Theoretical structures have been calculated for P-allylphosphonic diamide at the HF/3-21G(*V/HF/3-2lG(*> level for the gas phase neutral, 1, free anionic, 2, and lithiated species, 3 (Figures 1-3). The bonding in the phosphondiamide unit of the neutral species strongly suggests hyperconjugative interactions between the amide nitrogens and the C-P-O moiety. The free anion was found to exist in both E and Z forms in which the planar allylic ⌘-system is aligned with the P-O bond, with the E isomer, 2a, preferred by 1.8 kcal/mol. Inclusion of lithium was found to distort the allyl system and change the relative energy ordering such that the Z isomer, 3b, is preferred by 8.5 kcal/mol (3-21G(i|i)) or 5.5 kcal/mol (6-31+G* / /3-21G(*)). In both isomers the lithium atom makes both oxygen and carbon contacts. The anions were characterized by charge localization and highly ionic bonding. Comparisons of the theoretical conclusions with NMR spectroscopic data for the anions showed good agreement in charge distribution and structure of the anion.