

The square planar molecular geometry in chemistry describes the stereochemistry (spatial arrangement of atoms) that is adopted by certain chemical compounds. As the name suggests, molecules of this geometry have their atoms lying in a square about a central atom.

Relatively few compounds adopt this geometry, but these species are often notable and even useful, examples being the first noble gas compound XeF4 and the anticancer drug Cisplatin (PtCl2(NH3)2).

This geometry can be visualised as resulting from the removal of a pair of ligands from the z-axis of an octahedron, leaving four ligands in the x-y plane. For transition metal compounds, the orbital splitting diagram for square planar geometry can thus be derived from the octahedral diagram. The removal of the two ligands stabilizes the dz2 level, leaving the dx2-y2 level as the most destabilized. Consequenly the dx2-y2 remains unoccupied in complexes of metals with the d8 configuration. These compounds typically have 16 valence electrons (eight from ligands, eight from the metal). Examples include Vaska's complex, Zeise's salt and Crabtree's catalyst.