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Step-by-Step Explanation
Step 1: Examine the Electronic Configurations
• Manganese(II), Mn(II), typically has a $d^5$ electronic configuration.
• Nickel(II), Ni(II), typically has a $d^8$ electronic configuration.
Step 2: Analyze Statement (I)
Statement (I): "Both the complexes can be high spin."
• In an octahedral field with weak ligands, Mn(II) ($d^5$) usually remains high spin because pairing energy is relatively large, and all five d-electrons tend to remain unpaired.
• Tetrahedral Ni(II) ($d^8$) complexes often do not provide as large a splitting as octahedral fields, so they too are frequently high spin when the ligand is weak. Thus, both can be high spin under weak-field ligand conditions.
• Hence, statement (I) is correct.
Step 3: Analyze Statement (II)
Statement (II): "Ni(II) complex can very rarely be low spin."
• Tetrahedral Ni(II) ($d^8$) complexes have smaller splitting ($\Delta_t$) compared to octahedral complexes ($\Delta_o$). As a result, it is very rare to achieve a sufficiently strong ligand field that can force it into a low-spin arrangement in a tetrahedral geometry.
• Low-spin Ni(II) complexes are more commonly found in square-planar geometries with very strong field ligands, not in tetrahedral ones.
• Therefore, statement (II) is correct.
Step 4: Analyze Statement (III)
Statement (III): "With strong field ligands, Mn(II) complexes can be low spin."
• Mn(II) ($d^5$) in an octahedral field can become low spin only if the ligand field splitting ($\Delta_o$) is sufficiently large to overcome the pairing energy. Although it is more difficult for Mn(II) to be low spin compared to ions like Fe(II) or Co(III), it is possible in the presence of exceptionally strong field ligands.
• Hence, statement (III) is correct.
Step 5: Analyze Statement (IV)
Statement (IV): "Aqueous solution of Mn(II) ions is yellow in colour."
• In aqueous solution, Mn(II) typically imparts a pale pink colour, not yellow.
• Therefore, statement (IV) is incorrect.
Conclusion
Since statements (I), (II), and (III) are correct while (IV) is incorrect, the correct answer is:
(I), (II) and (III) only
