Problem 101 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following complexes is diamagnetic in nature? (1) \(\mathrm{K}_{2}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]\) (2) \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]\left(\mathrm{NO}_{3}\right)_{2}\) (3) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}\) (4) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{2}\) Select the correct answer (a) 1 and 2 (b) 2 and 3 (c) 1,2 and 4 (d) 1,3 and 4

Step-by-Step Solution

Answer

The correct answer is (d) 1, 3, and 4.

Step 1: Identify the central metal ions and their oxidation states

For each complex, determine the oxidation state of the central metal ion. This will help in determining the electron configuration and the type of hybridization.1. In \( \mathrm{K}_{2}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right] \), Ni is in the +2 oxidation state.2. In \( \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]\left(\mathrm{NO}_{3}\right)_{2} \), Ni is in the +2 oxidation state.3. In \( \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3} \), Co is in the +3 oxidation state.4. In \( \left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{2} \), Pt is in the +2 oxidation state.

Step 2: Determine the electron configuration and magnetic properties

Using the oxidation states, determine the electron configuration and whether each compound is diamagnetic (all electrons are paired) or paramagnetic (unpaired electrons present).1. \( \mathrm{Ni}^{2+} \) in \( \mathrm{K}_{2}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right] \) has the configuration \( [Ar]3d^8 \). The strong field ligand CN- causes pairing, resulting in a square planar geometry with no unpaired electrons, making it diamagnetic.2. \( \mathrm{Ni}^{2+} \) in \( \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]\left(\mathrm{NO}_{3}\right)_{2} \) is high spin and has the configuration \( [Ar]3d^8 \). It has unpaired electrons due to the weak field ligand \( \mathrm{H}_{2}\mathrm{O} \), making it paramagnetic.3. \( \mathrm{Co}^{3+} \) in \( \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3} \) has the configuration \( [Ar]3d^6 \). With NH3 as a strong field ligand, electrons pair completely in an octahedral geometry, making it diamagnetic.4. \( \mathrm{Pt}^{2+} \) in \( \left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{2} \) has the configuration \( [Xe]4f^{14}5d^8 \). NH3 causes full pairing in the square planar form, resulting in a diamagnetic complex.

Step 3: Identify the diamagnetic complexes

From the above deductions, identify the complexes that are diamagnetic:1. \( \mathrm{K}_{2}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right] \) is diamagnetic.2. \( \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]\left(\mathrm{NO}_{3}\right)_{2} \) is paramagnetic.3. \( \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3} \) is diamagnetic.4. \( \left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{2} \) is diamagnetic.

Step 4: Determine the correct answer option

Compare the diamagnetic complexes with the answer choices and select the one that matches: - Complexes 1, 3, and 4 are all diamagnetic. - Therefore, the correct answer is option (d) 1, 3, and 4.

Key Concepts

Coordination Chemistry

Coordination chemistry is a fascinating area of chemistry focused on the structure, bonding, and reactivity of compounds formed between metal ions and ligands. In complexes, metal ions are the central atoms that bind to molecules or ions called ligands.
Ligands are electron-rich substances capable of donating electron pairs to metals, forming coordinate covalent bonds.

• These ligands can be neutral molecules like water (\(\text{H}_2\text{O}\)) or ammonia (\(\text{NH}_3\)), or charged ions like cyanide (\(\text{CN}^−\)).
• The metal-ligand bonds form around a central metal atom or ion, and the complete entity is known as a coordination complex.

Coordination chemistry not only delves into the geometry of these complexes but also into their electronic properties.
Understanding how ligands interact with central metal ions helps predict the magnetic properties of the complex, such as whether they are paramagnetic or diamagnetic.

Ligand Field Theory

Ligand Field Theory (LFT) is an extension of Crystal Field Theory (CFT) that focuses on understanding the interaction between ligands and the d-orbitals of metal ions in coordination complexes. LFT considers the effects of ligand's electronic field on the energy levels of the metal's d-orbitals.
When ligands approach a metal ion, they can cause a splitting of the metal's degenerate d-orbitals into different energy levels. This can affect whether a complex will have unpaired electrons or not.

• Strong field ligands like CN\(^−\) and NH\(_3\), cause significant splitting, often leading to the complete pairing of electrons, which results in diamagnetic properties.
• Weak field ligands such as \(\text{H}_2\text{O}\) cause less splitting of the d-orbitals, often resulting in unpaired electrons, which make the complex paramagnetic.

By analyzing ligand strength using LFT, we can predict the magnetic nature of a complex—whether it is likely diamagnetic (no unpaired electrons) or paramagnetic (some unpaired electrons). This theory is critical for understanding many of the magnetic and spectral properties of coordination compounds.

Electron Configuration

Electron configuration is a way chemists describe the arrangement of electrons around the nucleus of an atom. In coordination chemistry, determining the electron configuration of the central metal ion is essential for predicting the electronic, magnetic, and structural properties of the complex.
Starting from the elemental form of the metal, electrons are removed or added to reach the oxidation state observed in the metal complex.

• For example, \(\text{Ni}^{2+}\) has the electron configuration \([\text{Ar}] 3d^8\), meaning it has lost two electrons from its 4s and 3d orbitals.
• The electron configuration directly influences the ion's bonding and magnetic properties.

Understanding electron configurations allows us to better predict whether a complex will contain paired or unpaired electrons, helping to identify diamagnetic or paramagnetic compounds.

Oxidation States

Oxidation state, also known as oxidation number, is a concept used in chemistry to describe the degree of oxidation (loss of electrons) of a chemical species. In coordination chemistry, oxidation states help determine how many electrons the metal ion has lost or gained.
This is crucial for predicting the configuration of electrons in the metal ion, which in turn affects the complex's properties.

• For instance, in the complex \(\text{K}_2[\text{Ni}(\text{CN})_4]\), nickel is in the +2 oxidation state, meaning it has lost 2 electrons compared to its elemental form.
• Knowing the oxidation state allows us to allocate electrons correctly when writing electron configurations.

By understanding the oxidation states, chemists can determine how the d-orbitals are filled or emptied, influencing the complex's magnetic behavior. Mastering oxidation states is fundamental to predicting and explaining the characteristics of transition metal complexes.