The rotational coordinates about the P-C bond in four different phosphorus(V)-substituted methyl anions have been studied in detail at the HFβ-21G(*)level, with stationary points characterized at levels equivalent to MP3/6-3 1+G*//HF/6-31+G*. The locations of local minima on the rotational coordinate were found to be dependent on opportunities for hyperconjugative stabilization. When amino substituents on phosphorus were geometrically unconstrained, two local minima were found. By contrast, when the amino groups were constrained to local geometries similar to those found in diazaphosphorinanes, the effects on the rotation coordinate were considerable. A single minimum and a higher rotation barrier were noted. Comparisons of static structure and rotational dynamics are made to both solution and solid-phase experimental data. The single-crystal X-ray structure of lithio-N, N, Nʺ,Nʺ, W-tetramethyl-P-benzylphosphondiamide Li+7- is reported.