Problem 93 - Chemistry: An Atoms-Focused Approach

Question

The iron(II) compound Fe(bipy) \(_{2}(\mathrm{SCN})_{2}\) is paramagnetic, but the corresponding cyanide compound \(\mathrm{Fe}(\text { bipy })_{2}(\mathrm{CN})_{2}\) is diamagnetic. Why do these two compounds have different magnetic properties?

Step-by-Step Solution

Answer

The magnetic properties of Fe(bipy)$_{2}(\mathrm{SCN})_{2}$ and $\mathrm{Fe}(\text { bipy })_{2}(\mathrm{CN})_{2}$ differ due to the nature of their ligands. Fe(bipy)$_{2}(\mathrm{SCN})_{2}$ is paramagnetic because the weak-field thiocyanate ligands do not cause pairing of the unpaired electrons in the d orbitals, while $\mathrm{Fe}(\text { bipy })_{2}(\mathrm{CN})_{2}$ is diamagnetic because the strong-field cyanide ligands cause the unpaired electrons in the d orbitals to pair up.

Step 1: Understand the concept of paramagnetism and diamagnetism

Paramagnetism is the property where a material is attracted to an external magnetic field because it has unpaired electrons. On the other hand, diamagnetism is the property where a substance is repelled by an external magnetic field due to all electrons being paired.

Step 2: Determine the oxidation state of iron in both compounds

In both compounds, the iron (Fe) has an oxidation state of +2. This is because the ligands bipyridine (bipy), thiocyanate (SCN), and cyanide (CN) are neutral, -1, and -1, respectively, and the sum of their charges must equal the charge on the complex ion.

Step 3: Write the electron configuration for Iron(II)

For the Fe(II) ion, we must first write the electron configuration for the neutral Fe atom. The electron configuration for iron (Z = 26) is: [Ar]3d\(^{6}\)4s\(^{2}\). When Fe loses two electrons to become Fe(II), the configuration changes into: [Ar]3d\(^{5}\)4s\(^{0}\) (or simply [Ar]3d\(^{5}\)).

Step 4: Analyze the role of the ligands in each compound

In Fe(bipy)\(_{2}(\mathrm{SCN})_{2}\), the thiocyanate (SCN) ligand is a weak-field ligand, which causes little pairing of the electrons in the iron's d orbitals. Therefore, the Fe(II) still has unpaired electrons, making the compound paramagnetic. In \(\mathrm{Fe}(\text { bipy })_{2}(\mathrm{CN})_{2}\), the cyanide (CN) ligand is a strong-field ligand, which causes the unpaired electrons in the d orbitals to pair up. This results in all the electrons being paired, making the compound diamagnetic.

Step 5: Final explanation

So, the difference in magnetic properties between these two compounds is due to the nature of their ligands. Fe(bipy)\(_{2}(\mathrm{SCN})_{2}\) is paramagnetic because the weak-field thiocyanate ligands do not cause pairing of the unpaired electrons in the d orbitals. In contrast, \(\mathrm{Fe}(\text { bipy })_{2}(\mathrm{CN})_{2}\) is diamagnetic because the strong-field cyanide ligands cause the unpaired electrons in the d orbitals to pair up, resulting in a diamagnetic property.

Key Concepts

Paramagnetism and Diamagnetism

When discussing magnetic properties, the concepts of paramagnetism and diamagnetism often come into play. Paramagnetic materials have at least one unpaired electron. This unpaired electron causes the material to be attracted to an external magnetic field.
On the other hand, diamagnetic materials have all their electrons paired. This gives them no net magnetic moment, causing them to be slightly repelled by a magnetic field.

• In paramagnetism, the unpaired electrons align with external magnetic fields.
• In diamagnetism, paired electrons create tiny currents resisting the applied field.

Understanding these differences is crucial when analyzing substances like coordination compounds, where electron pairing can change depending on their ligands.

Ligand Field Theory

Ligand field theory explains how ligands affect the distribution of electrons in a metal's d orbitals. When ligands come close to a metal ion, they can split the degeneracy of the d orbitals.
The extent of this splitting depends on the strength of the ligands.

• Strong-field ligands, such as CN⁻, cause large splitting, often resulting in electron pairing within the lower energy orbitals.
• Weak-field ligands, like SCN⁻, cause smaller splitting, allowing electrons to remain unpaired.

In our example, the \[ \text{Fe(bipy)}_{2}\text{(SCN)}_{2} \] compound with thiocyanate leads to a situation with unpaired electrons. Meanwhile, \[ \text{Fe(bipy)}_{2}\text{(CN)}_{2} \] with cyanide results in paired electrons because of the greater field strength of CN⁻.

Electron Configuration

Electron configuration helps in predicting how atoms interact in compounds based on where electrons are situated. For iron (Fe), with an atomic number of 26, its neutral atom configuration is \[\text{[Ar]}3d^{6}4s^{2}\].
When iron loses two electrons to form \(\text{Fe}^{2+}\), its configuration becomes \[\text{[Ar]}3d^{6}\]. This electron setup is key when it interacts with ligands.

• Electron occupancy in the 3d orbitals can change based on ligand strength.
• The proper understanding of electron filling using Hund's rule helps in predicting magnetic properties.

In this context, knowing the electron configuration allows us to see why \[ \text{Fe(bipy)}_{2}\text{(SCN)}_{2} \] is paramagnetic and why \[ \text{Fe(bipy)}_{2}\text{(CN)}_{2} \] is diamagnetic.

Oxidation States in Coordination Chemistry

In coordination chemistry, oxidation states help determine the electron count on a metal within a complex. The oxidation state of a metal can be inferred from the charges of its surrounding ligands.
For the coordination compounds in our example, iron maintains an oxidation state of +2.

• This is due to ligands such as bipy (neutral), SCN⁻ (-1), and CN⁻ (-1) summing up to the overall charge of the complex.
• The electron distribution and subsequent pairing depend greatly on the metal's oxidation state.

Thus, the consistent oxidation state of +2 across both compounds gives a baseline for comparing how different ligands affect electron configuration and, consequently, magnetic properties.