Preparation, Properties, And Reactivities Of Unprecedented Oxo-Sulfido Nb(Iv) Aqua Ions And Crystal Structure Of (Me2nh2)(6)[Nb-5(Mu(3)-S)(2)(Mu(3)-O)(2)(Mu(2)-O)(2)(Ncs)(14)]Center Dot 3.5h(2)O

By treatment of Zn-reduced ethanolic solutions of NbCl5 with HCl in the presence of sulfide followed by cation-exchange chromatography, two oxo-sulfido niobium aqua ions, the red [Nb-4(mu(4)-S)(mu(2)-O)(5)(H2O)(10)](4+) and the yellow-brown [Nb-5(mu(3)-S)(2)(mu(3)-O)(2)(mu(2)-O)(2)(H2O)(14)](8+), were isolated. Both readily form their respective thiocyanate complexes, for which the structure for the former has been previously reported. Brown crystals of (Me2NH2)(6)[Nb5S2O4(NCS)(14)] center dot 3.5H(2)O (1) were isolated in the case of the latter, and the structure was determined by X-ray crystallography (space group: a = 15.4018(5) angstrom, b = 21.1932( 8) angstrom, c = 22.0487( 8) angstrom, alpha = gamma = 90 degrees, beta = 103.4590(10)degrees, and R-1 = 0.0659). An unprecedented pentanuclear Nb5S2O48+ core is revealed in which short Nb - Nb distances (2.7995( 8) - 2.9111(8) angstrom) are consistent with metal - metal bonding. A stopped-flow kinetic study of the 1: 1 equilibration of NCS- with [Nb-4(mu(4)-S)(mu(2)-O)(5)(H2O)(10)](4+) has been carried out. Equilibration rate constants are independent of [H+] in the range investigated (0.5 - 2.0 M) and at 25 degrees C; k(f) = 9.5 M-1 s(-1), k(aq) = 2.6 x 10(-2) s(-1), and K = 365 M-1. Conditions with first NCS- and then [Nb-4(mu(4)-S)(mu(2)-O)(5)(H2O)(10)](4+) in excess revealed a statistical factor of 4, suggesting the presence of four kinetically equivalent Nb atoms. Attempts to study the 1: 1 substitution of NCS- with [ Nb-5(mu(3)-S)(2)(mu(3)-O)(2)(mu(2)-O)(2)(H2O)(14)](8+) showed signs of saturation kinetics. Quantum chemical calculations using the density functional theory (DFT) approach were performed on both the Nb4O5S4+ and Nb5O4S28+ naked clusters. The highest occupied and lowest unoccupied molecular orbitals have dominant Nb(4d) character. The HOMO for Nb4O5S4+ is a nondegenerate fully filled MO, whereas for Nb5O4S28+, it is a nondegenerate partially filled MO with one unpaired electron. EPR spectroscopy on [Nb-5(mu(3)-S)(2)(mu(3)-O)(2)(mu(2)-O)(2)(H2O)(14)](8+) shows that the molecule has total anisotropy (C-2v), with all three tensors, g(x) = 2.399, g(y) = 1.975, and g(z) = 1.531, resolved. No hyperfine interaction expected from the nuclear moment of I = 9/2 for Nb-93 was observed.