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) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)\right] \mathrm{Cl}_{3}\) (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: Understanding Diamagnetism

Diamagnetism occurs in complexes when all electrons are paired. First, identify if each complex has unpaired electrons by examining its electron configuration.

Step 2: Analyze Complex 1: \(\mathrm{K}_{2}\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]\)

Nickel in this complex is in the +2 oxidation state, giving it an electron configuration of \([Ar] 3d^8\). The strong field ligand \(CN^-\) will pair the electrons, leading to no unpaired electrons. Thus, this complex is diamagnetic.

Step 3: Analyze Complex 2: \(\left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]\left(\mathrm{NO}_{3}\right)_{2}\)

Nickel remains in the +2 oxidation state, \([Ar] 3d^8\). Water is a weak field ligand, which means it does not pair all electrons. As a result, this complex has unpaired electrons, making it paramagnetic.

Step 4: Analyze Complex 3: \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)\right] \mathrm{Cl}_{3}\)

Cobalt is in the +3 oxidation state here, giving it the electron configuration \([Ar] 3d^6\). \(NH_3\) is an intermediate field ligand causing pairing of all electrons, making the complex diamagnetic.

Step 5: Analyze Complex 4: \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{Cl}_{2}\)

Platinum in this complex has the +2 oxidation state, \([Xe] 5d^8\). \(NH_3\) encourages electron pairing, resulting in a full complement of paired electrons. This complex is diamagnetic.

Step 6: Conclusion: Find Diamagnetic Complexes

Based on the analysis, complexes 1, 3, and 4 are diamagnetic as they have all electrons paired.

Key Concepts

Complex Ion Analysis

Complex ion analysis is a crucial step to understanding how complex compounds behave, including their magnetic properties. A complex ion is a molecular entity formed when central metal atoms or ions bond with surrounding molecules or ions, known as ligands. This process results in a coordination compound with unique chemical and physical properties.

When analyzing complex ions, consider the following:

• Identify the central metal and its oxidation state, as this affects the electron configuration and distribution.
• Determine the nature of the ligands surrounding the metal ions. Ligands can influence the arrangement and pairing of electrons.
• Examine the coordination number, which is the number of ligand donor atoms bonded to the central metal ion.

These factors help predict whether a complex will be diamagnetic or paramagnetic based on whether electrons are paired or unpaired.

Electron Pairing

Electron pairing involves the redistribution of electrons in an atom's orbitals, especially when forming complex ions. The electron configuration and spin state of a metal ion change depending on the ligands bonded to it.

Why is electron pairing important?

• It dictates the magnetic properties of a compound. If all electrons in complexes are paired, the complex is diamagnetic and does not respond to magnetic fields.
• Unpaired electrons lead to paramagnetism, which causes attraction to magnetic fields due to the presence of one or more unpaired electrons.

The configuration of electrons is significantly altered by the field strength of the ligands, which can cause pairing or retain unpaired electrons.

Oxidation States

Understanding the oxidation state of a central metal ion in a complex is essential to determining its magnetic and chemical behavior. The oxidation state denotes the hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic.

Key points about oxidation states include:

• The oxidation state affects the electron count of the metal ion and hence its electron configuration.
• Electron configuration, in turn, tells us about the possible electron pairing. For example, Ni in a "+2" state will have a different configuration than Ni in a "0" state.
• Changing the oxidation state can lead to different magnetic properties, as seen in the paramagnetic and diamagnetic nature of complexes.

Analyzing oxidation states helps to predict electron arrangements and therefore whether electrons will be paired or unpaired in the complex.

Field Strength of Ligands

The field strength of ligands plays a pivotal role in the behavior of complex ions, particularly in the context of electron pairing and magnetic properties. The field strength is often described using the "spectrochemical series," which ranks ligands based on their ability to cause splitting of d-orbitals in transition metals.

Here’s why field strength matters:

• Strong field ligands, like CN⁻ and NH₃, cause a larger splitting of d-orbitals and often promote pairing of electrons. This leads to diamagnetic complexes.
• Weak field ligands, such as H₂O, result in smaller splitting, often maintaining unpaired electrons and resulting in paramagnetism.
• The choice of ligand will affect whether electrons adopt a low-spin or high-spin configuration, influencing the magnetic properties profoundly.

By understanding the field strength of ligands, one can predict and explain which complexes will have paired or unpaired electrons and whether they will be attracted to a magnetic field.