The denticity of an organic ligand, biuret is:

🧠 Read Biuret Like a Skeleton

Biuret is the condensation product of two ureas: $\mathrm{H_2N{-}CO{-}NH{-}CO{-}NH_2}$.

Look at every atom that could donate a lone pair to a metal:

• Two terminal $-\mathrm{NH_2}$ nitrogens — each has a lone pair. Donor count: 2.
• One central $-\mathrm{NH}-$ nitrogen — also has a lone pair. Donor count: 1.
• Carbonyl oxygens: in the standard biuret-ligand depiction (and in the famous biuret test for proteins), donation is through nitrogens, not oxygens.

Total N donors = 3 → biuret is tridentate.

🗺️ Map the Bonding

In the biuret test ($\mathrm{Cu^{2+}}$ + biuret in alkaline medium → violet colour), copper binds to all three nitrogen atoms simultaneously, forming two fused 5-membered chelate rings. That ring count requires three donor atoms — confirms tridentate.

⚡ Memorise Common Denticities

| Ligand | Denticity | |---|---| | $\mathrm{H_2O,\ NH_3,\ Cl^-,\ CN^-,\ CO}$ | 1 | | en, bpy, phen, ox, gly | 2 | | dien, biuret, terpy | 3 | | trien, EDTA-like trianions | 4 | | EDTA | 6 |

⚠️ Don't Pick "2" by Counting Just $-\mathrm{NH_2}$ Groups

A common slip: students see two terminal amines and write denticity = 2, missing the central $-\mathrm{NH}-$. The middle nitrogen is just as basic and just as ready to donate.

$\boxed{\text{Answer: (2) 3}}$