CBSE 2026 · Region 1 · Set 1 · Q30 · 4 marks

The Valence Bond Theory (VBT) explains the formation, magnetic behaviour and geometry of coordination compounds. 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. Answer the following questions :

(a)

In octahedral crystal field, energies of which d-orbitals will be raised when ligands approach the central metal atom/ion ? Give reason in support of your answer.

(b)

Using crystal field theory, write the electronic configuration of central metal atom/ion of the following :

(i)

$\displaystyle \left[\mathrm{CoF}_{6}\right]^{3-}$

(ii)

$\displaystyle \left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}$ [At. No. : $\displaystyle \mathrm{Co}=27$ ]

(c)OR(c)

$\displaystyle \left[\mathrm{NiCl}_{4}\right]^{2-}$ is paramagnetic while $\displaystyle \left[\mathrm{Ni}(\mathrm{CO})_{4}\right]$ is diamagnetic though both are tetrahedral. Why ? [Atomic No. : Ni = $\displaystyle 28$]

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.