Complexes and complex stability Questions in English

(A) According to the spectrochemical series, the order of field strength for the given ligands is: $I^{-} < Br^{-} < S^{2-} < SCN^{-} < Cl^{-} < F^{-} < OH^{-} < C_2O_4^{2-} < H_2O < NCS^{-} < EDTA^{4-} < NH_3 < en < CN^{-} < CO$.
Comparing the given options ($Br^{-}$, $OH^{-}$, $en$, $H_2O$), $Br^{-}$ occupies the position with the lowest field strength in the spectrochemical series.
Therefore, $Br^{-}$ is the weakest field ligand among the given options.

(B) According to the spectrochemical series, the order of field strength for the given ligands is: $S^{2-} < NCS^{-} < H_2O < NH_3$.
Among the given options, $NH_3$ has the highest field strength as it is a stronger field ligand compared to $H_2O$, $NCS^{-}$, and $S^{2-}$.

(C) According to the spectrochemical series, the order of increasing field strength for the given ligands is:
$I^- < Br^- < S^{2-} < SCN^- < Cl^- < N_3 < F^- < OH^- < C_2O_4^{2-} < H_2O < NCS^- < EDTA^{4-} < NH_3 < en < CN^- < CO$.
Comparing the given ligands: $C_2O_4^{2-} < H_2O < EDTA^{4-} < en$.
Thus, option $(C)$ is correct.

(C) $1$. The stability of a coordination complex is significantly enhanced by the chelate effect.
$2$. The ligand $C_2O_4^{2-}$ (oxalate) is a bidentate chelating ligand that forms five-membered stable rings with the central metal ion.
$3$. Among the given options, $[Fe(C_2O_4)_3]^{3-}$ forms a stable chelated structure, making it the most stable complex due to the chelate effect.