Crystal Field Theory: Square Planar Complexes: Videos & Practice Problems

Square planar complexes exhibit a complex crystal field splitting pattern, where orbitals along the x and y axes interact strongly with ligands, resulting in higher energy levels. The energy gradient of the first three orbitals indicates stronger interactions, while the last two are degenerate, having equal energy due to limited interaction. This leads to larger delta values (Δ) for square planar complexes, emphasizing their unique geometry and electronic properties, crucial for understanding coordination chemistry and ligand behavior.

Square planar complexes show the most complex splitting pattern.

Square planar complexes exhibit a unique and intricate crystal field splitting pattern due to the specific interactions of their d-orbitals with ligands. In these complexes, the orbitals that lie along the x and y axes experience the strongest interactions with the surrounding ligands, resulting in higher energy levels for these orbitals. The energy gradient observed in the first three d-orbitals illustrates this concept, where the shading indicates the strength of interaction; darker shading corresponds to stronger interactions and higher energy, while lighter shading signifies weaker interactions and lower energy.

The d-orbitals can be categorized based on their interactions with the axes. The first two orbitals, which are the d_x²-y² and d_z², have the highest energy due to their significant interactions with both the x and y axes. The d_x²-y² orbital interacts directly along the axes, while the d_z² orbital, although it may seem less relevant, also interacts through a ring or disc that intersects with both axes, albeit to a lesser extent.

The next two orbitals, d_yz and d_xz, are degenerate, meaning they possess the same energy level. These orbitals interact with only one of the two axes, with d_yz interacting with the y-axis and d_xz with the x-axis. This limited interaction results in lower energy levels compared to the first two orbitals.

The pronounced height difference in the crystal field splitting pattern of square planar complexes leads to larger Δ values, which is a measure of the energy difference between the split d-orbitals. Understanding how these orbitals interact with the x and y axes clarifies the complexity of the splitting pattern and the resulting energy levels in square planar complexes.

Example

In coordination chemistry, the geometry of metal complexes significantly influences their electronic properties and the resulting splitting patterns of d-orbitals. Among the various geometries, square planar complexes exhibit a unique splitting pattern due to the arrangement of ligands around the central metal ion.

When evaluating the splitting patterns, it is essential to consider the electron configuration of the metal ion involved. For instance, cadmium (Cd²⁺) has an electron configuration of [Kr] 4d¹⁰. As a d¹⁰ ion, cadmium typically adopts a tetrahedral geometry rather than square planar, which does not lead to the most complex splitting pattern.

On the other hand, nickel (Ni²⁺), when coordinated with four neutral ammonia (NH₃) ligands, has an electron configuration of [Ar] 3d⁸. This d⁸ configuration is characteristic of square planar complexes, which are known to exhibit more intricate splitting patterns due to the specific arrangement of ligands in relation to the d-orbitals.

Thus, the square planar geometry of the nickel complex results in the most complex splitting pattern energy, making it the correct choice when comparing the two options. This highlights the importance of understanding the relationship between the metal's oxidation state, electron configuration, and the resulting geometry in predicting the electronic behavior of coordination complexes.

Which of the following complexes will have the largest crystal field splitting energy?

• A. [Co(NH₃)₆]²⁺
• B. [Cr(NH₃)₃(H₂O)₃]²⁺
• C. [ZnCl₄]^2–
• D. [Ni(CN)₄]^2–

Answer: (D)

Here’s what students ask on this topic:

What is the crystal field splitting pattern in square planar complexes?

In square planar complexes, the crystal field splitting pattern is more complex compared to other geometries. The orbitals interact with ligands along and between the x and y axes, leading to varying energy levels. The first orbital has the highest energy due to strong interactions along both the x and y axes. The second highest energy orbital also interacts with both axes but to a lesser extent. The d_z² orbital, despite seeming out of place, interacts with the x and y axes through a disc-like region, placing it third in energy. The last two orbitals, d_yz and d_xz, are degenerate, each interacting with only one axis, resulting in equal energy levels. This complex splitting pattern results in large delta values for square planar complexes.

Why do square planar complexes have large delta values?

Square planar complexes have large delta values due to the significant difference in energy levels between the orbitals. The strong interactions of the orbitals with ligands along the x and y axes result in high energy for the first few orbitals. As the interactions decrease, the energy levels also decrease, creating a wide range of energy differences. This large difference in energy levels, or splitting pattern, is what leads to the large delta values characteristic of square planar complexes.

How do the d_z² orbitals interact in square planar complexes?

In square planar complexes, the d_z² orbital interacts with the x and y axes through a disc-like region. Although it might seem out of place, this interaction occurs because the disc lies along the x and y axes. The y-axis cuts through the disc vertically, and the x-axis cuts through it horizontally. This interaction is not as strong as the interactions of the first two orbitals with the axes, placing the d_z² orbital third in energy level.

What causes the degeneracy of the d_yz and d_xz orbitals in square planar complexes?

The d_yz and d_xz orbitals are degenerate in square planar complexes because they each interact with only one of the two axes (x and y) that are significant in this geometry. The d_yz orbital interacts with the y-axis, while the d_xz orbital interacts with the x-axis. Since these interactions are of equal strength and occur along only one axis, the energy levels of these two orbitals are the same, resulting in their degeneracy.

How does the interaction of orbitals with ligands affect their energy in square planar complexes?

In square planar complexes, the interaction of orbitals with ligands significantly affects their energy levels. Orbitals that interact strongly with ligands along the x and y axes have higher energy due to the increased repulsion between the electrons in the orbitals and the ligands. As the strength of the interaction decreases, the energy of the orbitals also decreases. This creates a gradient of energy levels, with the highest energy orbitals having the strongest interactions and the lowest energy orbitals having the weakest interactions.