Q.Classify each of the following ligands as a weak-field ligand or a strong-field ligand: $\text{F}^-$, $\text{en}$, $\text{H}_2\text{O}$, $\text{CO}$, $\text{Cl}^-$, $\text{CN}^-$.

[!TLDR] Halides and $\text{H}_2\text{O}$ are weak field; $\text{CO}$, $\text{CN}^-$, en are strong field. [!ANSWER] Weak field: $\text{F}^-$, $\text{H}_2\text{O}$, $\text{Cl}^-$. Strong field: en, $\text{CO}$, $\text{CN}^-$.

Placing each ligand on the spectrochemical series (Section 5.3): the halide ions $\text{F}^-$ and $\text{Cl}^-$ sit toward the weak-field end of the series (along with $\text{I}^-$ and $\text{Br}^-$), and $\text{H}_2\text{O}$ sits in the weak-to-intermediate region, well below ammonia. All three are classed as weak-field ligands — with a $d^4$-$d^7$ metal ion, they favour a high-spin configuration. On the other side, ethylenediamine (en), carbon monoxide ($\text{CO}$), and the cyanide ion ($\text{CN}^-$) all sit at the strong-field end of the series; $\text{CN}^-$ and $\text{CO}$ in particular produce some of the largest $\Delta_o$ values of any common ligands. All three are classed as strong-field ligands — with a $d^4$-$d^7$ metal ion, they favour a low-spin configuration. [!ANSWER] Weak-field: $\text{F}^-$, $\text{H}_2\text{O}$, $\text{Cl}^-$. Strong-field: en, $\text{CO}$, $\text{CN}^-$.

Compare each ligand's position on the spectrochemical series against the conventional weak/strong dividing line (roughly between $\text{H}_2\text{O}$ and $\text{NH}_3$).

Classifying $\text{H}_2\text{O}$ as strong field because it is a common, 'reactive' ligand — its actual splitting power is closer to the halides than to $\text{NH}_3$ or $\text{CN}^-$.