Ab initio MP2/aug'-cc-pVTZ calculations have been carried out on complexes H2C=(X)P:PXH2, for X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2. Three sets of complexes have been found on the potential surfaces. Conformation A complexes have A-P center dot center dot center dot P-A approaching linearity, with A the atom of X directly bonded to P. Conformation B complexes have A-P center dot center dot center dot P linear, but the P center dot center dot center dot P=C orientation of H2C=PX may differ significantly from linearity. Conformation C complexes are unique, since the pnicogen bond involves pi-electron donation and acceptance by H2C=PX. The order of binding energies of the three conformations of H2C=(X)P:PXH2 is C > A > B, with two exceptions. Although the binding energies of conformation C complexes tend to be greater than the corresponding conformation A complexes, intermolecular distances in conformation C tend to be longer than those in conformation A. Charge transfer stabilizes H2C=(X)P:PXH2 complexes. The preferred direction of charge transfer is from H2C=PX to PXH2. In conformations A and B, charge transfer occurs from a P lone pair on one molecule to an antibonding alpha* orbital on the other. However, in conformation C, charge transfer occurs from the pi. orbital of H2C=PX to the alpha*P-A orbital of PXH2, and from the lone pair on P of PXH2 through the pi-hole to the pi-*P=C orbital of H2C=PX. Changes in charges on P upon complexation do not correlate with changes in P-31 chemical shieldings. Computed EOM-CCSD spin-spin coupling constants correlate with P-P distances. At each distance, the ordering of (1P)J(P-P) is A > B > C. Binding energies and spin-spin coupling constants of conformation A complexes of (PH2X)(2) H2C=(X)P:PXH2, and (H2C=PX)(2) with A-P center dot center dot center dot P-A approaching linearity have been compared. For complexes with the more electronegative substituents, binding energies are ordered (PH2X)(2) > H2C=(X)P:PXH2 > (H2C=PX)(2), while the order is reversed for complexes formed from the more electropositive substituents. A plot of Delta E(PH2X)(2)/Delta E(H2C=PX)(2) versus Delta E[H2C=(X)-P:PXH2]/Delta E(H2C=PX)(2) indicates that there is a systematic relationship among the stabilities of these complexes. Complexes (PH2X)(2) tend to have larger spin-spin coupling constants and shorter P-P distances than H2C=(X)P:PXH2, which in turn have larger coupling constants and shorter P-P distances than (H2C=PX)(2), although there is some overlap. Complexes having similar P-P distances have similar values of (1P)J(P-P).