The set which does NOT have ambidentate ligand(s) is:

🧠 Spot the Three Names

Ambidentate ligands you must know cold: $\mathrm{NO_2^-}$, $\mathrm{NCS^-}$ (= $\mathrm{SCN^-}$), $\mathrm{CN^-}$. The "set without ambidentate" is the only set that contains none of these three. Scan the four sets, eliminate any that contain even one of the three names.

🗺️ Set-by-Set Scan

(1) $\mathrm{C_2O_4^{2-}}$, en, $\mathrm{H_2O}$ — all bidentate-via-same-atom or monodentate single donor. No ambidentate. (2) Contains $\mathrm{NCS^-}$ — has ambidentate. (3) Contains $\mathrm{NO_2^-}$ — has ambidentate. (4) Contains $\mathrm{NO_2^-}$ and $\mathrm{NCS^-}$ — clearly has ambidentate.

Only set (1) is ambidentate-free.

⚡ The "Three Names" Filter

Read each set looking for the three magic names ($\mathrm{NO_2^-}$, $\mathrm{SCN^-}/\mathrm{NCS^-}$, $\mathrm{CN^-}$). The first set without any of them is your answer.

⚠️ EDTA Is Not Ambidentate

EDTA confuses students because it has many donor atoms — but they all attach simultaneously (hexadentate chelate). It's poly-dentate, not ambi-dentate. Same trap with oxalate, glycinate, and en — multi-dentate ≠ ambidentate.

$\boxed{\text{Answer: (1) } \mathrm{C_2O_4^{2-}, \ en, \ H_2O}}$