Question

The d-electron configurations of Cr²⁺, Mn²⁺, Fe²⁺ and Co²⁺
are d⁴, d⁵, d⁶ and d⁷, respectively. Which one of the
following will exhibit minimum paramagnetic behaviour?
(a) [Mn(H₂O)₆]²⁺ (b) [Fe(H₂O)₆]²⁺
(c) [Co(H₂O)₆]²⁺ (d) [Cr(H₂O)₆]²⁺
(At, nos. Cr = 24, Mn = 25, Fe = 26, Co = 27)

Answer 1

Out of the given ions, one with the minimum number of unpaired electrons will exhibit minimum paramagnetic behaviour as:

1. Cr²⁺ will have 4 unpaired electrons in their orbital.

2. Mn²⁺ will have 5 unpaired electrons in their orbital.

3. Fe²⁺ will have 4 unpaired electrons in their orbital.

4. Co²⁺ will have 3(minimum) unpaired electrons in their orbital.

hence, (c) [Co(H₂O)₆]² is the correct answer.

Answer 2

[Co(H₂O)₆]²⁺ will have least paramagnetic behaviour.

Explanation:

• Paramagnetism is a phenomenon where the ion or radicle responds to the magnetic field.

• This phenomenon of paramagnetism is exhibited by the ions or radicles with unpaired electrons.

• The unpaired electrons can vibrate without any crossing effect from other unpaired electron to produce the magnetic field.

• The amount of paramagnetism seen in a ion or radicle is directly proportional to the number of unpaired electrons present in the species.

• In Mn²⁺, the configuration is d5.

• So it has 5 unpaired electrons.

• In Fe²⁺, the configuration is d6.

• So it has 4 unpaired electrons.

• In Cr²⁺, the configuration is d4.

• So it has 4 unpaired electrons.

• In Co²⁺ the configuration is d7.

• So it has 3 unpaired electrons.

• So Co²⁺ has the least amount of unpaired electrons in it.

• Thus it will be least paramagnetic.

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