Question

Which of the following species exhibits both LMCT and paramagnetism?

• A. MnO⁻₄
• B. MnO²⁻₄
• C. Cr₂O²⁻₇
• D. CrO²⁻₄

Hint:

Paramagnetism arises from unpaired electrons, while LMCT involves electron transfer between ligand and metal.

Answer 1

The Correct Option is B

To analyze the problem, we consider two properties:

• Ligand-to-Metal Charge Transfer (LMCT): LMCT occurs when there is a transfer of electrons from the ligand (oxygen) to the metal ion (Mn or Cr) due to a significant energy difference between the ligand and metal orbitals.
• Paramagnetism: Paramagnetic species have unpaired electrons in their electronic configuration.

Analysis of the given species:

• MnO⁻₄ (Permanganate ion): In MnO⁻₄, Mn is in the +7 oxidation state with no unpaired electrons (electronic configuration: 3d⁰). This species shows LMCT but is diamagnetic.
• MnO²⁻₄ (Manganate ion): In MnO²⁻₄, Mn is in the +6 oxidation state with one unpaired electron (electronic configuration: 3d¹). This species exhibits both LMCT and paramagnetism.
• Cr₂O²⁻₇ (Dichromate ion): In Cr₂O²⁻₇, Cr is in the +6 oxidation state with no unpaired electrons (electronic configuration: 3d⁰). This species is diamagnetic and shows LMCT.
• CrO²⁻₄ (Chromate ion): In CrO²⁻₄, Cr is also in the +6 oxidation state with no unpaired electrons. This species shows LMCT but is diamagnetic.

Conclusion: Among the given options, only MnO²⁻₄ exhibits both LMCT and paramagnetism due to the presence of one unpaired electron and the ability of oxygen ligands to transfer charge to Mn. Therefore, the correct answer is: 2. MnO²⁻₄.