JEE Main 2019 — Coordination Compounds Question with Solution

To determine which statements are correct, let's analyze each one individually in the context of Valence Bond Theory (VBT) as it applies to transition metal complexes.

Statement (I):

"Valence bond theory cannot explain the color exhibited by transition metal complexes."

Analysis:

Color in Transition Metal Complexes:

The colors of transition metal complexes arise from electronic transitions between different energy levels of the d-orbitals, specifically d-d transitions.

These transitions occur when an electron absorbs light energy and moves from a lower-energy d-orbital to a higher-energy d-orbital.

Valence Bond Theory Limitations:

VBT focuses on the hybridization of atomic orbitals to form covalent bonds.

It does not account for the splitting of d-orbitals into different energy levels in the presence of ligands (known as crystal field splitting).

Therefore, VBT cannot explain the origin of color in these complexes because it doesn't address the electronic transitions responsible for color.

Conclusion:

Statement (I) is correct.

Statement (II):

"Valence bond theory can predict quantitatively the magnetic properties of transition metal complexes."

Analysis:

Magnetic Properties:

The magnetic behavior of a complex depends on the number of unpaired electrons in the metal ion.

Quantitative prediction requires calculating the magnetic moment, often using the formula:

$\mu =\sqrt{n(n+2)}\text{Bohr Magnetons (BM)}$

where $n$ is the number of unpaired electrons.

Valence Bond Theory Capabilities:

VBT can provide a qualitative idea about the magnetic properties by indicating whether a complex is paramagnetic (unpaired electrons present) or diamagnetic (no unpaired electrons).

However, VBT does not offer the tools to quantitatively predict the exact magnetic moment.

Accurate quantitative predictions require more advanced theories like Crystal Field Theory (CFT) or Ligand Field Theory (LFT).

Conclusion:

Statement (II) is incorrect.

Statement (III):

"Valence bond theory cannot distinguish ligands as weak and strong field ones."

Analysis:

Weak and Strong Field Ligands:

Ligands are classified based on their ability to split the d-orbitals of the metal ion, influencing the pairing of electrons.

Strong field ligands cause a large splitting, often leading to low-spin complexes.

Weak field ligands cause small splitting, leading to high-spin complexes.

Valence Bond Theory Limitations:

VBT does not address the energy splitting of d-orbitals.

It assumes that all bonds are formed via overlap of orbitals without considering the effect of ligands on d-orbital energies.

Therefore, VBT cannot distinguish between weak and strong field ligands because it doesn't involve the spectrochemical series or orbital splitting concepts.

Conclusion:

Statement (III) is correct.

Final Answer:

Statements (I) and (III) are correct.

Statement (II) is incorrect.

Answer: Option D

(I) and (III) only