Why Fe3+ is more stable than Fe2+?

Ever wondered why Fe3+ is more stable than Fe2+? If you’re into chemistry, you might have seen these two forms of iron and wondered why they’re different. Learning about this can help us understand iron better and how it reacts with other chemicals.

It’s really important to know why Fe3+ and Fe2+ are different in stability. This affects lots of chemical stuff. It changes how iron ions act with other things, do redox reactions, and make compounds with iron more reactive. If we figure out why this happens, we can learn a whole lot more than just about iron.

You are going to solve this puzzle about why Fe³⁺ is more stable than Fe²⁺. Fe²⁺ and Fe³⁺ may differ in their oxidation state and physical and chemical properties. They may also differ in their names. Fe²⁺ is ferrous and Fe³⁺ is ferric. The compounds of Fe2+ and Fe³⁺ are ionic in nature.

So let’s dive right in .

Factors contributing to the stability of Fe3+ ion:

• High charge of Fe3+ ion
• Strong electrostatic attraction between Fe3+ ion and surrounding ions or molecules
• Formation of stable coordination complexes with ligands
• Presence of counterions to stabilize the charge balance
• Favorable crystal field splitting in solid-state compounds containing Fe3+ ion

F3+ ions are more stable than Fe2+ ions because of different factors. We will talk about these factors now.

The high charge density of Fe3+

One significant factor that contributes to the stability of Fe3+ is its high charge density. With three positive charges, Fe3+ has a stronger attraction for negatively charged species compared to Fe2+, which only carries two positive charges. This increased charge density allows Fe3+ ions to form stronger electrostatic interactions with surrounding ligands and other molecules, enhancing their overall stability.

Ligand interactions

Ligand interactions also play a crucial role in stabilizing Fe3+ ions. Ligands are molecules or ions that bond with metal ions like Fe3+. They form coordinate bonds by donating electron pairs to the metal ion, effectively surrounding it and reducing its reactivity. These ligands can be neutral molecules or anions, such as water (H2O) or chloride (Cl-).

When ligands stick to the iron atom in a special shape, it makes a strong and stable complex around the Fe3+ ion. The strength of the bond between the ligand and the metal affects how stable it is. Stronger bonds mean more stability.

Crystal field stabilization energy (CFSE)

The stability of Fe3+ is also affected by something called crystal field stabilization energy (CFSE). This happens when the d-orbitals of the iron atom interact with the ligands around it. In a complex with six ligands arranged in an octahedral shape, the d-orbitals split into higher-energy e.g. orbitals and lower-energy t2g orbitals.

The CFSE is the energy difference between these two sets of orbitals. When the ligands interact with the Fe3+ ion, they cause a redistribution of electrons within the d-orbitals, resulting in increased stability. The magnitude of CFSE depends on several factors such as ligand type and geometry.

Importance of half-filled d configuration in Fe3+

The presence of four unpaired electrons in its d orbitals makes Fe3+ highly susceptible to magnetic interactions. These unpaired electrons align their spins with an external magnetic field, resulting in enhanced magnetic properties. This phenomenon is widely exploited in various applications such as data storage devices and electromagnets.

Moreover, the half-filled d orbitals also contribute significantly to stabilizing complexes involving Fe3+. The empty spaces within these partially filled orbitals allow for effective coordination with ligands – atoms or molecules that bond to a central metal ion. Ligands can form coordinate bonds by donating lone pairs of electrons to vacant orbitals on Fe3+, resulting in complex formation.

Role of full d configuration in Fe2+ stability:

Firstly, the presence of a complete set of d orbitals provides more opportunities for effective overlap with ligand orbitals. This overlap leads to stronger bonding between the metal ion (Fe2+) and its surrounding ligands. As a result, complex formation becomes more favorable, stabilizing Fe2+ in these compounds.

Moreover, when all the d orbitals are completely filled, there is less room for electron promotion or excitation. This means that Fe2+ is less likely to undergo oxidation reactions compared to its partially filled counterpart, Fe3+. The reduced reactivity of Fe2+ stems from its stable electronic configuration and lower electron transfer potential.

Reason Why Fe3+ is more stable?

