Discussion
In cyclophosphazenes, (NPX2)3 (X = F, Cl, Br and Me), the strength of P-N π-bond varies with X in the order
Answer: A
The P-N π-bond strength varies as F \(\gt\) Cl \(\gt\) Br \(\gt\) Me.
The π component of the P-N bond in a phosphazene comes from donation of nitrogen lone-pair density towards phosphorus (classically described as dπ-pπ bonding; in modern terms, into P-X σ* orbitals). The more electronegative the groups on phosphorus, the more positive and electron-hungry the phosphorus becomes and the better it accepts this density. So the π bonding is strongest for X = F, weaker for Cl and Br, and weakest for the electron-releasing methyl group.
This follows the electronegativity order F \(\gt\) Cl \(\gt\) Br \(\gt\) C and shows up as shorter P-N bonds in (NPF2)3 than in (NPCl2)3. Any order that puts Me first, or Br ahead of F, reverses that trend.
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