The structure of P,P-bis(diisopropylamino)-4-phenyl-1,3,2 lambda(5)-diazaphosphete , 1a, has been determined by a single-crystal X-ray diffraction study (C19H33N4P, monoclinic system, space group P2(1), a = 9.482(1) Angstrom, b = 11.374(3) Angstrom, c = 9.668(2) Angstrom, beta = 97.16(1)degrees, Z = 2). According to quantum chemical calculations at an RHF level of optimization utilizing the 6-31g(d,p) basis set, 1a has a zwitterionic structure with the negative charge delocalized on the NCN allylic fragment and the positive charge localized at the phosphorus. Heterocycle 1a reacts with water and benzaldehyde affording N-phosphoranylbenzamidine 3 (95% yield) and the expected aza-Wittig adduct 4 (85% yield), respectively. Addition of 1 equiv of methyl trifluoromethanesulfonate and of 2 equiv of BH3 . THF to la affords cyclic phosphonium salt 5 (94% yield) and the bis(borane) adduct 6a (90% yield), respectively. Dimethyl acetylenedicarboxylate slowly reacts with 1a giving rise to 1,3,4 lambda(5)-diazaphosphinine, 9, in 70% yield. The X-ray crystal structures of products 2, 3, and 6a are reported (2 : C26H38N5P, monoclinic system, space group C2/c, a = 16.337(8) Angstrom, b = 19.810(2) Angstrom, c = 8.800(2) Angstrom, beta = 117.68(2)degrees, Z = 4. 3 : C19H35N4OP, orthorhombic system, space group P2(1)2(1)2(1), a = 9.090(1) Angstrom, b = 12.955(2) Angstrom, c = 17.860(3) Angstrom, Z = 4. 6a : C19H39B2N4P, orthorhombic system, space group P2(1)2(1)2(1), a = 10.340(1) Angstrom, b = 13.247(1) Angstrom, c = 16.996(1) Angstrom, Z = 4).