SYNTHESIS AND STRUCTURAL SYSTEMATICS OF NITROGEN BASE ADDUCTS OF GROUP-2 SALTS .1. SOME ADDUCTS OF GROUP-2 HALIDES WITH ACETONITRILE

Recrystallization of anhydrous Group 2 halides from anhydrous acetonit rile has yielded, in a number of cases, adducts MX(2).nMeCN suitable f or characterization by room-temperature single-crystal X-ray studies, which revealed novel stereochemical types. Magnesium(II) bromide yield s a trisolvate, ionic with disproportionated ligand arrays, as [Mg(NCM e)(6)] [MgBr4] (1), an unusual combination of homoleptic cationic and anionic species. Crystals are triclinic, PI, a 19.129(1), b 16.870(8), c 8.737(7) Angstrom, alpha 83.96(3), beta 87.93(4), gamma 65.24(2)deg rees, Z = 4 f.u.; R was 0.068 for N-o 3181 'observed' (I > 3 sigma(1)) reflections. Mg-N are 2.12(2)-2.18(2) Angstrom, and Mg-Br are 2.452(7 )-2.481(6) Angstrom. Calcium(II) bromide, by contrast, yields a disolv ate; an infinite polymer, with the two MeCN ligands trans in a quasi-o ctahedral coordination sphere, and the polymer comprising a... Ca(mu-B r)(2)Ca(mu-Br)(2)... linear array. Crystals of 1:2 CaBr2/MeCN (2) are orthorhombic, Pbam, a 14.252(3), b 8.539(2), c 4.2088(9) Angstrom, Z = 2 f.u.; R 0.043 for N-o 352. Ca-N is 2.46(1) Angstrom, and Ca-Br 2.87 65(9) Angstrom. CaI2 and SrI2 yield pentasolvates as neutral, molecula r species [(MeCN)(5)MX(2)] (3) and (4), with the halides tra ns in a p entagonal bipyramidal coordination sphere. They are isomorphous, ortho rhombic, Pcab, a approximate to 18.4, b approximate to 14.6, c approxi mate to 14.3 Angstrom, Z = 8 f.u.; R were 0.042 and 0.059 for N-o 2297 and 1489 respectively. Ca-I are 3.132(2), 3.116(2) Angstrom and Sr-I are 3.265(3), 3.238(3) Angstrom, while Ca-N are (range) 2.518(8)-2.566 (8) Angstrom and Sr-N 2.68(2)-2.73(2) Angstrom.