Phosphazene bases are strong non-metallic non-ionic and low-nucleophilic bases. They are stronger bases than regular amine or amidine bases. Protonation takes place at a doubly bonded nitrogen atom. Related to phosphazene bases are the Verkade bases, which feature P(III) with three amido substituents and a transannular amine. The pK_a's of tert-Bu−(H)N=P(−N=P(−NR₂)₃)₃]⁺, where R = Me and pyrrolidinyl, are 42.7 and 44, respectively. These are the highest pK_a measured for the conjugate acid of charge-neutral molecular base.^[2]

•  
  t-Bu-P₄
•  
  BEMP

Phosphazene bases are established reagents in organic synthesis. Most notably, they have been used as catalysts in the synthesis of poly(phthalaldehyde) polymer. Perhaps the best known phosphazene bases are BEMP with an acetonitrile pK_a of the conjugate acid of 27.6 and the phosphorimidic triamide t-Bu-P₄ (pK_BH⁺ = 42.7) also known as Schwesinger base after one of its inventors.^{[3][clarification needed]}

In one application t-Bu-P₄ is employed in a nucleophilic addition converting the pivaldehyde to the alcohol:^[4]

The active nucleophile is believed to be a highly reactive phosphazenium species with full negative charge on the arene sp² carbon.^{[clarification needed]}

Besides organic synthesis, phosphazene bases are used as basic titrants in non-aqueous acid–base titration. Their advantages for this are: they are very strong bases in many solvents and their conjugate acids are inert and non-HBD cations.^{[clarification needed]}

• Cyclodiphosphazane
• Hexachlorophosphazene
• Polyphosphazene