Structure of the trimethylaluminum dimer as determined by powder neutron diffraction at low temperature

The structure of Al-2(CD3)(6) has been determined at 4.5 EC by powder neutron diffraction. The molecule crystallizes in space group C2/c; Salient bond distances and angles areas follows: r(Al-C-b) = 2.145(7)/2.146(8) Angstrom, r(Al-C-t) = 1.945(6)/1.926(5) Angstrom, r(Al . . . Al) = 2.700(10) Angstrom, r(C-D) = 1.061(4)-1.118(5) Angstrom; angleC(b)-Al-C-b = 102.0(3)degrees, angleC(t)-Al-C-t = 125.8(3)degrees angleC(b)Al-C-t = 108.8(4)degrees, angleD-C-D = 99.0(4)-111.6(5)degrees. The bridging CD3 groups adopt a staggered conformation with respect to each other in a molecule with C-2h, symmetry, and there is no evidence in the structure for the involvement of the C-D bands in Al-CD3-Al bridge bonding. Variable-temperature studies show the lattice constant b to exhibit unusual behavior, initially decreasing with increasing temperature. This phenomenon is interpreted in terms of increasing thermal motion of the molecule with increasing temperature.