Reactions of a number of germylenes and dimethylsilylene with a phosphaalkene, 2,2-bis(trimethylsilyl)-1-phenyl-1-phosphaethene (1), were studied. The reaction of short-lived dimethylgermylene with 1 produced a phosphagermirane 3 (the first representative of a new class of heterocyclic compounds). Compound 3 was characterized in solution by 1H, 13C, 31P, and 29Si NMR spectroscopy. Subsequent reaction of 3 with dimethylgermylene results in 2,2,3,3-tetramethyl-4,4-bis(trimethylsilyl)-1-phenyl-2,3-digerma-1-phosphacyclobutane 4, which has not been reported so far. In order to rationalize different reactivities of germylenes towards alkenes and phosphaalkenes, the addition products of GeH2 to ethylene and phosphaethene (HP=CH2) were studied using the G2 computational scheme and DFT PBE technique. The adducts of GeMe2 (GeCl2) with HP=CH2 and of GeMe2 with PhP=C(SiH3)2 were also calculated by the DFT PBE method. According to calculations, the exothermicity, DE, of cycloaddition of GeH2 and GeMe2 to the phosphaalkenes HP=CH2 and PhP=C(SiH3)2 (43.5—39.7 kcal mol–1) is nearly twice as high as the exothermicity of cycloaddition of these germylenes to ethylene. In addition to the minimum corresponding to the three-membered cycle, a number of minima corresponding to quite stable donor-acceptor complexes in which the Ge atom is coordinated by the lone electron pair of the P atom in the phosphaalkene molecule were located on the potential energy surface of the germylene—phosphaalkene system. The complexation energy of the complex of GeH2 (GeMe2) with phosphaethene is 25.0 (16.9) kcal mol–1. For GeCl2, the exothermicity of cycloaddition to HP=CH2 decreases to 7.6 kcal mol–1 and the complexation energy decreases to 8.2 kcal mol–1.