squareplanar

Square planar is a molecular geometry in which a central atom is bonded to four ligands arranged at the corners of a square in one plane, yielding approximately 90-degree bond angles between adjacent bonds.

The arrangement is most common for transition metal complexes of d8 metals in strong-field environments. In such complexes, dsp2 hybridization can describe the bonding, forming four in-plane bonds. The dx2−y2 orbital is typically high in energy, and electrons pair in lower-energy orbitals to stabilize the planar arrangement. The result is a characteristic, two-dimensional coordination sphere that can influence reactivity and electronic properties.

Common metals and ligands. Typical metal centers include platinum(II), palladium(II), nickel(II), as well as copper(I) and gold(I in suitable environments). Ligands are often strong-field donors such as phosphines, amines, halides, cyanide, or carbon monoxide. A well-known example is cisplatin, [PtCl2(NH3)2], whose square-planar geometry is essential to its biological activity and mechanism of action. Square planar complexes frequently arise when ligands promote low-spin, compact bonding arrangements and when steric demands permit a planar arrangement around the metal.

Limitations and variations. While square planar geometry is common, it can distort or revert to other geometries (for example, tetrahedral or square pyramidal) if ligands are bulky, if the field is weaker, or if the metal center adopts a different electronic configuration. Interconversion among geometries can occur with changes in oxidation state, ligand set, or coordination number.

In coordination chemistry and organometallics, square planar complexes are important for catalysis, electrocatalysis, and medicinal chemistry, offering predictable ligand orientation and electronic structure that influence reactivity and selectivity.