Electronic structure of ReO3Me by variable photon energy photoelectron spectroscopy,absorption spectroscopy and density functional calculations

Valence photoelectron (PE) spectra have been measured for ReO(3)Me using a synchrotron source for photon energies ranging between 20 and 110 eV. Derived branching ratios (BR) and relative partial photoionization cross sections (RPPICS) are interpreted in the context of a bonding model calculated using density functional theory (DFT). Agreement between calculated and observed ionization energies (IE) is excellent. The 5d character of the orbitals correlates with the 5p --> 5d resonances of the associated RPPICS; these resonances commence around 47 eV. Bands with 5d character also show a RPPICS maximum at 35 eV. The RPPICS associated with the totally symmetric 4a(1) orbital, which has s-like character, shows an additional shape resonance with an onset of 43 eV. The PE spectrum of the inner valence and core region measured with photon energies of 108 and 210 eV shows ionization associated with C 2s, O 2s, and Re 4f and 5p electrons. Absorption spectra measured in the region of the O1s edge showed structure assignable to excitation to the low lying empty "d" orbitals of this d(0) molecule. The separation of the absorption bands corresponded with the calculated orbital splitting and their intensity with the calculated O 2p character. Broad bands associated with Re 4d absorption were assigned to (2)D(5/2) and (2)D(3/2) hole states. Structure was observed associated with the C1s edge but instrumental factors prevented firm assignment. At the Re 5p edge, structure was observed on the (2)P(3/2) absorption band resulting from excitation to the empty "d" levels. The intensity ratios differed from that of the O 1s edge structure but were in good agreement with the calculated 5d character of these orbitals. An absorption was observed at 45 eV, which, in the light of the resonance in the 4a(1) RPPICS, is assigned to a 4a(1) --> ne, na(2) transition. The electronic structure established for ReO(3)Me differs substantially from that of TiCl(3)Me and accounts for the difference in chemical behavior found for the two complexes.     

Synthesis, characterization, and materials chemistry of Group 4 silylimides

This paper focuses on the development of potential single source precursors for M-N-Si (M = Ti, Zr or Hf) thin films. The titanium, zirconium, and hafnium silylimides (Me(2)N)(2)MNSiR(1)R(2)R(3) [R(1) = R(2) = R(3) = Ph, M = Ti(1), Zr (2), Hf (3); R(1) = R(2) = R(3) = Et, M = Ti (4), Zr (5), Hf (6); R(1) = R(2) = Me, R(3) = (t)Bu, M = Ti (7), Zr (8), Hf (9); R(1) = R(2) = R(3) = NMe(2), M = Ti (10), Zr (11), Hf (12)] have been synthesized by the reaction of M(NMe(2))(4) and R(3)R(2)R(1)SiNH(2). All compounds are notably sensitive to air and moisture. Compounds 1, 2, 4, and 7-10 have been structurally characterized, and all are dimeric, with the general formula [M(NMe(2))(2)(μ-NSiR(3))](2), in which the μ(2)-NSiR(3) groups bridges two four-coordinate metal centers. The hafnium compound 3 possesses the same basic dimeric structure but shows additional incorporation of liberated HNMe(2) bonded to one metal. Compounds 11 and 12 are also both dimeric but also incorporate additional μ(2)-NMe(2) groups, which bridge Si and either Zr or Hf metal centers in the solid state. The Zr and Hf metal centers are both five-coordinated in these species. Aerosol-assisted CVD (AA-CVD) using 4-7 and 9-12 as precursors generates amorphous films containing M, N, Si, C, and O; the films are dominated by MO(2) with smaller contributions from MN, MC and MSiON based on XPS binding energies.     

