Pentacoordinated phosphoranes, PH4X, substituted with a full range of first- and second-row groups have been studied with ab initio computations. The apically and equatorially substituted trigonal-bipyramidal (TB) and apically substituted square-pyramidal (SP) conformations were fully optimized with the 3-21G(*) and 6-31G* basis sets. Frequency calculations were performed at 6-31G* on the optimized structures. Correlation corrections through the MP4 level were carried out on these 6-31G* structures. Energies calculated with zero-point energy corrections, i.e., at MP4SDTQ/6-31G* + ZPE, provide relative energies of various isomers. The apically substituted SP structures for PH4X (X = Li, Na, BeH, and MgH) are the most stable. The intrinsic apicophilicities of the first- and second-row groups are derived and are compared to available experimental data. The apicophilicities (in kcal mol-1) are OH (0.4) > SH (-0.1) > CH3 (-0.9) > PH2 (-3.3) > NH2 (-7.2) > SiH3 (-8.6). Due to their high degree of ion pair character, PH4F and PH4Cl are unsuitable as models for relative energy comparisons, π interaction energies were evaluated by calculating various PH4X (X = NH2, PH2, OH, SH) conformations. Natural bond orbital (NBO) analysis on these conformers shows that the π interaction is due chiefly to nx → σP-H*. Inductive and π bonding contributions are estimated for the first-row group substituents. The effects of substitution (e.g., relative energies) that are dominated by inductive interactions correlate linearly with group electronegativities.