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Square planar molecular geometry
Square planar molecular geometry
Square planar molecular geometry
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Square planar molecular geometry
ExamplesXenon tetrafluoride, Potassium tetrachloroplatinate
Point groupD4h
Coordination number4
Bond angle(s)90°
μ (Polarity)0
Structure of cisplatin, an example of a molecule with the square planar coordination geometry.

In chemistry, the square planar molecular geometry 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.

Examples

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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.

Splitting of d-orbitals

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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 (D4h) transition metal complexes can be derived from the general octahedral (Oh) splitting diagram, in which the dz2 and the dx2−y2 orbitals are degenerate and higher in energy than the degenerate set of dxy, dxz and dyz orbitals. When the two axial ligands are removed to generate a square planar geometry, the dz2 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 dz2 orbital is still higher in energy than the dxy, dxz and dyz orbitals because of the torus shaped lobe of the dz2 orbital. It bears electron density on the x- and y-axes and therefore interacts with the filled ligand orbitals. The dxy, dxz and dyz 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 D4h. 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 dxz and dyz orbitals to be higher in energy than the dz2 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]

See also

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References

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Square planar molecular geometry

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from Grokipedia
Square planar molecular geometry is a common structural motif in chemistry where a central atom forms four bonds to surrounding atoms or ligands arranged at the corners of a square, all lying within a single plane, resulting in bond angles of 90° between adjacent bonds and 180° between opposite bonds. This arrangement arises from the valence shell electron pair repulsion (VSEPR) theory in main-group compounds with an octahedral electron geometry (steric number 6) featuring two lone pairs positioned trans to each other along the z-axis, as seen in xenon tetrafluoride (XeF₄), where the central xenon atom utilizes sp³d² hybridization to form the bonds. In coordination chemistry, square planar geometry is prevalent among four-coordinate transition metal complexes, particularly those with d⁸ electron configurations such as Ni(II), Pd(II), Pt(II), Rh(I), Ir(I), and Au(III), stabilized by large crystal field splitting energies (Δ) from strong-field ligands that favor low-spin states and dsp² hybridization. Notable examples include the anticancer drug cisplatin (cis-[Pt(NH₃)₂Cl₂]) and tetrachloroplatinate(II) ([PtCl₄]²⁻), where the planar structure influences reactivity, stereochemistry, and electronic properties like intense color due to d-orbital splitting patterns (with dₓ₂₋ᵧ₂ as the highest energy orbital). These complexes often exhibit nonpolar character if symmetrically substituted and play key roles in catalysis, luminescence, and bioinorganic applications, distinguishing square planar from competing tetrahedral geometries by higher ligand field stabilization.

Fundamentals

Definition

Square planar molecular geometry describes a coordination arrangement in which a central atom is bonded to four ligands that lie in a single plane, with the central atom positioned at the geometric center of the square formed by the ligands at its corners. This configuration corresponds to a coordination number of four, distinguishing it from other tetrahedral arrangements of the same . In this geometry, the ligands are equivalently positioned, resulting in an idealized symmetry classified under the D_{4h} , which includes a principal four-fold rotation axis perpendicular to the molecular plane and multiple mirror planes. Square planar geometry is particularly prevalent in coordination chemistry for complexes exhibiting d^8 configurations, where it provides electronic stability. This arrangement can be conceptually related to octahedral geometry as a derivative form achieved by vacating two trans axial positions.

Geometric Characteristics

In square planar molecular geometry, four ligands surround the central atom in a single plane, resulting in bond angles of exactly 90° between adjacent ligands. This arrangement differs from , which features bond angles of approximately 109.5°. The structure is inherently planar, with the central atom and all ligands coplanar and exhibiting no out-of-plane deviations in symmetric configurations. In ideal square planar complexes featuring identical ligands, the central atom-ligand bond lengths are equal. Deviations from this equality can arise due to differences in ligand types, often causing in-plane distortions such as elongation or shortening of specific bonds. Square planar molecules with identical ligands belong to the D4hD_{4h} point group, which includes a principal four-fold rotation axis (C4C_4) perpendicular to the molecular plane passing through the central atom, a horizontal mirror plane (σh\sigma_h) that bisects the plane of the molecule, and four two-fold rotation axes (C2C_2) in the molecular plane—two aligned with bonds to opposite ligands and two bisecting angles between adjacent ligands.