Crystal structures, electronic structures, and physical properties of Tl4MQ4 (M = Zr or Hf; Q = S or Se)

The ternary thallium chalcogenides of the general formula Tl(4)MQ(4) (M = Zr or Hf; Q = S or Se) were obtained from high-temperature reactions without air. These sulfides and selenides are isostructural, crystallizing in the triclinic system with space group P1 and Z = 5, in contrast to Tl(4)MTe(4) compounds that adopt space group R3. The unit cell parameters for Tl(4)ZrS(4) are as follows: a = 9.0370(5) ?, b = 9.0375(5) ?, c = 15.4946(9) ?, α = 103.871(1)°, β = 105.028(1)°, γ = 90.138(1)°, and V = 1183.7(1) ?(3). In contrast to the corresponding tellurides, the sulfides and selenides exhibit edge-shared MQ(6) octahedra, propagating along the c axis in a zigzag manner. All elements occur in the most common oxidation states, according to the formulation (Tl(+))(4)M(4+)(Q(2-))(4). Electronic structure calculations predict energy band gaps of 1.7 eV for Tl(4)ZrS(4) and 1.3 eV for Tl(4)ZrSe(4), which are in accordance with the large resistivity values observed experimentally.     

Molecular spectroscopy of uranium(IV) bis(ketimido) complexes. rare observation of resonance-enhanced raman scattering from organoactinide complexes and evidence for broken-symmetry excited states

Electronic absorption and resonance-enhanced Raman spectra for ketimido (azavinylidene) complexes of tetravalent uranium, (C(5)Me(5))(2)U[-N=C(Ph)(R)](2) (R = Ph, Me, and CH(2)Ph), have been recorded. The absorption spectra exhibit four broad bands between 13 000 and 24 000 cm(-1). The highest-energy band is assigned to the ketimido-localized p( perpendicular)(N)-->pi(N=C) transition based on comparison to the spectra of (C(5)H(5))(2)Zr[-N=CPh(2)](2) and (C(5)Me(5))(2)Th[-N=CPh(2)](2). Upon excitation into any of these four absorption bands, the (C(5)Me(5))(2)U[-N=C(Ph)(R)](2) complexes exhibit resonance enhancement for several Raman bands attributable to vibrations of the ketimido ligands. Raman bands for both the symmetric and nominally asymmetric N=C stretching bands are resonantly enhanced upon excitation into the p( perpendicular)(N)-->pi(N=C) absorption bands, indicating that the excited state is localized on a single ketimido ligand. Raman excitation profiles for (C(5)Me(5))(2)U[-N=CPh(2)](2) confirm that at least one of the lower-energy electronic absorption bands (E(max) approximately 16300 cm(-1)) is a charge-transfer transition between the U(IV) center and the ketimido ligand(s). The observations of both charge-transfer transitions and resonance enhancement of Raman vibrational bands are exceedingly rare for tetravalent actinide complexes and reflect the strong bonding interactions between the uranium 5f/6d orbitals and those on the ketimido ligands.     

Synthesis and structure of (Ph4P)2MCl6 (M = Ti, Zr, Hf, Th, U, Np, Pu)

High-purity syntheses are reported for a series of first, second, and third row transition metal and actinide hexahalide compounds with equivalent, noncoordinating countercations: (Ph(4)P)(2)TiF(6) (1) and (Ph(4)P)(2)MCl(6) (M = Ti, Zr, Hf, Th, U, Np, Pu; 2-8). While a reaction between MCl(4) (M = Zr, Hf, U) and 2 equiv of Ph(4)PCl provided 3, 4, and 6, syntheses for 1, 2, 5, 7, and 8 required multistep procedures. For example, a cation exchange reaction with Ph(4)PCl and (NH(4))(2)TiF(6) produced 1, which was used in a subsequent anion exchange reaction with Me(3)SiCl to synthesize 2. For 5, 7, and 8, synthetic routes starting with aqueous actinide precursors were developed that circumvented any need for anhydrous Th, Np, or Pu starting materials. The solid-state geometries, bond distances and angles for isolated ThCl(6)(2-), NpCl(6)(2-), and PuCl(6)(2-) anions with noncoordinating counter cations were determined for the first time in the X-ray crystal structures of 5, 7, and 8. Solution phase and solid-state diffuse reflectance spectra were also used to characterize 7 and 8. Transition metal MCl(6)(2-) anions showed the anticipated increase in M-Cl bond distances when changing from M = Ti to Zr, and then a decrease from Zr to Hf. The M-Cl bond distances also decreased from M = Th to U, Np, and Pu. Ionic radii can be used to predict average M-Cl bond distances with reasonable accuracy, which supports a principally ionic model of bonding for each of the (Ph(4)P)(2)MCl(6) complexes.     

