Various equilibrium structures for the XYPN and XYP2 isomeric systems (X, Y=H, F and Cl) have been determined by means of ab initio molecular orbital calculations. Geometries were optimized at the MP2/6-31G** level while relative energies were estimated at the CISDQ/6-311G** + ZPE level. In general, monohalogenenation destabilizes phosphinonitrenes, XYP-N, or phosphinophosphinidenes, XYP-P, relative to their corresponding XP=NY or XP=PY isomers, whereas perhalogenation stabilizes the former appreciably. The perfluoro effect in F2PN is quite strong, making it the lowest energy form among F2PN isomers. The effect of the chlorine atom is less pronounced. F2PN is best regarded as a phosphonitrile, F2P=N, characterized by a hypervalent phosphorus atom. Its peculiar stability is also attributable to its pronounced ionic character and to the intrinsic strength of the P-F bond. Perfluorophosphinophosphinidene (F2P-P) is also stabilized but remains less stable than FP=PF; the P-P bond in the singlet F2P-P has a character between double and triple. Monohalogenophosphinophosphinidenes, HFP-P and HCIP-P, probably have singlet ground electronic states. The cis-effect favouring a Z isomer is operative in XP=NX but not in XP=PX. The vibrational spectrum of F2P=N has also been predicted.