(7-Phenyl-7-phosphadispiro[126.96.36.199]heptane)pentacarbonyltungsten (8), a phosphatriangulane, was synthesized from bicyclopropylidene. Its single- crystal X-ray structure determination is reported. Comparison of the crystal structure data with those of the related phosphaspiropentane 7 and phosphirane 6 complexes suggests that the phosphirane ring tightens when the number of spiro atoms is increased. This is supported by the B3LYP and MP2/6- 31G* computed geometries of the uncomplexed parent systems. Ab initio calculated heats of formation and strain energies (SE) are reported for the parent phosphirane 11, phosphaspiropentane 12, and phosphatriangulane 13 using both G2MP2 theory and ring separation reactions. Our best estimates for the ΔH(f) of 11, 12, and 13 are 18.3, 48.4, and 78.2 kcal/mol, respectively, with corresponding SE values of 21.3, 54.7, and 87.9 kcal/mol. For comparison, the slightly modified G2MP2 method was also applied to cyclopropane 1, spiropentane 2, and triangulane 3 to give respective ΔH(f) values of 12.6, 44.3, and 75.3 kcal/mol, with corresponding SEs of 28.0, 64.6, and 100.5 kcal/mol, all of which are in excellent agreement with reported experimental data. These strain energies suggest that the excess strain per spiro atom is 5.3 kcal/mol for phospha[n]triangulanes, which is smaller than the 8.6 kcal/mol determined from the heat of combustion measurements for the [n]triangulanes.