Energies of minima and TS as well as characteristic NMR chemical shifts are reported for the parent unsaturated phospho-organic molecules with CP2 backbone. Within the 14 relevant isomers of CH4P2, the two most stable structures are 1,2-diphospha-1-propene, 3, and diphosphirane (cyclo-CH2(PH)(2), 1, E-rel = 8 kJ/mol). The relative energies in kJ/mol at MP2/6-31G(d,p) are 84 for 1,3-diphospha-2-propene, 54 for 1,2-diphospha-2-propene, and for the phosphinidenes, they are 63 (P-PH-CH3) and 102 (P-CH2-PH2), Although the potential. intermediate products (PH-CH2-PH, PH-PH-CH2, and cyclo-CH2-PH2-P) of diphosphirane rearrangements have the relative energies 182, 157, and 158 kJ/mol, respectively, and other minima were found to have E-rel between 287 and 322 kJ/mol. Rupture of endocyclic bonds of In displays a preference for opening the P-C bond toward the CH2-PH-PH structure, which is stabilized by allyl conjugation. The lowest energy pathway for isomerization of 1 has a barrier of 213 kJ/mol and goes via the intermediate cyclo-(CH2)(PH2)(P) toward PH2-P=CH2. The calculated energy barriers of CH3-P=PH, 3, indicate that this isomer should be kinetically stable as isolated molecule.