Answer :

Square planar geometry is more common for d[tex]^8[/tex] metal complexes due to a combination of electronic and energetic factors that involve the distribution of electrons among the metal's d orbitals.

  1. Crystal Field Stabilization Energy (CFSE):

    When a metal ion with d[tex]^8[/tex] configuration forms a complex, it can either adopt a square planar or a tetrahedral geometry. In square planar coordination, the splitting of the d orbitals is significant because the square planar arrangement greatly affects the energy levels of the d orbitals. There is a large energy separation between the highest energy d orbital(s) and those at lower energy, particularly in square planar complexes. The CFSE is generally higher for square planar than for tetrahedral due to the stabilization that occurs when the electrons occupy lower energy d orbitals.

  2. Orbital Splitting Patterns:

    In square planar complexes, the splitting pattern is such that the dx[tex]^2[/tex]-y[tex]^2[/tex] orbital is nonbonding or antibonding with the highest energy. This orbital remains unoccupied in a d[tex]^8[/tex] configuration, which leads to a much lower overall energy and higher stability for the complex. The other d orbitals (dxy, dxz, dyz, and dz[tex]^2[/tex]) are at lower energy levels, allowing electrons to fill them more favorably.

  3. Ligand Field Strength:

    The type of ligands also plays a role. Strong field ligands, such as those in the square planar complexes, can cause larger splitting of the d orbitals. This often leads to low spin configurations and stabilization of the square planar shape due to the pairing of electrons in the lower energy orbitals.

  4. Steric and Electronic Factors:

    Sterically, the square planar geometry can be favorable because it provides a more effective overlap between the metal d orbitals and ligand orbitals, resulting in stronger bonds. Electrostatic factors might also play a part, where the removal of two axial ligands as compared to an octahedral geometry allows the square planar configuration to occur more freely without steric hindrance.

Square planar complexes are particularly common for some metals like Ni(II), Pd(II), and Pt(II), especially with strong field ligands, due to these electronic stabilization effects. This makes the square planar arrangement energetically favorable for d[tex]^8[/tex] metal ions in many cases.

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Rewritten by : Barada