Square Planar Complex in Coordination Chemistry

A strong understanding of the square planar complex is essential for mastering coordination chemistry, especially for JEE Main. In these compounds, four ligands arrange themselves around a central transition metal ion in a single plane, with 90° bond angles, forming a distinct square shape. Recognizing their structure, hybridisation, and d-orbital splitting patterns helps solve conceptual and application-based questions typical in entrance exams. The square planar geometry also connects directly with electronic configuration, especially for d⁸ metal ions such as Ni²⁺, Pd²⁺, and Pt²⁺.

What is a Square Planar Complex?

Square planar complexes are a type of coordination compound in which four ligands are symmetrically positioned at the corners of a square, all in one plane, around a central metal ion. This arrangement yields a bond angle of 90° between adjacent ligand-metal-ligand directions. Square planar geometry is particularly common for transition metals with a d⁸ configuration, where crystal field effects and hybridization lower the energy for this specific shape as compared to tetrahedral alternatives.

How to Identify a Square Planar Complex

To determine if a coordination compound adopts square planar geometry, follow these evidence-based checks:

• Check the central metal’s electronic configuration: d⁸ transition metals (like Ni²⁺, Pd²⁺, Pt²⁺) are prime candidates.
• Look for strong-field ligands (e.g., CN^-, NH₃, Cl^-) which can induce pairing of electrons in the d orbitals, making square planar more stable than tetrahedral.
• For coordination number 4, compare with possible tetrahedral geometry.
• If hybridization is confirmed as dsp², expect square planar structure for the complex ion.
• Empirical evidence like diamagnetism (all electrons paired) often supports square planar geometry.

Key Examples and Formulas: Square Planar Complexes

Learning core examples helps with quick identification and recall in exams. Typical square planar complexes are listed below, with formula and center metal’s configuration highlighted for JEE:

The classic example for exam purposes is [Ni(CN)₄]²⁻, featuring strong-field cyanide ligands and a diamagnetic, square planar structure. The square planar geometry of [NiCN4]2- is frequently asked in objective-type questions.

Hybridisation and Bonding in Square Planar Complexes

The hybridization involved in a square planar complex is dsp². Here, one d orbital (typically d_x²-y²), one s, and two p orbitals combine to produce four equivalent hybrid orbitals lying in a plane. This arrangement allows the metal to accept electron pairs from four ligands, producing strong bonding and the square configuration.

• dsp² hybridization: 1 d (usually d_x²-y²), 1 s, 2 p orbitals (p_x, p_y).
• The central metal must have at least one empty d orbital, which is often achieved by pairing up d electrons through ligand field interaction.
• This hybridisation is mainly seen in d⁸ or occasionally d⁹ ions with strong field ligands.

The formation of square planar hybridization [Ni(CN)₄]²⁻ complex, involving 3d4sp2 hybridization of Ni2+ ion orbitals, illustrates the process for JEE questions.

Crystal Field Splitting in Square Planar Complexes

In square planar complexes, the five d orbitals experience different repulsions due to the arrangement of the four ligands in a plane, causing them to split into energy levels. The d_x²-y² orbital is most destabilized because it points directly at the ligands, whereas d_z², d_xy, d_xz, and d_yz are less affected. The splitting energy (Δ_sp) is larger than in tetrahedral fields and comparable to the octahedral field with removed axial ligands.

• Order of increasing energy: d_xz, d_yz < d_z² < d_xy < d_x²-y².
• d_x²-y² orbital lies in the same plane as the ligands and thus is highest in energy.
• Complete pairing of electrons is favored in strong-field, square planar cases, often leading to diamagnetic behavior.

Crystal field splitting in the square planar complexes is often visualized in questions requiring orbital energy sequence or magnetic property prediction.

Square Planar vs Tetrahedral Geometry: Key Differences

A frequent JEE Main question type is to contrast square planar and tetrahedral geometries for coordination compounds with coordination number 4. The table below summarizes their core differences for quick revision.

Common Pitfalls and Exam-Style Applications

JEE aspirants sometimes mistakenly assign square planar geometry to all d⁸ or 4-coordinate metal ions, overlooking electronic factors or ligand strength. Always confirm strong-field ligand presence and check for dsp² hybridization. Application problems may require you to:

• Predict geometry based on ligands: [Ni(CN)₄]²⁻ is square planar, [NiCl₄]²⁻ is tetrahedral due to Cl^- being a weak field ligand.
• Use magnetic properties: Diamagnetic 4-coordinate d⁸ complexes are often square planar.
• Draw d-orbital splitting diagrams—and use them to justify shapes.
• Apply valence bond theory or crystal field theory to deduce hybridization.

TIP: Always check if the central ion is a late transition metal (like Ni, Pd, Pt) and the complex is diamagnetic. If so, suspect square planar geometry for the compound.

Summary and Quick Revision Points

• Square planar complexes have four ligands placed at 90° around a central metal in a plane.
• Favored by d⁸ ions like Ni²⁺, Pd²⁺, Pt²⁺ with strong-field ligands.
• dsp² hybridization is characteristic of square planar geometry.
• Crystal field splitting puts d_x²-y² highest in energy for these complexes.
• Compare carefully with tetrahedral: sp³ hybridization and weak field ligands indicate non-planar shapes.
• Classic example: [Ni(CN)₄]²⁻ is square planar and diamagnetic.

To dive deeper, refer to Coordination Compounds, Hybridization, Coordination Compounds Mock Test, and Chemical Bonding and Molecular Structure on Vedantu for more solved questions and additional conceptual clarity. Mastering the concepts of square planar complexes will strengthen your confidence in d- and f-block elements and complex ion geometry. This understanding is vital for high scores in the JEE Main Chemistry paper.