Question

Which of the following is the correct order of increasing field strength of ligands to form coordination compounds?

• A. ${\mathrm{CN}}^{-}<{\mathrm{C}}_2{\mathrm{O}}_2^{-4}<{\mathrm{SCN}}^{-}<{\mathrm{F}}^{-}$
• B. ${\mathrm{SCN}}^{-}<{\mathrm{F}}^{-}<{\mathrm{CN}}^{-}<{\mathrm{C}}_2{\mathrm{O}}_2^{-4}$
• C. ${\mathrm{F}}^{-}<{\mathrm{SCN}}^{-}<{\mathrm{C}}_2{\mathrm{O}}_2^{-4}<{\mathrm{CN}}^{-}$
• D. ${\mathrm{SCN}}^{-}<{\mathrm{F}}^{-}<{\mathrm{C}}_2{\mathrm{O}}_2^{-4}<{\mathrm{CN}}^{-}$

Answer 1

The correct option is D

${\mathrm{SCN}}^{-}<{\mathrm{F}}^{-}<{\mathrm{C}}_2{\mathrm{O}}_2^{-4}<{\mathrm{CN}}^{-}$

Explanation for correct option

(D)${\mathrm{SCN}}^{-}<{\mathrm{F}}^{-}<{\mathrm{C}}_2{\mathrm{O}}_2^{-4}<{\mathrm{CN}}^{-}$

Increasing Field Strength Of Ligands To Form Coordination Compounds:

• Some ligands are able to produce strong fields in which case, the splitting will be large whereas others produce weak fields and consequently result in small splitting of d orbitals.
• Weak Field Ligands are observed at the start of the series, and Strong Field Ligands are observed at the end.
• Strong field ligands help in the pairing of the electrons.
• In general, ligands can be ordered in the following sequence in terms of increasing field strength:
• In accordance with the spectrochemical series,

$$\begin{gathered} {\mathrm{I}}^{-}<{\mathrm{Br}}^{-}<{\mathrm{SCN}}^{-}<{\mathrm{Cl}}^{-}<{\mathrm{S}}^{2-}<{\mathrm{F}}^{-}<{\mathrm{OH}}^{-}<{\mathrm{C}}_2{{\mathrm{O}}_4}^{2-}<{\mathrm{H}}_2\mathrm{O}<{\mathrm{NCS}}^{-} \\ <{\mathrm{EDTA}}^{4-}<{\mathrm{NH}}_3<\mathrm{en}<{\mathrm{CN}}^{-}<\mathrm{CO} \end{gathered}$$

• According to this series, the ligand is arranged in option (D).
• Hence, the correct option is (D).

Explanation for incorrect option

The ligands are not arranged according to spectrochemical series in options (A), (B), and (C). Hence, (A), (B), and (C) are incorrect.

Hence, option (D) is correct. The increasing field strength of ligands to form coordination compounds is ${\mathrm{SCN}}^{-}<{\mathrm{F}}^{-}<{\mathrm{C}}_2{\mathrm{O}}_2^{-4}<{\mathrm{CN}}^{-}$.