Strong-Field vs Weak-Field Ligands: Videos & Practice Problems

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.

The crystal field splitting energy, denoted as Δ, in octahedral complexes is significantly influenced by the type of ligands attached to the metal cation. Strong field ligands lead to a larger crystal field splitting energy, resulting in a greater energy gap between the lower and higher orbitals. This larger Δ indicates that the orbitals are less degenerate, meaning they have different energy levels. Conversely, weak field ligands produce a smaller Δ, where the orbitals remain more degenerate, exhibiting similar energy levels.

To categorize ligands based on their field strength, it is essential to memorize the order of strong and weak field ligands. Strong field ligands, which create a large Δ, include cyanide (CN^-), nitrate (NO₃^-), ethylenediamine (en), and ammonia (NH₃). A helpful mnemonic to remember this order is "Larry cannot enter the neighborhood," where 'Larry' represents large Δ, 'cannot' stands for cyanide, 'enter' for ethylenediamine, and 'neighborhood' for ammonia.

On the other hand, weak field ligands, associated with a smaller Δ, start with water (H₂O) and include the halogens from group 7A of the periodic table: fluorine (F^-), chlorine (Cl^-), bromine (Br^-), and iodine (I^-). This classification helps in understanding the electronic structure and properties of various metal complexes, as the strength of the ligand field directly affects the stability and reactivity of the complexes.

Example

In coordination chemistry, the crystal field splitting energy, denoted as Δ, varies depending on the geometry of the complex and the nature of the ligands involved. Among common geometries, tetrahedral complexes exhibit the smallest Δ, followed by octahedral complexes, and square planar complexes have the largest Δ. This hierarchy is crucial for understanding the electronic properties of transition metal complexes.

When analyzing complexes with octahedral geometry, the ligands play a significant role in determining the magnitude of Δ. For instance, in a scenario where iron is coordinated to six ligands, the specific ligands can influence the crystal field splitting. In contrast, a complex with copper(I) coordinated to four chloride ions can either adopt a tetrahedral or square planar geometry. However, since copper(I) has an electron configuration of argon 3d¹⁰, it typically forms a tetrahedral complex, which is characterized by the smallest Δ.

To identify the complex with the largest crystal field splitting energy, one must consider the nature of the ligands in octahedral complexes. Strong field ligands, such as cyanide (CN^-), produce a greater Δ compared to weaker field ligands like water (H₂O) and ammonia (NH₃). Therefore, in a comparison of octahedral complexes, the presence of cyanide as a ligand would result in the largest crystal field splitting energy, making it the correct choice for the complex with the highest Δ.

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

Answer: (C)

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).