Element-element bonds. IX. Structures of tetrakis(trifluoromethyl)diphosphane and -diarsane: Experimental and theoretical investigations

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 = 6 81.6(2) pm, c = 802.8(3) pm, alpha = 108.58(1)degrees, beta = 99.66(1)degre es, gamma = 103.29(1)degrees, wR2 = 0.204) and -diarsane (3c, mp -52 degree s 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, r eveal 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 2 c 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 tho se determined for the crystalline state. As a result of quantum chemical ca lculations at Hartree-Fock and hybrid density functional levels of theory u sing 6-311+G* basis sets, the gauche conformer of hydrazine derivative 1c a nd 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 sligh tly more stable. Variations of calculated E-E and E-C bond lengths are anal yzed and compared with corresponding values of the parent compounds E2H4 (1 a to 3a) as well as the tetramethyl derivatives 1b to 3b.