Compounds 6a and 6b were unambiguously characterized using multinuclear NMR spectroscopy as well as by SC-XRD analysis. The 31 P NMR spectra show the characteristic chemical shifts of P -18.5 and -16.8 for 6a and 6b, respectively, and are consistent with the formation of pentacoordinate phosphorus species. The molecular structure of 6a is shown in Figure . The construction of the P-N3-C4-O1 ring is confirmed and the phosphorus atom adopts a distorted trigonal bipyramidal geometry that has the N2 and O1 atoms at the axial positions (N2-P-O1 = 158.3(1)º) and the N3 atom and two C atoms in the trigonal plane. The N3-P (1.673(3) Å) and N2-P (1.899(2) Å) bonds in 6a are notably elongated compared with those in 3a, suggesting a decrease in their bond orders. In contrast, the length of the B1-N3 bond remains almost unchanged (ca. 1.56 Å) through the transformation from 3a to 6a.