Question

Weak-field ligands tend to give high-spin complexes, but strongfield ligands tend to give low-spin complexes. Explain.

Short answer

Weak-field ligands give high-spin due to small splitting, while strong-field ligands give low-spin due to large splitting.

Step-by-step solution

Understanding High-Spin and Low-Spin Complexes

In coordination chemistry, metal complexes with d-orbitals can adopt either a high-spin or low-spin electron configuration. A high-spin complex means more unpaired electrons, while a low-spin complex has fewer unpaired electrons due to electron pairing in lower energy orbitals.

Role of Ligands

Ligands are ions or molecules attached to the central metal atom in a complex. The nature of these ligands influences the electron configuration of the metal by affecting the crystal field splitting energy ( abla").

Crystal Field Splitting

Crystal field theory describes how ligands fill the d-orbitals of a metal ion. The arrangement of ligands around the metal ion creates an electric field that splits the degenerate d-orbitals into two energy levels: the lower-energy t2g and the higher-energy eg.

Weak-field Ligands

Weak-field ligands create a small crystal field splitting ( abla<10Dq"), resulting in less separation between the t2g and eg orbitals. Therefore, electrons tend to occupy higher energy eg orbitals, leading to many unpaired electrons or a high-spin configuration.

Strong-field Ligands

Strong-field ligands cause a large crystal field splitting ( abla>10Dq"). This considerable energy difference encourages electrons to pair up in the lower-energy t2g orbitals, reducing the number of unpaired electrons and resulting in a low-spin configuration.

Conclusion

The strength of the field created by the ligand influences the electronic configuration of the metal ion. Weak-field ligands create high-spin complexes due to small splitting of d-orbitals, while strong-field ligands create low-spin complexes due to large splitting and electron pairing in lower-energy orbitals.

Key concepts

High-spin complexes

In coordination chemistry, high-spin complexes are characterized by a greater number of unpaired electrons in the metal's d-orbitals. This occurs when electrons fill the available orbitals with minimal pairing as possible, to minimize energy according to Hund's rule.
High-spin complexes occur in situations where the energy required to pair electrons in an orbital is greater than the energy needed to place them in a higher energy orbital that remains unpaired.
With high-spin configurations, the magnetic properties are typically more pronounced due to the presence of these unpaired electrons, which contribute to a higher magnetic moment.
This is often seen when the ligands surrounding the central metal ion are known as weak-field ligands.

Low-spin complexes

Conversely, low-spin complexes have fewer unpaired electrons compared to their high-spin counterparts. In these complexes, electrons are more likely to pair up in the lower energy orbitals before they fill higher energy orbitals, leading to fewer unpaired electrons.
The tendency to pair up occurs when the energy cost for pairing is less than the energy of occupying an additional orbital at higher energy.
Low-spin configurations are often associated with strong-field ligands, which create a large energy gap between orbitals, encouraging electron pairing in the lower energy orbitals first. As a result, these complexes are usually less magnetic than high-spin complexes due to fewer unpaired electrons.

Crystal field splitting

Crystal field theory offers insight into how metal ions, when surrounded by ligands, experience a split in their d-orbital energies, a concept known as crystal field splitting. The surrounding ligands generate an electrostatic field that influences the energy levels of these orbitals.
The d-orbitals are degenerate—that is, they have the same energy without any external influence.
Upon interaction with ligands, these orbitals split into two distinct groups: the t2g orbitals (lower energy) and the eg orbitals (higher energy).
The extent of this split, or crystal field splitting energy ( abla), is crucial, as it affects whether a complex becomes high-spin or low-spin. This splitting energy is largely determined by the nature of the ligands involved.

Weak-field ligands

Weak-field ligands play a pivotal role by causing only a minimal crystal field splitting. For these ligands, the energy ( abla) between the t2g and eg orbitals is small.
This small splitting means that the energy required to keep electrons unpaired in the higher eg orbitals is less than that for electron pairing in the t2g orbitals.
Thus, electrons occupy the higher energy eg orbitals singly, resulting in more unpaired electrons and leading to the formation of high-spin complexes.
Examples of weak-field ligands include halides like Cl⁻, and other species that typically do not produce a strong crystal field effect.

Strong-field ligands

Strong-field ligands significantly impact the energy levels of metal center d-orbitals by inducing a large crystal field splitting. For these ligands, the energy gap ( abla) between t2g and eg orbitals is substantial.
This larger separation encourages electron pairing in the lower energy t2g orbitals, as the energy cost for pairing is less than promoting an electron to the eg level.
This behavior favors the formation of low-spin complexes, where paired electrons reduce the number of unpaired electrons. Examples of strong-field ligands include CN⁻, and CO, which typically create a strong field and lead to low-spin configurations.