We expected the four-fold oxidation of 1 with four equivalents of Me3SiN3 to be facile given that the analogous reaction of (Me2N)3P is complete at 55 o C in a few hours. To our surprise, while single and double oxidation of 1 occurred smoothly (Scheme 1), giving 1' and 1'', complete oxidation was not achieved even upon refluxing with Me3SiN3 in toluene overnight. The reaction progress can be easily monitored through signal multiplicities observed in the 31 P NMR spectra (Figure ), and doing so over two half lives revealed the bimolecular rate constants of successive oxidations to be >100, 14.3, 2.3, and 0.15 M -1 h -1 (Figure , SI). The dramatic deceleration as a function of extent of oxidation is likely an electronic rather than steric effect, as it is also observed when the reaction is performed with a less hindered benzyl azide. Even using a 10-fold excess of Me3SiN3, the reaction proved to be lethargic, requiring 12 weeks at 100 o C to achieve quantitative conversion to the tetraphosphinimine 2. Following removal of excess Me3SiN3 and recrystallization, 2 was reproducibly isolated in 60-80% crystalline yield and comprehensively characterized.