Question

The following table indicates the number of unpaired electrons in the crystal field diagrams for some complexes. Complete the table by classifying each species as weak field, strong field, or insufficient information. $$ \begin{array}{|lcc|} \hline \text { Species } & \begin{array}{l} \text { Unpaired } \\ \text { Electrons } \end{array} & \text { Classification } \\ \hline \mathrm{Fe}(\mathrm{CNS})_{6}{ }^{4-} & 0 & \\ \mathrm{CoCl}_{4}^{2-} & 3 & \\ \mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}{ }^{3+} & 5 & \\ \mathrm{Fe}(\mathrm{CN})_{6}{ }^{4-} & 0 & \\ \hline \end{array} $$

Short answer

The complexes can be classified as follows based on the number of unpaired electrons: - \(Fe(CNS)_{6}^{4-}\): This complex has 0 unpaired electrons and is considered a strong field complex. - \(CoCl{_4}^{2-}\): This complex, with 3 unpaired electrons, is a weak field complex. - \(Fe(H_{2}O)_{6}^{3+}\): This complex has 5 unpaired electrons, making it a weak field complex. - \(Fe(CN)_{6}^{4-}\): This complex, showing 0 unpaired electrons, is a strong field complex.

Step-by-step solution

Identify high-spin and low-spin complexes

We need to determine whether each complex is high-spin (with most or all electrons unpaired) or low-spin (with most or all electrons paired). Unpaired electrons result in a high-spin configuration, while paired electrons are characteristic of low-spin configurations.

Classify each species as weak field or strong field

Now that we have determined the spin configuration of each complex, we can classify them as weak field (high spin) or strong field (low spin), based on the number of unpaired electrons. - \(Fe(CNS)_{6}^{4-}\): 0 unpaired electrons. This complex has a low-spin configuration, meaning it is a strong field complex. - \(CoCl{_4}^{2-}\): 3 unpaired electrons. This complex has a high-spin configuration, meaning it is a weak field complex. - \(Fe(H_{2}O)_{6}^{3+}\): 5 unpaired electrons. This complex has a high-spin configuration, meaning it is a weak field complex. - \(Fe(CN)_{6}^{4-}\): 0 unpaired electrons. This complex has a low-spin configuration, meaning it is a strong field complex. Here's the completed table: $$ \begin{array}{|lcc|} \hline \text { Species } & \begin{array}{l} \text { Unpaired } \\ \text { Electrons } \end{array} & \text { Classification } \\ \hline \mathrm{Fe}(\mathrm{CNS})_{6}{ }^{4-} & 0 & \text { Strong Field } \\ \mathrm{CoCl}_{4}^{2-} & 3 & \text { Weak Field } \\ \mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}{ }^{3+} & 5 & \text { Weak Field } \\ \mathrm{Fe}(\mathrm{CN})_{6}{ }^{4-} & 0 & \text { Strong Field } \\ \hline \end{array} $$

Key concepts

Unpaired Electrons

In the realm of crystal field theory, unpaired electrons play a crucial role in determining the magnetic properties of a complex. Simply put, unpaired electrons are electrons that do not share an orbital with another electron, hence affecting the spin state and magnetic properties of a molecule. The presence of unpaired electrons often results in paramagnetism, which means the complex can be attracted by an external magnetic field.

• When electrons are paired, they occupy the same orbital but with opposite spins, resulting in no net magnetic moment.
• When electrons are unpaired, they occupy separate orbitals or the same orbital with parallel spins, contributing to a net magnetic moment.

Detecting the number of unpaired electrons helps in categorizing complexes into high-spin or low-spin states. More unpaired electrons signify a high-spin state, while fewer or none indicate a low-spin state.

High-spin and Low-spin Complexes

High-spin and low-spin complexes are classifications based on the spin state of the electrons within a complex. These terms refer to how electrons are arranged in the d orbitals of transition metals when they interact with ligands in a crystal field.

• High-spin complexes: These complexes have a larger number of unpaired electrons, which occurs when the energy gap between the crystal field splitting is small. The electrons prefer to occupy higher energy orbitals rather than pair up in lower energy orbitals.
• Low-spin complexes: Here, the energy gap is significant, leading electrons to pair up in the lower energy orbitals rather than occupying higher ones. This means fewer unpaired electrons.

This spin state influences the magnetic and spectroscopic properties of the complex. High-spin complexes are usually associated with weak field ligands, whereas low-spin complexes are linked with strong field ligands.

Weak Field and Strong Field Ligands

Ligands affect the electronic structure of a complex through their ability to either induce a large or small splitting of the d orbitals, known as the crystal field splitting. This difference is attributed to the ligand field strength.

• Weak field ligands: These ligands create smaller splitting. This results in high-spin configurations as the electrons remain unpaired, occupying the higher energy orbitals, leading to paramagnetism.
• Strong field ligands: They cause larger splitting, which results in low-spin configurations. Electrons pair up in the lower energy orbitals, usually leading to diamagnetism.

Understanding whether a ligand is a weak or strong field is essential for predicting the behavior and properties of complexes. Examples of strong field ligands include CN⁻ and CO, while weak field ligands include halides like Cl⁻ and H₂O.