The new 1,3,2,4-diazadiphosphetidines cis- and trans-[(MeNH)PNMe]2 (8, 9), cis- and trans-[(EtNH)PNEt]2 (10, 11), cis- and trans-[(i-PrNH)PN-i-Pr]2 (13,14), [(MeNH)P(S)NMe]2 (16), trans-[(EtNH)P(S)NEt]2 (17), [(i-PrNH)P(S)N-i-Pr]2 (19), cis-[(t-BuNH)P(S)N-t-Bu]2 (20), and the cage compounds P4(NEt)6 (12) and P4S4-(NEt)6 (18) have been prepared and characterized. Structures of 17 and 20 have been determined by X-ray single-crystal analysis : 17, orthorhombic, Pbcn, a = 12.558(2) angstrom, b = 8.889(1) angstrom, c = 14.432(2) angstrom, V = 1611.1(4) angstrom3, Z = 4, R = 0.039, R(w) = 0.052; 20, monoclinic, P2(1)/n, a = 10.728(2) angstrom, b = 15.503(3) angstrom, c = 14.652(2) angstrom, beta = 103.7-degrees, V = 2367 (7) angstrom3, Z = 4, R = 0.049, R, = 0.078. Equilibration of cis/trans mixtures give equilibrium constants (K(cis/trans)) for 8/9, 10/11, and 13/14 of 3, 8, and 15, respectively. Conformational structural properies of the P(III) and P(V) 1,3,2,4-diazadiphosphetidines 8-11, 13, 14, 16, 17, 19, and 20, along with cis-[(t-BuNH)-PN-t-Bu]2 (5), cis-[(PhNH)PNPh]2 (4), cis-(t-BuNH)P(N-t-Bu)2PCl (15), and trans-[(PhNH)P(S)NPh]2 (7), have been examined invariable-temperature P-31 and H-1 NMR studies. Trans isomers are temperature independent to -90-degrees-C; however, the cis isomers 8, 10, 13, 15, and 20 undergo restricted exo-N(H)R group rotation. Rotational barriers are in the 9.5-12.9 kcal/mol range. Correlation of 2J(PNH) coupling constants of P-N(H)R groups with previously measured 2J data and X-ray structural data, along with X-ray data obtained in this study for 17 and 20, allow specific conclusions to be made as to the lowest energy conformation(s).