Crystal Field Theory: Square Planar Complexes - Online Tutor, Practice Problems & Exam Prep

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 show the most complex splitting pattern. We're going to say orbitals on and between the X&Y axes have the strongest interactions with ligands. Remember, the stronger the interaction with the ligands, the higher the energy of the orbital. If we take a look here at this image, we can see that there's a gradient that forms with the 1st 3 orbitals. It starts off very shaded initially, and then it tapers off and gets lighter as we move towards the right, signifying a lesser interaction with the ligands and therefore lower energy for our orbitals.

The first will make sense because in this one we're dealing with interactions that are on or along the axis. The X axis interacts with two lobes here as it cuts through, and then the Y axis interacts with two lobes as it cuts here. The next one should have the second highest energy because it interacts with both X&Y axes, still just not as much as the first one. dz² might seem misplaced here, but there is some interaction occurring with the X&Y axis. If you look in closely, you'll see this ring or kind of disk. This one lies and interacts with the X&Y axes. If you look, you can see the Y cutting through it, and here you can see the X cutting through it. So it's along the axis. It just doesn't have as great interaction as the previous two orbitals, so that's why it's here.

The last two, they are degenerative. They have the same energy because they're each only interacting with one of the two axes that we care about. X&Y dyz is interacting with the Y axis and dxz is interacting with the X axis. Now as you can see, this crystal field splitting pattern is pretty tall, and the difference in height here is why square planar complexes have the largest delta values. Now again, their splitting pattern is a bit more complex, but as you go through each one and see how they interact on the organ with the X&Y axes, it all starts to make sense.

Example

Which one of the following complexes shows the most complex splitting pattern energy? Remember this occurs with square planar complexes and if we look at our options, B&D are both out because in both we have 6 donor atoms connecting to our metal cation. Those could not give us a square planar complex.

The answer is going to be either A or C because in both of those we have 4 logins which have the potential of being either tetrahedral or square planar. In the first one, if we take a look at the cadmium ion, it's cadmium 2 plus its electron configuration when you look at it would be Krypton 4D¹⁰. Now here it is AD¹⁰ ion. And remember if you're D¹⁰ you are tetrahedral O. This will not give us the correct geometry.

For C we have nickel here connected to formonias which are neutral. So the two plus charge is coming from the nickel itself. If we do its electron configuration it would be argon 3D⁸. Now it is a D⁸ ion and that means that its geometry is square planar.

So option C would give us a squared planar geometry which results in the most complex splitting pattern energy.

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–

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.