Question

An aqueous solution of iron (II) sulfate is paramagnetic. If \(\mathrm{NH}_{3}\) is added, the solution becomes diamagnetic. Why does the magnetism change?

Short answer

The magnetism changes because ext{NH}_{3} causes d electrons in iron to pair up, shifting from paramagnetic to diamagnetic.

Step-by-step solution

Understanding Paramagnetism

Paramagnetism occurs in substances with unpaired electrons. Iron (II) sulfate, when dissolved in water, forms ext{Fe}^{2+} ions. These ions have unpaired electrons in their d orbitals, which makes the solution paramagnetic.

Effect of Ammonia Addition

When ext{NH}_{3} is added to the solution, it acts as a ligand, creating a complex with the ext{Fe}^{2+} ions. This new complex involves the d electrons forming covalent bonds with ammonia's lone pair.

Formation of Octahedral Complex

The ext{NH}_{3} ligands surround the ext{Fe}^{2+} ion forming an octahedral complex. In this process, the d orbital electrons pair up because ammonia is a strong field ligand.

Role of Ligand Field Theory

According to ligand field theory, a strong field ligand like ext{NH}_{3} causes the splitting of d orbitals. This results in all the d electrons pairing up, leading to no unpaired electrons in the new configuration.

Reason for Diamagnetism

With no unpaired electrons in the iron complex, the substance becomes diamagnetic, meaning it is repelled by a magnetic field, rather than attracted as in the paramagnetic state.

Key concepts

Paramagnetism

Magnetism is a fascinating property of matter that refers to how a substance responds to a magnetic field. Paramagnetism is one type of magnetism. This phenomenon occurs in materials that have unpaired electrons. These electrons, spinning on their own axes, create small magnetic fields. In a paramagnetic material, when an external magnetic field is applied, these little magnetic fields align with it.
This alignment causes the substance to be attracted to the magnetic field. An everyday example of a paramagnetic substance is a dissolved solution of iron(II) sulfate, which has one or more unpaired electrons in its orbitals.
These unpaired electrons are the main reason the solution shows paramagnetic behavior.

Diamagnetism

In contrast to paramagnetism, diamagnetism occurs in substances where all of the electrons are paired. Paired electrons have their magnetic fields cancel each other out, leaving no unpaired electrons or net magnetic moment. As a result, diamagnetic materials are not attracted to a magnetic field; rather, they are slightly repelled by it.
This is an intriguing property because it contrasts with the attractive behavior seen in paramagnetic substances. In the case of our exercise, adding ammonia (\(\text{NH}_3\)) to the iron(II) sulfate solution eventually transforms the system into a diamagnetic state. This happens because the resulting complex no longer has any unpaired electrons after completing its reaction.

Ligand Field Theory

Ligand field theory is a vital concept in coordination chemistry, especially in identifying the behavior of metal ions in complexes. It explains how ligands - molecules or ions - interact with metal ions' d orbitals, resulting in the splitting of these d orbitals. The extent of this splitting can greatly depend on the ligand's nature.
Strong field ligands, such as ammonia (\(\text{NH}_3\)), cause a more significant splitting of the d orbitals. They often lead to high energy differences between the split d orbitals and can cause electron pairing within these orbitals.
Understanding these interactions is key to understanding why certain complexes display specific magnetic properties such as paramagnetism or diamagnetism.

Electron Pairing

Electron pairing is an essential process where unpaired electrons in an atom or ion are paired together. This process often results from strong magnetic or electric fields typically introduced by strong field ligands.
Pairing changes the magnetic properties of a substance. Consider an iron(II) ion surrounded by ligands like \(\text{NH}_3\). As the ligand forms covalent bonds with the metal ion, the electrons in the d orbitals are subjected to repulsion and increased energy states, forming pairs, if energetically favorable.
When unpaired electrons pair up, they no longer contribute to magnetism, shifting the substance to be diamagnetic due to an absence of unpaired electrons.

Octahedral Complex

In coordination chemistry, the term octahedral complex describes the spatial arrangement of ligands around a central metal ion when six ligands form bonds with it. This geometery is based on an octahedron, where the central metal ion sits in the middle, and the ligands occupy the vertices.
For instance, when ammonia is added to an aqueous solution of iron(II) sulfate, it forms an octahedral complex by surrounding the \(\text{Fe}^{2+}\) ion.
This octahedral arrangement is significant because it influences the electronic configuration of the central metal ion and often alters its magnetic properties. When strong field ligands, like ammonia, adhere in an octahedral arrangement, significant changes in electronic and magnetic properties can occur, as observed in the transition from paramagnetic to diamagnetic behavior.