Question

In a square planar complex which \(d\) orbital is at lowest energy? Which \(d\) orbital is at highest energy? (Assume the ligands lie along the \(x\) and \(y\) axes.)

Short answer

In a square planar complex, the \(d_{z^2}\) orbital is at the lowest energy, and the \(d_{x^2-y^2}\) orbital is at the highest energy.

Step-by-step solution

Understand the geometry of the square planar complex

In a square planar complex, the metal is at the center of a square plane with ligands at the four corners. The ligands' positions along the axes affect the splitting of the metal's d orbitals due to ligand field theory.

Recall the order of d orbital splitting

For square planar complexes, the typical order of energy levels for the d orbitals, from lowest to highest energy, is: \(d_{z^2} < d_{xy} < d_{xz} = d_{yz} < d_{x^2-y^2}\). This order is influenced by the ligands' positions at 90 degrees relative to each other along the x and y axes.

Key concepts

d orbital splitting

The concept of *d orbital splitting* is central to understanding the behavior of square planar complexes. In transition metals, d orbitals undergo energy level changes when they form complexes with ligands. When a metal ion is surrounded by ligands, such as in a square planar configuration, the electrostatic interactions and arrangements between the ligands and central metal ion lead to the splitting of the degenerate d orbitals (those with the same energy). This splitting occurs because some orbitals experience more repulsion from ligands positioned directly along their axes, leading to higher energy levels, while others experience less, leading to lower energy levels. This is described by ligand field theory, which provides a detailed explanation of how these interactions occur.

ligand field theory

*Ligand field theory* (LFT) plays a key role in understanding the electronic structure of coordination compounds, especially those with complex shapes such as square planar geometries. LFT is an adaptation and extension of crystal field theory that takes into account the covalent nature of the metal-ligand bond. It suggests that ligands are not only point charges that repel electrons in the metal d orbitals but also have interactions based on orbital overlap. In a square planar complex, LFT predicts how the ligand arrangement affects d orbital energy levels:

• Ligands positioned directly on the x- and y-axes cause significant repulsion for the d_x²-y² orbital.
• This orbital ends up at the highest energy level because its lobes face the ligands directly.
  
• Other orbitals, such as d_z², are affected less directly, resulting in a lower energy state.
  

By understanding these principles, one can predict the square planar complexes' electronic properties and reactivity.

coordination chemistry

*Coordination chemistry* is a fascinating field that explores how metal ions interact with surrounding molecules or ions, known as ligands, to form coordination compounds. In square planar complexes, coordination chemistry is particularly interesting due to the geometrical arrangement and distribution of ligands around a central metal:

• The metal is usually a transition metal, which provides the d orbitals that participate in coordinate bonding.
• Ligands are arranged in a square plane, creating unique electronic environments differing from other geometries such as tetrahedral or octahedral.
  

These interactions are critical in various applications, including catalysis, materials science, and bioinorganic chemistry. Understanding how ligands affect d orbital energies in square planar complexes allows chemists to tailor these compounds for specific purposes, enhancing their functionality and effectiveness in chemical processes.