Examples

Transition Metal Complexes

Square planar geometry is a prominent structural motif in four-coordinate transition metal complexes, particularly those featuring d^8 metal centers such as Ni(II), Pd(II), and Pt(II). These complexes adopt this arrangement due to the favorable ligand field stabilization energy provided by the square planar d-orbital splitting pattern, which pairs electrons in lower-energy orbitals. This geometry is especially prevalent for second- and third-row s, where larger ionic radii and stronger metal-ligand interactions favor square planar over tetrahedral coordination compared to first-row analogs. In coordination chemistry, such complexes play key roles in , , and medicinal applications, with ligands like CN^-, Cl^-, and NH_3 commonly coordinating at 90° angles around the central ion. A classic example is the tetracyanonickelate(II) ion, [Ni(CN)_4]^{2-}, where Ni^{2+} (d^8) is coordinated by four strong-field cyanide ligands, resulting in a diamagnetic, low-spin complex with yellow coloration. This ion is typically prepared by reacting NiCl_2 with excess KCN in aqueous solution, forming K_2[Ni(CN)_4] as a stable salt. Another representative is the tetrachloropalladate(II) ion, [PdCl_4]^{2-}, featuring Pd^{2+} (d^8) bound to four chloride ligands in a square planar arrangement that renders it diamagnetic. It is synthesized by dissolving PdCl_2 in concentrated HCl, yielding salts like K_2[PdCl_4], and exhibits reactivity useful in cross-coupling reactions. The tetraammineplatinum(II) ion, [Pt(NH_3)_4]^{2+}, exemplifies third-row d^8 behavior, with Pt^{2+} coordinated equatorially by four ligands in a square planar structure. This complex, which is diamagnetic and colorless in solution, is prepared by heating K_2[PtCl_4] with aqueous , followed by precipitation as the chloride salt. It serves as a precursor for other Pt(II) derivatives, including anticancer agents like analogs. While d^8 configurations dominate square planar transition metal chemistry, notable exceptions include d^9 complexes such as [Cu(NH_3)_4]^{2+}, where Cu^{2+} adopts square planar geometry with ligands due to Jahn-Teller distortion effects in the absence of axial coordination.

Main Group and Other Cases

Square planar geometry in main group elements is relatively uncommon, as these systems typically favor tetrahedral or arrangements due to valence shell repulsion (VSEPR) preferences for AX4 or AX4E configurations. However, certain hypervalent p-block compounds adopt the square planar shape under AX4E2 VSEPR notation, where the central atom accommodates four ligands and two lone pairs in an octahedral electron geometry, with the lone pairs positioned axially to minimize repulsion./Chemical_Bonding/VSEPR_Theory/VSEPR_-_AX4E2) A prominent example is tetrafluoride (XeF4), where the central atom is bonded to four atoms in a square planar arrangement, with the two lone pairs occupying trans axial positions. This structure was confirmed through vibrational spectroscopy, revealing characteristic Raman and bands consistent with D4h symmetry. The Xe-F bond lengths are approximately 1.95 , and the molecule exhibits no dipole moment due to its symmetry. Similar geometries occur in other p-block anions, such as the tetrafluoroiodate(III) ion (IF4⁻) and the tetrachlorotellurate(IV) ion (TeCl4²⁻), both classified as AX4E2. In IF4⁻, the central iodine atom forms four equivalent I-F bonds at 90° angles within the plane, with axial lone pairs, as determined by spectroscopic and computational methods aligning with VSEPR predictions. For TeCl4²⁻, the tellurium center coordinates four chlorides in a square, with Te-Cl bond lengths around 2.45 Å, and the structure maintains planarity despite the larger size of chlorine ligands, again following AX4E2 electron arrangement. These cases highlight how expanded octets in period 4 and 5 elements enable the octahedral electron framework necessary for square planarity. In , square planar configurations are rare for main group or low-oxidation-state d-block centers like Ni(0) or Pd(0), which typically prefer tetrahedral due to d10 counts. However, bulky s can enforce planarity by steric constraints; for instance, tris(perfluoroaryl) s stabilize square planar Ni(0) complexes through strong σ-acceptor interactions, as evidenced by showing Ni-B bonds and D4h-like symmetry. Such examples underscore the role of ligand design in overriding default geometries in these systems. Overall, square planar main group cases remain exceptional compared to the prevalence in d8 complexes.

Theoretical Explanation

Crystal Field Theory

In crystal field theory, the square planar geometry arises from the interaction between a central metal and four s positioned in the xy-plane at the corners of a square, leading to a specific splitting of the five degenerate -orbitals into distinct levels. This splitting occurs because the ligands create an electrostatic field that raises the of -orbitals pointing directly toward them while lowering those oriented away. The orbital dx2−y2d_{x^2 - y^2}, with its lobes directly along the x and y axes, experiences the strongest repulsion from the ligands and thus has the highest . Next is the dxyd_{xy} orbital, whose lobes lie between the ligand axes in the xy-plane, resulting in moderate repulsion. The dz2d_{z^2} orbital, with its primary lobes along the z-axis where no ligands are present, has lower , followed by the degenerate pair dxzd_{xz} and dyzd_{yz}, which have lobes extending out of the xy-plane and thus minimal interaction with the ligands, placing them at the lowest level. The diagram for this splitting can be represented as follows, with relative normalized to the barycenter (the of the unsplit d-orbitals set at zero), where Δsp\Delta_{sp} is defined as the difference between the highest (dx2−y2d_{x^2 - y^2}) and lowest (dxz/dyzd_{xz}/d_{yz}) orbitals: dx2−y2(+0.705Δsp)dxy(+0.131Δsp)dz2(−0.246Δsp)dxz,dyz(−0.295Δsp)\begin{align*} &d_{x^2 - y^2} \quad (+0.705 \Delta_{sp}) \\ & \\ &d_{xy} \quad (+0.131 \Delta_{sp}) \\ & \\ &d_{z^2} \quad (-0.246 \Delta_{sp}) \\ & \\ &d_{xz}, d_{yz} \quad (-0.295 \Delta_{sp}) \end{align*} These relative spacings reflect the progressive decrease in ligand repulsion as orbital orientation shifts away from the xy-plane. For d⁸ ions, this splitting pattern provides significant stabilization in square planar geometry. The eight electrons fill the lower four orbitals completely—four in the degenerate dxz/dyzd_{xz}/d_{yz} pair, two in dz2d_{z^2}, and two in dxyd_{xy}—leaving the high-energy dx2−y2d_{x^2 - y^2} orbital empty. Since Δsp\Delta_{sp} is typically large (approximately 1.7 times larger than the tetrahedral splitting and comparable to or larger than the octahedral Δo\Delta_o), pairing energy is overcome, resulting in a low-spin, diamagnetic configuration with all electrons paired. This electronic arrangement is observed in complexes such as [PtCl₄]²⁻.

