 5 -Oxazaphosphetane 6a gradually decomposed to yield N-phosphinoyl isocyanate 7 along with the concomitant formation of 8a at room temperature. These results thus demonstrate the direct observation of a  5 -oxazaphosphetane species as the intermediate in the transformation of CO2 using phosphine imides. Given that the thermolysis of 6a would be promoted by the formation of the thermodynamically favorable P=O bond, we then explored the cleavage of P-O1 bond in 6a. The reaction between 6a and HB(C6F5)2 afforded 9a in 97% yield (Figure ) through the coordination of the carboxylate moiety to the borane (O2-B2 = 1.518(4) Å, Figure ). Accordingly, reconfiguration of the geometry around the phosphorus center to form a distorted tetrahedron occurs along with the shortening of the N2-P bond (1.720(3) Å) and the elongation of the B1-N3 bond (1.608(5) Å). The N3-P bond length was almost unchanged at 1.642(3) Å in comparison with that of 6a and thus significantly longer than in 3a. NMR analyses also gave an insight into the electronic state of the P atom in 9a. The resonance at P 59.9 indicates that the electron density on the P atom in 9a is nearly identical with 3a, although that of 9a should be slightly higher. Based on these experimental data, we conclude that the electron delocalization between the N3 atom and the adjacent carbonyl group partially weakens the negative hyperconjugation between the N3 and P atoms, while the increased -donation from the imidazolium unit to the P atom eventually compensates for the loss of electron density on the P atom. Results of the AIM analyses also support this conclusion, as exemplified by increased values of  (0.16), ∇ 2  (0.30), and G/ (1.30) at the BCP of the N3-P bond in 9a (Figures S15).