On the energy hypersurfaces of the anions HP4- and CH2P3- at the RMP2(fc)/6-31+G(d) level, the isomers with triphosphaallyl moiety are the lowest energy structures. For these free 1-X-2,4-(P-B)(2)-3-P-A(-) anions characteristic P-31 NMR chemical shifts, ?P-31, are predicted to be (for X = PH, 1, delta(31)P(P-A) = 517, delta(31)P(P-B) = 424, and delta(31)P(P-X) = 50; for X = CH2, 4, delta(31)P(P-A) = 611, delta(31)P(P-B) = 450). The observed delta(exp) P-31 values for HP4- (Na/K, DME) completely disagree with the delta(31)P calculated at GIAO/MP2/6-311+G(d)//RMP2(fc)/6-31+G(d) for structure 1. The rotational average of the phosphinidyltriphosphirene structures (P-3-PH-, 3) agree better with the delta(exp) P-31 than those with a bicyclo[1.1.0]hydrogentetraphosphanide backbone, 2. MO analysis can rationalize the extreme endo/exo effect (Delta delta(31)P = 455 ppm) on the chemical shift in the exocyclic PH group of 3. The lowest energy geometry of the anion 3 has E-rel of 31 kJ mol(-1) relative to 1. The most favored 3 + Na+ structure is only 15 kJ mol(-1) above the lowest energy HP4Na minimum, 2 + Na+ with Na+ in endo and H in exo orientation of the bicyclo-P-4 framework (E-rel of 1 + Na+ is 13 kJ mol(-1)). In most HP4Na structures the Na+ changes the P-31 NMR chemical shifts towards higher field with respect to the bare anions.