Factors Determining Geometry

Square planar geometry is predominantly favored in four-coordinate transition metal complexes with a d⁸ electron configuration, particularly when paired with strong-field ligands that promote low-spin states and maximize crystal field stabilization energy (CFSE). In such systems, the d-orbital splitting pattern results in a large energy gap (Δsp\Delta_{sp}) between the lower-lying orbitals (dxyd_{xy}, dxzd_{xz}, dyzd_{yz}) and the higher-energy dx2−y2d_{x^2-y^2} orbital, allowing all eight electrons to occupy the lower orbitals without pairing in antibonding levels, yielding a CFSE of approximately -1.41 Δsp\Delta_{sp} relative to the barycenter. This electronic stabilization outweighs the energy cost of ligand pairing for strong-field ligands like CN⁻, which have high ligand field strength and induce larger splitting compared to weak-field halides like Cl⁻. Steric factors also play a role, though they often compete with electronic preferences; the 90° bond angles in square planar arrangements lead to greater - repulsion than the 109.5° angles in tetrahedral geometry, making bulky ligands more likely to favor tetrahedral distortions in first-row d⁸ metals like Ni(II). However, in cases where electronic stabilization is dominant, such as with moderately sized ligands, the planarity is maintained to minimize overall strain. High from π-acceptor ligands further reinforces planarity by enhancing orbital overlap and reducing effective steric crowding through backbonding. Periodic trends significantly influence geometry adoption, with second- and third-row transition metals (e.g., Pd(II), Pt(II)) exhibiting a stronger preference for square planar structures than first-row counterparts (e.g., Ni(II)) due to inherently larger crystal field splitting parameters (Δ\Delta) and lower pairing energies (P) arising from better ligand-metal orbital overlap and relativistic effects. For instance, even weak-field ligands induce sufficient Δ\Delta in Pt(II) to stabilize square planar forms, whereas Ni(II) requires strong-field ligands to overcome the smaller Δ\Delta and higher P. The overall charge on the complex and the nature of counterions can modulate by altering the effective ligand and through ion-pairing or lattice effects; higher positive charge on the metal increases Δ\Delta, promoting square planar adoption, while counterions like large anions (e.g., BF₄⁻) may stabilize planar forms via reduced electrostatic repulsion in the coordination plane compared to smaller counterions that encourage distortions.

References

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  5. https://www.sciencedirect.com/topics/chemistry/point-group-d4h
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  7. https://employees.csbsju.edu/cschaller/Principles%20Chem/molecules/IMothershapes.htm
  8. https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/09%3A_Molecular_Geometry_and_Bonding_Theories/9.02%3A_The_VSEPR_Model
  9. https://www.sciencedirect.com/science/article/abs/pii/S001085450700152X
  10. https://www.chemtube3d.com/xef4-symmetry-d4h/
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  12. https://home.iitk.ac.in/~madhavr/CHM102/Notes_on_Coordination_Compounds.pdf
  13. http://www.adichemistry.com/jee/qb/coordination-chemistry/1/q1.html
  14. https://pubchem.ncbi.nlm.nih.gov/compound/61439
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  16. https://www.researchgate.net/post/Can_anyone_help_with_the_preparation_of_PtCl2_to_PtNH34Cl2_in_detail
  17. https://fiveable.me/key-terms/intro-chem/%5Bcunh34%5D2
  18. https://pubs.acs.org/doi/10.1021/jacs.4c15892
  19. https://pubs.acs.org/doi/10.1021/ja00896a007
  20. https://www.bartleby.com/questions-and-answers/the-molecular-shape-of-tecl42-is__/ff00d89d-d31f-4a4d-b000-8850aafa9dc1
  21. https://upcollege.ac.in/Upload/econtent/1286.pdf
  22. https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_and_Websites_(Inorganic_Chemistry)/Crystal_Field_Theory/Tetrahedral_vs._Square_Planar_Complexes
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  24. https://pubs.rsc.org/en/content/articlehtml/2012/dt/c2dt31273e
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