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Question: Answered & Verified by Expert
Among the ligands $\mathrm{NH}_3$, en, $\mathrm{CN}^{-}$and $\mathrm{CO}$ the correct order of their increasing field strength, is :
ChemistryCoordination CompoundsBITSATBITSAT 2023 (Memory Based Paper 1)
Options:
  • A $\mathrm{NH}_3 < $ en $ < \mathrm{CN}^{-} < \mathrm{CO}$
  • B $\mathrm{CN}^{-} < \mathrm{NH}_3 < \mathrm{NH}_3 < $ en
  • C en $ < \mathrm{CN}^{-} < \mathrm{NH}_3 < \mathrm{CO}$
  • D $\mathrm{CO} < \mathrm{NH}_3 < $ en $ < \mathrm{CN}^{-}$
Solution:
2795 Upvotes Verified Answer
The correct answer is: $\mathrm{NH}_3 < $ en $ < \mathrm{CN}^{-} < \mathrm{CO}$
Ligands can be arranged in a series in the orders of increasing field strength as given below:
Weak field ligands:
$\mathrm{I}^{-} < \mathrm{Br}^{-} < \mathrm{S}^{2-} < \mathrm{SCN} < \mathrm{Cl}^{-} < \mathrm{N}_3^{-}, \mathrm{F}^{-}$ $ < $ Urea, OH $ < $ oxalate
Strong field ligands
$\mathrm{O}^{--} < \mathrm{H}_2 \mathrm{O} < \mathrm{NCS}^{-} < \text {EDTA } < \mathrm{Py}, \mathrm{NH}_3 < $ en $=\mathrm{SO}_3^{-} < $bipy, Phen $ < \mathrm{NO}_2^{-} < \mathrm{CH}_3^{-}$ $ < \mathrm{C}_6 \mathrm{H}_5^{-} < \mathrm{CN}^{-} < \mathrm{CO}$
Such a series is termed as spectrochemical series. It is an experimentally determined series based on the absorption of light by complexes with different ligands.

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