The single crystal structures of the 2 isoelectronic and isosteric compds. tetramethylaminoborane (triclinic, space group P.hivin.1) and tetramethylethene (triclinic, space group P.hivin.1) were detd. by an x-ray study at 110 K. At. coordinates are given. The distances of the central bonds are refined to 140.31(1) pm (B:N) and 134.8(1) pm (C:C), the CXXC-torsion angles (XX:CC, BN) are smaller than 1 Deg in both compds. A bond-length-bond-order correlation for the aminoboranes [Me2N]nBMe3-n with n = 1, 2, and 3 gives a regression coeff. of 0.9977; the same is found for the CNC-angle as a function of the CBNC-torsion angle. Also, the comparison among these aminoboranes show that the variation of CBNC-torsion angles has more influence on bond lengths at small angles.