The molecular structures of the diphosphines P-2[CH(SiH3)(2)](4), P-2[C(SiH3)(3)](4), P-2[SiH(CH3)(2)](4), and P-2[Si(CH3)(3)](4) and the corresponding radicals P[CH(SiH3)(2)](2), P[C(SiH3)(3)](2), P[SiH(CH3)(2)](2), and P[Si(CH3)(3)](2) were predicted by theoretical quantum chemical calculations at the HF/3-21G*, B3LYP/3-21G*, and MP2/6-31+G* levels. The conformational analyses of all structures found the gauche conformers of the diphosphines with C-2 symmetry to be the most stable. The most stable conformers of the phosphido radicals were also found to possess C2 symmetry. The structural changes upon dissociation allow the release of some of the energy stored in the substituents and therefore contribute to the decrease of the P-P bond dissociation energy. The P-P bond dissociation enthalpies at 298 K in the compounds studied were calculated to vary from -11.4 kJ mol(-1) (P-2[C(SiH3)(3)](4)) to 179.0 kJ mol(-1) (P-2[SiH(CH3)(2)](4)) at the B3LYP/3-21G* level. The MP2/6-31+G* calculations predict them to be in the range of 52.8-207.9 kJ mol(-1). All the values are corrected for basis set superposition error. The P-P bond energy defined by applying a mechanical analogy of the flexible substituents connected by a spring shows less variation, between 191.3 and 222.6 kJ mol(-1) at the B3LYP/3-21G* level and between 225.6 and 290.4 kJ mol(-1) at the MP2/6-31+G* level. Its average value can be used to estimate bond dissociation energies from the energetics of structural relaxation.