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 xy 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).^[1]

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.

Splitting of the energy of the d-orbitals in square planar transition metal complexes

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 lope 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_z_² 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.^[2]

References

↑ Miessler, G. L.; Tarr, D. A. Inorganic Chemistry (3rd ed.). Pearson/Prentice Hall. ISBN 0-13-035471-6.
↑ Börgel, Jonas; Campbell, Michael G.; Ritter, Tobias (2016-01-12). "Transition Metal d-Orbital Splitting Diagrams: An Updated Educational Resource for Square Planar Transition Metal Complexes". Journal of Chemical Education. 93 (1): 118–121. ISSN 0021-9584. doi:10.1021/acs.jchemed.5b00542.

External links

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

Molecular geometry
Coordination number 2	Linear Bent
Coordination number 3	Trigonal planar Trigonal pyramidal T-shaped
Coordination number 4	Tetrahedral Square planar Seesaw
Coordination number 5	Trigonal bipyramidal Square pyramidal Pentagonal planar
Coordination number 6	Octahedral Trigonal prismatic Pentagonal pyramidal Distorted octahedral
Coordination number 7	Pentagonal bipyramidal
Coordination number 8	Square antiprismatic
Coordination number 9	Tricapped trigonal prismatic Capped square antiprismatic

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