Match List I with List II:
|
List I (Complex ion) |
List II |
| A. \([CoCl(NH_3)_5]^{2+}\) | I. 500 nm |
| B. \([Co(NH_3)_5(H_2O)]^{3+}\) | II. 535 nm |
| C. \([Co(NH_3)_6]^{3+}\) | III. 475 nm |
| D. \([Co(CN)_6]^{3-}\) | IV. 310 nm |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A-II, B-I, C-III, D-IV
As the ligand around \( \ce{Co^{3+}} \) becomes a stronger field ligand (\( \ce{Cl^{-}} \) < \( \ce{H2O} \) < \( \ce{NH3} \) < \( \ce{CN^{-}} \)), the crystal field splitting Δo increases, so the complex absorbs light of higher energy, i.e., shorter wavelength. The wavelengths absorbed are:
(A) [CoCl(NH3)5]2+ — (II) 535 nm. One weak-field \( \ce{Cl^{-}} \) ligand gives the smallest splitting and the longest wavelength (complex looks violet).
(B) [Co(NH3)5(H2O)]3+ — (I) 500 nm. \( \ce{H2O} \) is stronger than \( \ce{Cl^{-}} \) but weaker than \( \ce{NH3} \) (complex looks red).
(C) [Co(NH3)6]3+ — (III) 475 nm. Six \( \ce{NH3} \) ligands give a larger splitting (complex looks yellow-orange).
(D) [Co(CN)6]3− — (IV) 310 nm. \( \ce{CN^{-}} \) is the strongest field ligand; absorption is in the ultraviolet, and the complex is pale yellow.
NCERT (2026-27 edition):
Colour in Coordination Compounds – …
