Square planar molecular geometry

Idealized structure of a compound with square planar coordination geometry.

Structure of cisplatin, an example of a molecule with the square planar coordination geometry.

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 positioned at the corners of a square on the same plane about a central atom.

Relationship to other geometries

XeF₂, with a linear geometry, which can gain two fluorine atoms to form...

XeF₄, with a square planar geometry, which can gain two fluorine atoms to form...

XeF₆, with a distorted octahedral geometry.

Linear

The addition of two ligands to linear compounds, ML₂, can afford square planar complexes. For example, XeF₂ adds fluorine to give square planar XeF₄.

Tetrahedral molecular geometry

In principle, square planar geometry can be achieved by flattening a tetrahedron. As such, the interconversion of tetrahedral and square planar geometries provides an intramolecular pathway for the isomerization of tetrahedral compounds. This pathway does not operate readily for hydrocarbons, but tetrahedral nickel(II) complexes, e.g. NiBr₂(PPh₃)₂, undergo this change reversibly.

Octahedral geometry

Square planar geometry can also be achieved by 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 crystal field splitting diagram for square planar geometry can thus be derived from the octahedral diagram. The removal of the two ligands stabilizes the d_z² level, leaving the d_x²-y² level as the most destabilized. Consequently the d_x²-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

Numerous compounds adopt this geometry, examples being especially numerous for transition metal complexes. The noble gas compound XeF₄ 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.

See also

• AXE method
• Molecular geometry

External links

• 3D Chem - Chemistry, Structures, and 3D Molecules
• IUMSC - Indiana University Molecular Structure Center
• Interactive molecular examples for point groups
• [1] - Coordination numbers and complex ions