X-ray structure determinations of tetrakis(trifluoromethyl)diphosphane (2c, mp -82 degrees C, triclinic, P (1) over bar; Z = 1, a = 529.7(3) pm, b = 681.6(2) pm, c = 802.8(3) pm, alpha = 108.58(1)degrees, beta = 99.66(1)degrees, gamma = 103.29(1)degrees, wR2 = 0.204) and -diarsane (3c, mp -52 degrees C, monoclinic, P2(1)/c; Z = 2, a = 769.5(3) pm, b = 750.0(3) pm, c = 960.7(2) pm, beta = 105.26(1)degrees, wR2 = 0.115), both at -100(3)degrees C, reveal the molecules to adopt the trans conformation in the solid. Compared with the tetramethyl derivatives, the E-E (224.6(2)/246.3(1) pm, E = P, As) and E-C (188.3(4)/201.3(7) pm) bonds are elongated by 4.5/4.8 pm and 3.4/3.4 pm, respectively. From gas electron diffraction studies of diphosphane 2c a mixture of 85(10)% trans and 15(10)% gauche conformers can be deduced; diarsane 3c shows the trans form exclusively. The molecular parameters (E-E, 224.8(11)/245.2(6); E-C, 189.6(4)/201.2(4) pm) agree excellently with those determined for the crystalline state. As a result of quantum chemical calculations at Hartree-Fock and hybrid density functional levels of theory using 6-311+G* basis sets, the gauche conformer of hydrazine derivative 1c and the trans conformer of diarsane 3c are clearly lowest in energy. However, for diphosphane 2c the gauche and not the trans form is found to be slightly more stable. Variations of calculated E-E and E-C bond lengths are analyzed and compared with corresponding values of the parent compounds E2H4 (1a to 3a) as well as the tetramethyl derivatives 1b to 3b.