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Chemistry · 2025 · 4 marks
CBSE 2025 · Region 7 · Set 1 · Q29
The Crystal Field Theory (CFT) of coordination compounds is based on the effect of different crystal fields (provided by the ligands taken as point…
The Crystal Field Theory (CFT) of coordination compounds is based on the effect of different crystal fields (provided by the ligands taken as point charges) on the degeneracy of d-orbital energies of the central metal atom/ion. The splitting of the d-orbitals provides different electronic arrangements in strong and weak crystal fields. In tetrahedral coordination entity formation, the d-orbital splitting is smaller as compared to the octahedral entity. Answer the following questions :
(a)
On the basis of CFT, explain why $\displaystyle \left[\mathrm{Ti}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3}$ complex is coloured ? What happens on heating the complex $\displaystyle \left[\mathrm{Ti}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3}$ ? Give reason. [Atomic no. : $\displaystyle \mathrm{Ti}=22$ ]
(b)(i)
What is crystal field splitting energy ?
OR
(b)
(ii)
On the basis of $\displaystyle \Delta_{0}$ and P (pairing energy), how can you differentiate between a strong field ligand and a weak field ligand ?
(c)
Why are low spin tetrahedral complexes rarely observed ?
Marking-scheme solution
(a)
Due to presence of one unpaired electron in t2g which gets excited to eg / Due to
excitation energy t2g $\displaystyle 1$→ eg1, it gives colour. (d-d transition)
When heated, water is lost therefore crystal field splitting does not occur and it becomes
colourless.
(b)
The energy required to split the degenerate d-orbitals into two sets of orbitals (t2g and
eg). /The difference of energy between the two sets of d-orbitals t2g and eg due to the
presence of ligands in a definite geometry .
(b)(ii)
Δo < P, weak filed ligand
Δo > P, strong field ligand
(c)
Because the orbital splitting energies are not sufficiently large for forcing pairing / Due to low
crystal field splitting energy.
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CBSE Class 12 Chemistry past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.