Strong-Field vs Weak-Field Ligands - Online Tutor, Practice Problems & Exam Prep

Crystal field splitting energy (Δ) in octahedral complexes varies with the type of ligand. Strong field ligands, such as cyanide, create larger Δ, resulting in greater energy gaps between orbitals. Conversely, weak field ligands, like iodine, lead to smaller Δ and more degenerate orbitals. Memorization of strong field ligands can be aided by the phrase "Larry cannot enter the neighborhood," while weak field ligands include water and halogens. Understanding these concepts is crucial for grasping coordination chemistry and the behavior of complex ions.

Strong-Field Ligands result in a large Δ and Weak-Field Ligands result in a small Δ.

Here we can say that the crystal field splitting energy or Δ of the octahedral complexes depends on the ligand. Now, depending on the type of ligand attached to our metal cation, it can influence the magnitude and size of our crystal field splitting energy. Here we're going to say that strong field ligands attached to metal cations result in large crystal field splitting energy. So if we take a look here, we'd see that the distance is between our lower orbitals to our higher level orbitals. There's a bigger gap. That's a larger Δ.

Next, we'd say that weak field ligands typically have result in a smaller or small crystal field splitting energy. Here our Δ is much smaller and we say in these cases we'd have orbitals that are more degenerate, similar or same energy. Now here when we're talking about strong field versus weak field ligands, we have these ones in particular, these are the ones that you need to commit to memory. So over here we have our large Δ. These are the largest Δ with cyanide being at the very end. And over here we have our smallest Δ with iodine.

Over here on this end. Now our strong field ligands, how do we memorize the order of them? Well, all you have to recall is this memory tool and it is that Larry cannot enter the neighborhood. So Larry Large can CN Cyanide, no deals with nitrate. Enter is ethylenediamine the neighborhood. So here we have ammonia. These represent our strong field ligands and then our weak field ligands we started with water and then you might notice that the rest of the ions are halogens.

If you look at group 7A on the periodic table, you'll see you have fluorine, chlorine, bromine and iodine. So this is listed as we go down group 7A, so fluoride, chloride, bromide and iodide. So just remember that Larry cannot enter the neighborhood. Then for the weak we have water and then look at group 7A flooring down to iodine. This gives us the order from strong field ligands to weak field ligands.

Example

Identify the complex with the largest crystal field splitting energy, otherwise known as Delta. So when it comes to Delta, remember that tetrahedral has the smallest venopahedrals in the middle and then square planar is the highest. Now if we take a look the first three each has iron connected to six ligands, so all of these are octahedral in nature.

The last one though has four chlorides connected to copper, so there's the potential of it being tetrahedral or square planar. So let's figure that one out first. So here this has to do with the copper plus one ion, because here it's trying to basically work with the four chloride ions. But since there's more negative charge, that's why we have a remainder of three minus neutral copper.

Its electron configuration is argon 4S¹3D¹⁰, remember, is an exception. Plus one means we've lost one electron from the highest shell number, so the 4S electron will be gone. So Copper one is Argon 3D¹⁰. We have a D¹⁰ ion, and because it's D¹⁰, that means that it is tetrahedral. Tetrahedral has the smallest crystal field slitting energy, so this would not be our answer.

It's going to be one of these three here they're all octahedral. So how do we determine? Well, remember for octahedral species, the ligand that attaches determines or effects our crystal field splitting energy. So all you have to remember is that Larry cannot enter the neighborhood. So here the strongest field ligand would give us the highest or largest crystal field splitting energy and cannot is the beginning of our memory tool.

So cyanide here would be the strongest field ligand when compared to water and ammonia. So here the answer would have to be option C. So C would be our answer. It would produce the largest crystal field splitting energy.

Arrange the following complexes in an ascending order of the magnitude of Δ.

a) [Cr(NH₃)₆]²⁺

b) [Cr(en)₃]²⁺

c) [Cr(NO₂)₆]^4–

d) [Cr(H₂O)₄Cl₂]

• A. a < c < b < d
• B. c < b < a < d
• C. d < a < b < c
• D. b < c < d < a

Here’s what students ask on this topic:

What is the difference between strong-field and weak-field ligands?

Strong-field ligands result in a large crystal field splitting energy (Δ), creating a significant gap between the lower and higher orbital levels in an octahedral complex. This means that the electrons are more likely to pair up in the lower energy orbitals. Examples of strong-field ligands include cyanide (CN⁻), nitrate (NO₃⁻), ethylenediamine (en), and ammonia (NH₃). Weak-field ligands, on the other hand, lead to a smaller crystal field splitting energy, resulting in more degenerate orbitals with similar energy levels. This means that electrons are more likely to occupy higher energy orbitals before pairing up. Examples of weak-field ligands include water (H₂O) and halogens such as fluorine (F⁻), chlorine (Cl⁻), bromine (Br⁻), and iodine (I⁻).

How does the type of ligand affect the crystal field splitting energy in octahedral complexes?

The type of ligand attached to the metal cation in an octahedral complex significantly influences the crystal field splitting energy (Δ). Strong-field ligands, such as cyanide (CN⁻) and ammonia (NH₃), create a large Δ, resulting in a significant energy gap between the lower (t₂g) and higher (e₉) orbitals. This causes electrons to pair up in the lower energy orbitals. Weak-field ligands, such as water (H₂O) and halogens (F⁻, Cl⁻, Br⁻, I⁻), produce a smaller Δ, leading to more degenerate orbitals with similar energy levels. In this case, electrons are more likely to occupy higher energy orbitals before pairing up.

What is the mnemonic to remember the order of strong-field ligands?

The mnemonic to remember the order of strong-field ligands is 'Larry cannot enter the neighborhood.' This stands for: Larry (large), cannot (cyanide, CN⁻), enter (ethylenediamine, en), the (nitrate, NO₃⁻), neighborhood (ammonia, NH₃). This mnemonic helps recall the sequence of ligands that produce a large crystal field splitting energy (Δ) in octahedral complexes.

Which ligands are considered weak-field ligands?

Weak-field ligands include water (H₂O) and halogens such as fluorine (F⁻), chlorine (Cl⁻), bromine (Br⁻), and iodine (I⁻). These ligands produce a smaller crystal field splitting energy (Δ) in octahedral complexes, resulting in more degenerate orbitals with similar energy levels. The order of weak-field ligands can be remembered by looking at Group 7A of the periodic table, starting from fluorine and moving down to iodine.

Why do strong-field ligands result in a larger crystal field splitting energy?

Strong-field ligands result in a larger crystal field splitting energy (Δ) because they create a stronger electrostatic interaction with the metal cation in an octahedral complex. This strong interaction causes a significant energy difference between the lower (t₂g) and higher (e₉) orbitals. As a result, the energy gap (Δ) is larger, leading to electrons pairing up in the lower energy orbitals. Examples of strong-field ligands include cyanide (CN⁻), nitrate (NO₃⁻), ethylenediamine (en), and ammonia (NH₃).