Problem 10 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

For octahedral Mn(II) and tetrahedral Ni(II) complexes, consider the following statements: (I) both the complexes can be high spin. (II) Ni(II) complex can very rarely below spin. (III) with strong field ligands, Mn(II) complexes can be low spin. (IV) aqueous solution of \(\mathrm{Mn}(\mathrm{II})\) ions is yellow in color. The correct statements are : (a) (I) and (II) only (b) (I), (III) and (IV) only (c) (I), (II) and (III) only (d) (II), (III) and (IV) only

Step-by-Step Solution

Answer

The correct answer is (c) (I), (II), and (III) only.

Step 1: Analyze Mn(II) Complex

Manganese (II) in an octahedral complex typically has a high spin configuration due to weak-field ligands, or a low spin configuration in the presence of strong-field ligands. Hence, statement (I) and statement (III) regarding Mn(II) being potentially high spin or low spin with strong field ligands as options are possible.

Step 2: Evaluate Ni(II) Complex

Nickel (II) in a tetrahedral complex is usually high spin since the ligand field splitting is lower than that in octahedral complexes. It's uncommon for Ni(II) tetrahedral to be low spin, making statement (II) about the rarity of low spin conditions typically true. Statement (I) about being high spin also applies to Ni(II) complexes.

Step 3: Investigate Color of Mn(II) in Aqueous Solution

Manganese (II) in aqueous solution usually appears pale pink, not yellow. Therefore, statement (IV) about the yellow color is incorrect.

Step 4: Identify Correct Statements

Given the analysis of the statements: (I) both Mn(II) and Ni(II) complexes can be high spin (correct), (II) Ni(II) complexes are rarely low spin (correct), and (III) Mn(II) can be low spin with strong field ligands (correct), we find that statements (I), (II), and (III) are correct.

Key Concepts

High Spin Complexes

High spin complexes arise when a metal ion in a coordination complex has electrons that fill in the highest energy orbitals first. These complexes are typically associated with weak field ligands, such as water or fluoride, that do not produce a significant splitting of the d-orbitals in the metal ion.

Here’s how it works: the electrons prefer to occupy different orbitals as much as possible due to the minimization of electron repulsion. In a high spin complex, the energy difference (also known as the ligand field splitting energy, \( \Delta \)) between the two sets of d-orbitals is small, allowing the electrons to remain unpaired in higher energy orbitals instead of pairing up in lower ones.

• This results in a higher total spin state, with more unpaired electrons.
• Examples include common elements like Manganese (Mn) in octahedral complexes when paired with weak field ligands.

High spin states often lead to interesting magnetic properties because of the unpaired electrons, causing compounds to be paramagnetic.

Low Spin Complexes

Low spin complexes, in contrast, form when strong field ligands, such as cyanide or carbon monoxide, cause a large splitting of the metal’s d-orbitals. This large energy gap makes it energetically more favorable for electrons to pair up in the lower energy orbitals rather than occupy higher energy orbitals.

When this occurs, fewer unpaired electrons are present, which significantly changes the magnetic properties. The resulting complexes are often diamagnetic because all electrons are paired.

• An example is Manganese (II) in octahedral complexes, but only when strong field ligands are present, forcing the electrons to pair up.
• This can also lead to changes in the color and reactivity of the complex due to differences in electronic structure.

Understanding these differences is key to predicting the physical properties of coordination compounds.

Ligand Field Theory

Ligand field theory (LFT) is a concept used to describe the breaking of orbital degeneracy in transition metal complexes due to the presence of ligands. It combines elements of crystal field theory and molecular orbital theory to explain the electronic structure and properties of these complexes.

Ligands, which are molecules or ions that donate an electron pair to the metal, interact with the metal’s d-orbitals, causing rearrangement of electron distribution. The extent of this interaction varies with different ligands and affects the splitting energy, \( \Delta \).

• Strong field ligands result in a larger splitting energy and typically form low spin complexes.
• Weak field ligands cause a smaller splitting energy, favoring high spin states.

In LFT, the geometry of the complex, such as octahedral or tetrahedral, also influences the degree of splitting. This theory helps predict and explain various properties such as magnetic behavior, color, and reactivity, making it a foundational concept in coordination chemistry.