THE SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF THE [NB(SE)(3)(SBU)-BU-T](2-), [NB(O)(SE-2)SEH](2-), [(NB(O)(SE-2)(2))(2)SE-4](4-) AND ([NB(SE)(3)](2)O)(4-) ANIONS - NEW ELEMENTARY SELENONIOBATES

The reaction of NbCl5, (NaSBu)-Bu-t, Se and Et4NCl in CH3CN in a 1:6:1 :1.6 molar ratio affords crystalline (Et4N)(2)[Nb(Se)(3)((SBu)-Bu-t)], 1. The same salt is obtained in about 25% yield by the oxidation of [ Nb(S)((SBu)-Bu-t)(4)](-) with 1 equiv. of Se. Oxidation of 1, or its P h4P+ analog with 1.5 equiv. of Se in the presence of water, results in the formation of the [Nb(O)(Se-2)(2)SeH)](2-) anion, 2, that can be i solated as Ph4P+ or Et4N+ salts. The Se-H vibration in the Et4N+ salt of 2 is found at 2526 cm(-1) and the proton in the anion is easily exc hanged with D3COD. The (Et4N)(4)[(Nb(O)(Se-2)(2))(2)Se-4] dimer, 3 was obtained by the oxidation of (Et4N)(2)[Nb(O)(Se-2)(2)SeH)] with 2 equ iv. of elemental selenium. The reaction of (Et4N)(2)[Nb(Se)(3)((SBu)-B u-t)] with water in CH3CN, led to isolation of the ore-bridged dimer, (E4N)(4)[(Se)(3)Nb-O-Nb(Se)(3)], 4. The crystal structures of 1-4 have been determined. Single crystal X-ray diffraction data for 1-4 were c ollected on a Nicolet P3/F diffractometer using Mo-K alpha radiation a nd the structures served using a combination of heavy atom Patterson t echniques, direct methods and Fourier techniques. The number of parame ters refined and the final R factors obtained for each structure at th e conclusion of refinement are. (1) 280, 0.0463; (2); 518, 0.0941; (3) , 318, 0.0886 and (4), 205, 0.0625 respectively. The anion in 1 exhibi ts tetrahedral geometry while the structure of the anion in 2 shows th e Nb5+ ion in a pentagonal pyramidal geometry with the two eta(2)-Se-2 (2-) and the eta(1)-SeH- ligands in the equatorial plane and the oxo l igand in the apical position. The Nb atom is located 0.70 Angstrom abo ve the plane defined by the five equatorial Se atoms. In 2, the Nb=O, Nb-SeH and Nb-Se-2 bonds are found at 1.671(11), 2.608(4) and 2.60(1) Angstrom, respectively. The structure of the anion in 3 shows 2 [Nb(O) (Se-2)(2)] units, bridged by a tetraselenido ligand to give a dimer wi th pentagonal pyramidal [Nb(O)(Se-2)Se] subunits and the oxo ligands i n an anti conformation. In 4 the mu-oxo ligand of the anion is located on a crystallographic centre of symmetry and the Nb-O bond, in the li near Nb-O-Nb unit is 1.916(1) Angstrom. The two Nb(Se)(3) units are or iented in a staggered fashion to give an idealised D-3d symmetry. (C) 1997 Elsevier Science Ltd.