Colour of Complexes — d-d transitions
Coordination Compounds
15
JEE Qs
8%
Hard
60
min
Master the spectrochemical series and the concept of complementary colours to accurately predict the colour of a given transition metal complex based on its ligand and metal oxidation state.
🧮 Key Formulas
✅ Key Points for JEE
- 1The colour of transition metal complexes primarily arises from d-d transitions, where an electron absorbs energy from visible light to jump between split d-orbitals (t2g and eg in octahedral/tetrahedral fields).
- 2The energy absorbed (Δo or Δt, Crystal Field Splitting Energy) corresponds to a specific wavelength of incident light; the colour observed is its complementary colour.
- 3The magnitude of Δ (and thus the colour) is influenced by the nature of the ligand (stronger ligands = larger Δ), the oxidation state of the metal ion (higher oxidation state = larger Δ), and the geometry of the complex.
- 4Complexes with d^0 or d^10 electronic configurations, or those with completely filled/empty split d-orbitals where d-d transitions are forbidden, are colourless (e.g., Sc3+, Zn2+).
- 5Using the spectrochemical series is crucial to predict the relative magnitude of Δ and thereby the absorbed and observed colours for different ligands.
⚠️ Common Mistakes
- ✕Confusing the colour of light absorbed by the complex with the colour that is observed (the complementary colour).
- ✕Incorrectly predicting colour for complexes with d^0 or d^10 configurations, or for high spin d^5 complexes where all d-orbitals are half-filled (e.g., Mn^2+).
- ✕Incorrectly applying the spectrochemical series or failing to consider the oxidation state of the metal when comparing Δ values and predicting colours.
📝 Practice Questions
See allQ74.In the Claisen-Schmidt reaction to prepare, dibenzalacetone from 5.3 g of benzaldehyde, a total of 3.51 g of product was obtained. The percentage yield in this reaction was ______ %.
Q61.The calculated spin-only magnetic moments of K3 [Fe(OH)6] and K4 [Fe(OH)6] respectively are : (1) 3.87 and 4.90 B.M. (2) 4.90 and 5.92 B.M. (3) 4.90 and 4.90 B.M. (4) 5.92 and 4.90 B.M.
Q54.Identify the homoleptic complexes with odd number of d electrons in the central metal : (A) [FeO4]2− (B) [Fe(CN)6]3− (C) [Fe(CN)5NO]2− (D) [CoCl4]2− (E) [Co(H2O)3 F3] Choose the correct answer from the options given below : (1) (A), (B) and (D) only (2) (C) and (E) only (3) (B) and (D) only (4) (A), (C) and (E) only
Q68.In which of the following complexes the CFSE, Δo will be equal to zero? (1) [Fe(en)3]Cl3 (2) K4 [Fe(CN)6] (3) [Fe(NH3)6]Br2 (4) K3 [Fe(SCN)6]
Q70.From the magnetic behaviour of [NiCl4]2− (paramagnetic) and [Ni(CO)4] (diamagnetic), choose the correct geometry and oxidation state. (1) [NiCl4]2−: NiII , tetrahedral [Ni(CO)4] : NiII , (2) [NiCl4]2− : NiII , square planar [Ni(CO)4] : Ni(0) square planar , square planar (3) [NiCl4]2−: NiII , tetrahedral [Ni(CO)4] : Ni(0), (4) [NiCl4]2−: Ni(0), tetrahedral [Ni(CO)4] : Ni(0), tetrahedral square planar
Q53.The correct order of the following complexes in terms of their crystal field stabilization energies is : (1) [Co(NH3)4]2+ < [Co(NH3)6]2+ < [Co(en)3]3+ < [Co(NH3)6]3+ (2) [Co(NH3)6]2+ < [Co(NH3)6]3+ < [Co(NH3)4]2+ < [Co(en)3]3+ (3) [Co(en)3]3+ < [Co(NH3)6]3+ < [Co(NH3)6]2+ < [Co(NH3)4]2+ (4) [Co(NH3)4]2+ < [Co(NH3)6]2+ < [Co(NH3)6]3+ < [Co(en)3]3+
NCERT Chapters
- Class 12 Chemistry Ch 9: Coordination Compounds