Our interest in the design, development, and application of chiral, phosphorus-based anionic reagents5 has led to a study of their structure. Recent reports from these laboratories have disclosed the X-ray structures of phosphonamide-stabilized carbanions Li+land Li+2along with the solution aggregation and anion hybridization state.6 In both cases the anions have the following characteristics: (1) dimeric structure, (2) no C-Li contact, (3) nearly planar sp2 carbanionic carbon, (4) parallel conformation (0 = 0, Figure l), and ( 5 ) a low barrier of rotation around the phosphorus-carbon bond. The low barrier (<8 kcal/ mol) was a serious concern for the design of effective chiral auxiliaries. Since the origin of the barrier is the splitting between the 2ex and 2ey acceptor orbitals on phosphorus (which is very small, due to the similar electronegativity of nitrogen and oxygen,' Figure 1, X = 0), we chose to increase the magnitude of the splitting (and thus the barrier) by enhancing the electronic dissimilarity of the phosphorus ligands and thus turned our attention to thiophosphonamides*-10 3 and 4 (Figure 1, X = S).