CBSE 2025 · Region 7 · Set 1 · Q29 · 4 marks
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)What is crystal field splitting energy ?(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 ?
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)
What is crystal field splitting energy ?
(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 .
(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 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.