SYNTHESES OF THE URANIUM COMPLEXES [U(N(SIME3)(2))(2)(N(SIME3)(SIME2CH2B(C6F5)(3)))] AND [U(C(PH)(NSIME3)(2))(2)(MU(3)-BH4)(2)] - DETERMINATION OF HYDROGEN POSITIONS BY SINGLE-CRYSTAL X-RAY AND NEUTRON-DIFFRACTION

The complex [U{N(SiMe3)(2)}(2){N(SiMe3)(SiMe2CH2B(C6F5)(3))}] (1) is f ormed in the reaction between the hydride complex [U{N(SiMe3)(2)}(3)(H )] and B(C6F5)(3), and H-2 is evolved. The X-ray [C36H53BF15N3Si6U . 3 .5C(6)D(6), triclinic, space group P (1) over bar, Z = 2, 90 K, a = 14 .065(1) Angstrom, b = 14.496(1) Angstrom, c = 18.759(1) Angstrom, alph a = 82.898(1)degrees, beta = 74.415(1)degrees, gamma = 62.919(1)degree s] and neutron structure [C36H53BF15N3Si6U . 3.5C(6)D(6), triclinic, s pace group P (1) over bar, Z = 2, 20 K, a = 13.993(1) Angstrom, b = 14 .383(1) Angstrom, c = 18.720(1) Angstrom, alpha = 82.810(1)degrees, be ta = 74.200(1)degrees, gamma = 63.053(1)E] of compound 1, which crysta llizes with 3.5 molecules of C6D6 per asymmetric unit, show the electr on deficiency of the uranium atom to be effectively compensated by the formation of multicenter bonds between U and three Si-CH2 units of th e amido ligands. The reaction of the uranium complex [U{C(Ph)(NSiMe3)( 2)}(2)(Cl)(2)] with [Na(BH4)] gives the complex [U{C(Ph)(NSiMe3)(2)}(2 ){mu(3)-BH4}(2)] (2). The X-ray structure of 2 [C26H54B2N4Si4U, monocl inic, space group C2/c, Z = 4, 90 K, a = 21.613(1) Angstrom, b = 9.233 (1) Angstrom, c = 18.132(1) Angstrom, beta = 98.803(1)degrees] proves unequivocally the mu(3) coordination of the BH4 moieties. In both sing le-crystal X-ray structure determinations, all hydrogen and deuterium atoms could be located and isotropically refined, including those whic h are directly coordinated to the uranium. The reliability of the refi ned hydrogen and deuterium positions for compound 1 is confirmed by co mparison of the X-ray and neutron structure determinations. The abilit y to locate the hydrogen and deuterium positions in these uranium comp ounds by single-crystal X-ray diffraction is due to good crystal quali ty, the measurement of data at low temperature, and the use of image p late technology for data collection.