CRYSTAL STRUCTURE AND X-RAY CRYSTALLOGRAPHIC ANALYSIS OF THE BINUCLEAR CLUSTER THIOCYANATE COMPLEX OF NIOBIUM(IV)

摘要

The structure of a binuclear niobium(IV) thiocyanate complex (Bu4N)(4)[Nb-2(mu(2)-S-2)(2)(NCS)(8)]center dot H2O (1 center dot H2O) is studied by X-ray crystallography. Compound 1 center dot H2O crystallizes in the orthorhombic crystal system (space group Pbca) with unit cell parameters a = 23.2139(4) angstrom, b = 25.2962(4) angstrom, c = 32.5478(6) angstrom, V = 19112.8(6) angstrom(3), R = 0.0592. The electronic structures of (Bu4N)(4)[Nb-2(mu(2)-S-2)(2)(NCS)(8)] and (Bu4N)(4)[Nb-2(mu(2)-Se-2)(2)(NCS)(8)] complexes are investigated by X-ray spectroscopy techniques and compared with the structures of polymeric chalcogenides Nb2S4Br4 and Nb2Se4Br4. By XPS it is shown that effective charges on atoms of the {Nb-2(mu-Q(2))(2)}(4+) (Q = S, Se) core do not change when passing from Br- to NCS- ligands. The HOMO and LUMO structures of (Bu4N)(4)[Nb-2(mu(2)-S-2)(2)(NCS)(8)] and (Bu4N)(4)[Nb-2(mu(2)-Se-2)(2)(NCS)(8)] are analyzed by XPS and XANES techniques. The largest contributions of S 3p states of NCS- ligands to the occupied molecular orbitals are found to be near the upper boundary of the valence band while the contributions from dichalcogenide bridging ligands have a more uniform distribution. For the conduction band, the contributions of S 3p states of NCS- ligands are located deeper whereas the contributions from dichalcogenide bridging ligands are near the lower boundary of the conduction band. The interaction of Nb 4d and S 3p/Se 4p states of dichalcogenide bridges for the HOMO has an antibonding character, which is bonding for all the other occupied molecular orbitals and antibonding for all unoccupied molecular orbitals.