CRYSTALLOGRAPHIC CHARACTERIZATION OF DIHALOGENODIMETHYLSELENIUM COMPOUNDS, ME(2)SEX(2) (X=CL, BR OR I) AND THE DEPENDENCE OF THEIR STRUCTURES ON THE NATURE OF THE HALOGEN

The reaction of Me(2)Se with I-2 in diethyl ether produced the charge- transfer complex Me(2)Se-I-I, which has a short I-I distance of 2.916( 3) Angstrom, when compared to the analogous Group 16 compound Me(3)As- I-I, reflecting the weaker donor power of Se compared to As towards di iodine. Dimethyl selenide reacted with Br, to produce the regular trig onal-bipyramidal molecule Me(2)SeBr(2) with. Sr-Br bond lengths of 2.5 46(4) and 2.551(4) Angstrom, i.e. virtually identical. However, the di chloro-compound, Me(2)SeCl(2), prepared from the reaction of SO2Cl2 wi th Me(2)Se, shows significant variations in Se-Cl bonds [2.349(5), 2.4 08(6), 2.351(6) and 2.405(5) A (two inequivalent molecules in the asym metric unit)]. This is due to significant long Se . Cl contacts which lie within the van der Waals radii. These results illustrate the struc tural changes observed for the Me(2)SeX(2) (X = I, Br or Cl) series in the solid state upon changing the nature of the halogen.