Problem 116 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following are paramagnetic tetrahedral species? (a) \(\left[\mathrm{FeCl}_{4}\right]^{-}\) (b) \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}\) (c) \(\left[\mathrm{NiCl}_{4}\right]^{2-}\) (d) \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\)

Step-by-Step Solution

Answer

(a) \\([\text{FeCl}_4]^-\\) and (c) \\([\text{NiCl}_4]^{2-}\\) are paramagnetic.

Step 1: Understand Paramagnetism

Paramagnetism occurs in species with one or more unpaired electrons. To determine this, we need to analyze the electron configuration of the metal ions and check if unpaired electrons are present.

Step 2: Analyze each Complex

For each complex, determine the oxidation state of the central metal ion, find the electron configuration, and check if there are any unpaired electrons.

Step 3: Evaluate \\([\text{FeCl}_4]^-\\)

Fe is in a +3 oxidation state. The electron configuration is [Ar] 3d\(^{5}\). There are 5 unpaired electrons, making it paramagnetic.

Step 4: Evaluate \\([\text{Ni}(\text{CN})_4]^{2-}\\)

Ni is in a +2 oxidation state. In the presence of strong field ligands like CN, the electron configuration is [Ar] 3d\(^{8}\), which results in paired electrons due to crystal field splitting. It is diamagnetic.

Step 5: Evaluate \\([\text{NiCl}_4]^{2-}\\)

Ni is again in a +2 oxidation state. With weak field ligands like Cl, the electron configuration remains [Ar] 3d\(^{8}\), with 2 unpaired electrons, making it paramagnetic.

Step 6: Evaluate \\([\text{Zn}(\text{NH}_3)_4]^{2+}\\)

Zn is in a +2 oxidation state. The electron configuration is [Ar] 3d\(^{10}\) with all electrons paired, making it diamagnetic.

Step 7: Conclusion

The paramagnetic tetrahedral species are the ones with unpaired electrons. From the analysis, these are \([\text{FeCl}_4]^-\) and \([\text{NiCl}_4]^{2-}\).

Key Concepts

Unpaired Electrons

When it comes to understanding paramagnetism in coordination compounds, unpaired electrons play a crucial role. An atom or ion is considered paramagnetic if there are one or more unpaired electrons in its outer shell. These unpaired electrons align with external magnetic fields, causing the material to be magnetically attracted.
In the context of coordination compounds, the presence of unpaired electrons in the d-orbitals of transition metals often determine if a compound is paramagnetic or not.
For instance, in the complex \( ext{[FeCl}_4]^ -\), iron's configuration includes five unpaired electrons, making it strongly paramagnetic.

Here’s a simple rule to remember:

• Unpaired electrons = Paramagnetic
• All paired electrons = Diamagnetic

Identifying unpaired electrons is essential in determining the magnetic properties of coordination complexes.

Electron Configuration

Electron configuration is a method used to represent the arrangement of electrons within an atom. Understanding electron configuration is vital for determining the number of unpaired electrons.
For transition metals involved in coordination compounds, the focus is often on the d-block electrons.
Each electron is filled sequentially in the orbitals according to their energy levels, which are influenced by the ligand field. For example:

• In \( ext{[FeCl}_4]^ -\), iron (Fe) in its +3 oxidation state has an electron configuration of [Ar] 3d\(^{5}\), indicating 5 unpaired electrons.
• Conversely, in \( ext{[Ni(CN)}_4]^{2-}\), nickel (Ni) with a +2 oxidation state and strong field ligands like CN results in a [Ar] 3d\(^{8}\) configuration, implying that all electrons are paired.

This configuration directly influences whether a compound will exhibit paramagnetism or diamagnetism.

Oxidation States

The oxidation state of a metal in a coordination compound is crucial in determining its electron configuration and, subsequently, its magnetic properties. The oxidation state represents the charge of the metal after accounting for the electron sharing between the metal and ligands.
Knowing the oxidation state helps establish the number of d electrons that are left with the metal ion after forming the complex.
For instance:

• In \( ext{[FeCl}_4]^{-}\), the oxidation state of Iron is +3, making it ext{Fe}^{3+}, which influences its electron configuration to be [Ar] 3d\(^{5}\).
• For \( ext{[Zn(NH}_3)_4]^{2+}\), Zinc is in the +2 state, leading to a [Ar] 3d\(^{10}\) configuration with all electrons paired.

Determining the oxidation state is a foundational step for predicting the magnetic behavior of complexes.

Crystal Field Theory

Crystal field theory provides a model that helps predict the electronic structure of metal ions in coordination compounds and how they affect magnetic properties. This theory considers how ligands affect the energy of the metal d orbitals, leading to a split in energy levels.
The extent of this splitting depends on the ligand field strength:

• Strong field ligands (like CN\(^-\)) cause a significant energy split, often resulting in electron pairing in lower energy orbitals, leading to diamagnetism.
• Weak field ligands (like Cl\(^-\)) cause smaller splits, allowing electrons to remain unpaired.

For example, in \( ext{[Ni(CN)}_4]^{2-}\), CN\(^-\) causes a strong field splitting, and all d electrons are paired, while in \( ext{[NiCl}_4]^{2-}\), Cl\(^-\) leads to 2 unpaired electrons due to weaker splitting.
Understanding crystal field theory is pivotal to explaining the paramagnetic or diamagnetic nature of coordination compounds.