CBSE 2026 · Region 2 · Set 1 · Q25 · 3 marks
(a)$\displaystyle \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$ is diamagnetic whereas $\displaystyle \left[\mathrm{CoF}_{6}\right]^{3-}$ is paramagnetic. Justify the statement. [Atomic number of $\displaystyle \mathrm{Co}=27$ ](b)Write the electronic configuration for $\displaystyle \mathrm{d}^{4}$ ion if $\displaystyle \Delta_{0}>\mathrm{P}$ on the basis of crystal field theory.
(a)
$\displaystyle \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$ is diamagnetic whereas $\displaystyle \left[\mathrm{CoF}_{6}\right]^{3-}$ is paramagnetic. Justify the statement. [Atomic number of $\displaystyle \mathrm{Co}=27$ ]
(b)
Write the electronic configuration for $\displaystyle \mathrm{d}^{4}$ ion if $\displaystyle \Delta_{0}>\mathrm{P}$ on the basis of crystal field theory.
Marking-scheme solution
(a)
In [Co(NH3)$\displaystyle 6$]$\displaystyle 3$+, NH3 being a strong field ligand, causes pairing.
∴ no unpaired electrons hence it is diamagnetic.
In [CoF6]$\displaystyle 3$−, F− being a weak field ligand cannot cause pairing. It has unpaired
electrons and is paramagnetic.
(b)
t2g $\displaystyle 4$ eg°
Coordination CompoundsBonding in Coordination CompoundsUnderstandshort_answerhard
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CBSE Class 12 Chemistry past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.