If we imagine the electronic configuration of  Fe²⁺ and Fe³⁺, we should observe that Fe³⁺ is more stable. In the configuration of Fe²⁺, the 2 electrons are removed from the 4s orbital because the nucleus attracts other prior orbitals with more attraction so it is easy to remove. on the other hand, the half-filled and completely-filled orbitals are more stable than that of partially filled. so in a configuration of Fe³⁺, there is half filled d orbital and in Fe²⁺ there is a partially filled d orbital.

Fe2+ (Ferrous):

• It is pale yellow and turns violet when reacting with water. 
• Fe²⁺ shows the paramagnetic property. If it forms a low-spin complex it may be diamagnetic. Paramagnetic property is the property that shows that it has an unpaired electron in its outermost shell.
• Fe²⁺ oxidizes to the Fe³⁺ ion in the presence of KMnO4, and HNO3. In addition to KMnO4, the pink color it decolorizes shows the exact oxidation of Fe2+.
• Another colored complexes test to distinguish between Fe²⁺ and Fe³⁺ is that Fe²⁺ forms a red-orange color when reacting with amine ligands.

Fe3+ (Ferric):

• It is yellow-brown when reacting with the water.
• Fe³⁺ shows paramagnetic behavior. 
• Fe³⁺ gives the read solution when reacting with the thiocyanate ions. This method is used to flake blood in movies and films.
• The basic difference between Fe²⁺ and Fe³⁺ is the number of electrons. 

General configuration of Fe:

Iron is a chemical element with the atomic number 26. Its oxidation states range from -2 to +6. But now here I will illustrate the configuration of Fe²⁺ and Fe³⁺. The general configuration of Fe is [Ar] 3d⁶, 4s².

One key factor contributing to the increased stability of Fe3+ is the presence of a half-filled d orbital. According to Hund’s rule, electrons tend to occupy separate orbitals with parallel spins before pairing up. In this case, with five electrons in the 3d orbital of Fe3+, each electron occupies a separate orbital with parallel spins, adhering to Hund’s rule. This arrangement minimizes electron-electron repulsion and maximizes the overall stabilization energy.

To further illustrate the significance of this electronic configuration, consider the comparison between Fe2+ and Fe3+. The electronic configuration for Fe2+ is [Ar] 3d^6, which means there are two unpaired electrons in the d orbital. These unpaired electrons result in higher electron-electron repulsion and decreased stabilization energy compared to Fe3+. Consequently, Fe2+ is less stable than its counterpart.

Variable Electrovalence:

Some elements show more than one valency is known as variable valency. For example, in ferric sulfate Fe₂(SO₄)₃, the valency of Fe is 3. In ferrous sulfate FeSO₄, the valency of Fe is 2. Variable electrovalency depends upon the stability of the core. If all the valence electrons are removed by an atom, the residue obtained is called the core. The core obtained from the normal element is more stable if it has 2 or 8 electrons. The electronic configuration of the sodium atom is:

1s², 2s², 2p⁶, 3s¹

When sodium removes its 3s¹ electron and attains the electronic configuration of noble gas (neon), the residue obtained is called the core. The remaining residue is stable if it has two or eight electrons or vice versa. The d-block elements show variable valency.

Example Along Period

Fe₂₆

Fe₂₆ = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d⁶

When two electrons are removed from Fe, then we get;

Fe²⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁶

We remove 4s² in Fe₂₆ because the fourth shell is completely removed and the nucleus attracts the remaining three shells with greater force. When three electrons are removed from Fe₂₆ than we get;

Fe³⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁵

This Fe³⁺ is more balanced than Fe²⁺ because half-filled and completely filled is more stable than partially-filled.

Co₂₇

Co₂₇ = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d⁷

When two electrons are removed, then we get;

Co²⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁷

When three electrons are removed from Co₂₇, then we get;

Co³⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁶

Co²⁺ is more stable than Co. When we compare Co²⁺ and Co³⁺, both are equally stable but Co³⁺ is slightly more stable than Co²⁺.

Ni₂₈

Ni₂₈ = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d⁸

Ni²⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁸

Ni³⁺ = 1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁷

Ni²⁺ is more stable than Ni. Ni²⁺ is more common and more stable than Ni³⁺ because it is only two steps away from completion and completes the valence shell.