Question

A classmate says, "A weak-field ligand usually means the complex is high spin." Is your classmate correct? Explain.

Short answer

Yes, your classmate is correct. Generally, weak-field ligands cause small splits in the d-orbital energy levels, leading to high-spin complexes. This is because electrons tend to occupy all available d-orbitals with parallel spins before pairing up due to the small energy gap between the orbitals. However, it is important to note that this statement may not be true for every case, as there could be exceptions depending on the specific metal ion and ligands involved.

Step-by-step solution

Understand weak-field and strong-field ligands

A ligand is a molecule or ion that binds to a central metal atom or ion, forming a coordination complex. Ligands can be classified as weak-field or strong-field ligands based on their ability to split the d-orbital energy levels in a transition metal complex. Weak-field ligands cause a small splitting of the d-orbitals, while strong-field ligands cause a large splitting of the d-orbitals. The crystal field stabilization energy (CFSE) is used to quantify the energy stabilization resulting from this splitting.

Understand high-spin and low-spin complexes

High-spin and low-spin complexes refer to the distribution of electrons in the d-orbitals of a metal complex. For a high-spin complex, electrons will occupy all available d-orbitals with parallel spins (i.e., unpaired electrons) before pairing up. This happens when the energy difference between the d-orbitals is small. On the other hand, a low-spin complex will have electrons filling the lower-energy d-orbitals, pairing up before occupying the higher-energy d-orbitals. Low-spin complexes are formed when the energy difference between the d-orbitals is large. The difference in the energy gap between the orbitals is largely due to the ligands and their ability to split the d-orbitals.

Establish the relationship between ligands and their complexes

Now that we understand the terms "weak-field ligand," "strong-field ligand," "high-spin complex," and "low-spin complex," we can establish their relationships. Generally, weak-field ligands bind to metal centers in such a way that the energy gap between the d-orbitals remains small. Therefore, weak-field ligands usually result in high-spin complexes. Conversely, strong-field ligands produce large energy gaps in the d-orbitals, which, in turn, result in low-spin complexes.

Evaluate the accuracy of the classmate's statement

The classmate's statement, "A weak-field ligand usually means the complex is high spin," is generally correct, as weak-field ligands are known to cause small splits in the d-orbital energy levels, leading to high-spin complexes. However, it is crucial to remember that this statement is not universally true for every case, as there may be some exceptions depending on the specific metal ion and ligands involved.

Key concepts

Weak-field Ligands

Weak-field ligands are special because of their effect on a metal's d-orbitals. When these ligands attach to a metal ion, they only cause a small energy splitting of the d-orbitals. This minor splitting is primarily due to the weak electrostatic repulsion between the ligand's electrons and the metal's d-electrons. As a consequence, the energy difference between the lower and higher d-orbitals is not substantial.

Characteristics of weak-field ligands include:

• Causing small splitting of d-orbital energies.
• Being generally less effective in stabilizing the metal-ligand bond.
• Participating in high-spin complexes more frequently.

Common examples of weak-field ligands are halides (like Cl^- and Br^-), as well as hydroxide (OH^-). All these ligands have a tendency to form coordination complexes without causing drastic changes in the arrangement of d-electrons.

High-spin Complexes

High-spin complexes play a vital role in understanding electron configurations in metal ions. In a high-spin complex, the energy gap between split d-orbitals is small. This allows electrons to occupy all the orbitals with parallel spins as much as possible before pairing up. As such,

• Electrons are less likely to pair in lower-energy orbitals.
• The complex tends to display unpaired electrons, leading to paramagnetic properties.

High-spin formations are often associated with weak-field ligands. When these weak-field ligands bind to a metal ion, the d-orbitals have a minimal energy split, promoting high-spin configurations. This is because there's insufficient energy gain from pairing electrons in lower-energy orbitals.

One can typically observe high-spin complexes in first-row transition metals, where the energy difference added from pairing is greater than the energy required to overcome the small d-orbital splitting.

d-orbital Splitting

d-orbital splitting is a cornerstone concept in Crystal Field Theory. It explains how ligands influence metal ion d-orbitals in a coordination complex. When ligands approach, they interact with these orbitals, splitting their energy levels into distinctive sets.

The extent of this splitting varies depending on:

• The type of ligand (weak-field versus strong-field).
• The geometry of the complex (octahedral, tetrahedral, etc.).
• The nature of the metal itself.

In octahedral complexes, the presence of ligand fields breaks the degeneracy of the metal's d-orbitals into two higher-energy orbitals and three lower-energy orbitals. The degree to which this occurs is known as the Crystal Field Splitting Energy ( deltaoct delta_{oct} ).

Weak-field ligands result in lesser splitting, leading to high-spin complexes where unpaired electrons dominate. Conversely, strong-field ligands can cause significant splitting, favoring low-spin states with paired electrons.