Construct a linear five-atom backbone P-N4-N2-N1 sequence, ending in a terminal nitrogen N3.  

1. Phosphorus: single-bonded to N4 and to two mono-anionic oxygens. No other substituents.  
2. N4: single-bonded to P, double-bonded to N2.  
3. N2: double-bonded to N4, single-bonded to N1.  
4. N1: single-bonded to N2, double-bonded to terminal nitrogen N3.  
5. N3: terminal, only the double bond to N1.  

No rings, no chiral centres, no additional heteroatoms or hydrogens beyond those implied by valence.