Read the passage carefully and answer.
The crystal field theory (CFT) is an electrostatic model which considers the metal-ligand bond to be ionic arising purely from electrostatic interactions between the metal ion and the ligand. Ligands are treated as point charges in case of anions or dipoles in case of neutral molecules. The five d orbitals in an isolated gaseous metal atom/ion have same energy, i.e., they are degenerate. This degeneracy is maintained if a spherically symmetrical field of negative charges surrounds the metal atom/ion. However, when this negative field is due to ligands (either anions or the negative ends of dipolar molecules like \( NH_3 \) and \( H_2O \)) in a complex, it becomes asymmetrical and the degeneracy of the d orbitals is lifted. It results in splitting of the d orbitals. The pattern of splitting depends upon the nature of the crystal field.
The colour of coordination compounds is attributed to
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → d-d transition
In a complex the ligands split the d orbitals of the metal ion into sets of different energy. The gap between them matches the energy of visible light. When light falls on the complex, an electron in the lower set absorbs light of that energy and jumps to the higher set. This jump within the d orbitals is a d–d transition, and the complex shows the colour complementary to the light absorbed.
In \( \ce{[Ti(H2O)6]^3+} \), Ti (Z = 22) is \( \mathrm{[Ar]\,3d^{2}\,4s^{2}} \), so \( \ce{Ti^3+} \) is \( \mathrm{3d^{1}} \). Without ligands there is no splitting and no colour: anhydrous \( \ce{CuSO4} \) is white, while \( \ce{CuSO4.5H2O} \) is blue.
Why the other options are incorrect
Option (1) s-p transition — Incorrect. The ligand field splits the d orbitals; s to p jumps are not what gives coordination compounds their colour.
Option (3) d-f transition — Incorrect. The d-block ions concerned have no partly filled f orbitals to take part.
Option (4) p-p* transition — Incorrect. Jumps into antibonding (π*) orbitals belong to organic molecules with multiple bonds, not to the crystal field picture of complexes.
NCERT (2026-27 edition):
Colour in Coordination Compounds – …
