NEET 2020 Chemistry Question 116 Solution

The crystal field splitting parameter ($$\Delta_o$$) depends on the field produced by the ligand. An experimentally determined series of ligands arranged in the order of increasing field strength (ability to split $$d$$-orbitals) is called the spectrochemical series.

$$\text{I}^- < \text{Br}^- < \text{SCN}^- < \text{Cl}^- < \text{S}^{2-} < \text{F}^- < \text{OH}^- < \text{C}_2\text{O}_4^{2-} < \text{H}_2\text{O} < \text{NCS}^- < \text{edta}^{4-} < \text{NH}_3 < \text{en} < \text{CN}^- < \text{CO}$$

Extracting the four ligands in the question: 1. $$\text{SCN}^-$$ (thiocyanate, S-donor): Appears early, weak field. 2. $$\text{F}^-$$ (fluoride): Weak field halide, stronger than &#x27; in math mode at position 13: \text{SCN}^-̲. 3." style="color:#cc0000">\text{SCN}^-$. 3.\text{C}_2\text{O}_4^{2-}$$(oxalate, O-donor bidentate): Stronger than fluoride. 4.$$\text{CN}^-$$ (cyanide, C-donor pseudohalide): Very strong field ligand near the end of the series.

$$\text{SCN}^- < \text{F}^- < \text{C}_2\text{O}_4^{2-} < \text{CN}^-$$