Tris(pyrazolyl)borate amidoborane complexes of the group 4 metals

Treatment of the tris(pyrazolyl)borate metal triamides Tp'M(NMe(2))(3), where Tp' = (C(3)H(3)N(2))(3)BH (Tp) or (3,5-Me(2)C(3)HN(2))(3)BH (Tp*) and M = Ti, Zr and Hf, with the Br?nsted acidic Lewis adduct (C(6)F(5))(3)B·NH(3) in toluene solution leads to the formation of Tp'M(NMe(2))(2){NH(2)B(C(6)F(5))(3)} complexes. The exception to this was the attempted preparation of Tp*Ti(NMe(2))(2){NH(2)B(C(6)F(5))(3)} which was unsuccessful. Where Tp' = Tp and M = Ti and Zr and where Tp' = Tp* and M = Zr the complexes have been characterized by single crystal X-ray diffraction methods, revealing the first examples of octahedral amidoborane complexes of the group 4 metals. Attempts to drive the reactions to completion resulted in competing preferential hydrolysis of the amidoborane group, regenerating (C(6)F(5))(3)B·NH(3).     

Perfluoroterephthalate bridged complexes with M-M quadruple bonds: ((t)BuCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)C(t)Bu)(3), where M = Mo or W. Studies of solid-state,molecular, and electronic structure and correlations with electronic and Raman spectral data

The compounds [((t)BuCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)C(t)Bu)(3)], M(4)PFT, where M = Mo or W, are shown by model fitting of the powder X-ray diffraction data to have an infinite "twisted" structure involving M.O intermolecular interactions in the solid state. The dihedral angle between the M(2) units of each molecule is 54 degrees. Electronic structure calculations employing density functional theory (Gaussian 98 and ADF2000.01, gradient corrected and time dependent) on the model compounds (HCO(2))(3)M(2)(mu-O(2)CC(6)F(4)CO(2))M(2)(O(2)CH)(3), where M = Mo or W, reveal that in the gas phase the model compounds adopt planar D(2)(h) ground-state structures wherein M(2) delta to bridge pi back-bonding is maximized. The calculations predict relatively small HOMO-LUMO gaps of 1.53 eV for M = Mo and 1.22 eV for M = W for this planar structure and that, when the "conjugation" is removed by rotation of the plane of the C(6)F(4) ring to become orthogonal to the M(4) plane, this energy gap is nearly doubled to 2.57 eV for M = Mo and 2.18 eV for M = W. The Raman and resonance Raman spectra of solid M(4)PFT and of Mo(4)PFT in THF solution are dominated by bands assigned to the bridging perfluoroterephthalate (pft) group. The intensities of certain Raman bands of solid W(4)PFT are strongly enhanced on changing the excitation line from 476.5 nm (off resonance) to 676.5 nm, which is on resonance with the W(2) delta --> CO(2) (pft) pi transition at ca. 650 nm. The resonance enhanced bands are delta(s)(CO(2)) (pft) at 518 cm(-)(1) and its first overtone at 1035 cm(-)(1), consistent with the structural change to W(4)PFT expected on excitation from the ground to this pi excited state. The electronic transitions for solid Mo(4)PFT (lowest at 410 nm) were not accessible with the available excitation lines (457.9-676.5 nm), and no resonance Raman spectra of this compound could be obtained. For Mo(4)PFT in THF solution, it is the band at 399 cm(-)(1) assigned to nu(MoMo) which is the most enhanced on approach to resonance with the electronic band at 470 nm; combination bands involving the C(6)F(4) ring-stretching mode, 8a, are also enhanced.     

Group-4 eta(1)-pyrrolyl complexes incorporating N,N-di(pyrrolyl-alpha-methyl)-N-methylamine

Syntheses and properties of group-4 complexes incorporating the tridentate, dianionic ligand N,N-(dipyrrolyl-alpha-methyl)-N-methylamine, dpma, have been investigated. Addition of 1 equiv of H(2)dpma to Ti(NMe(2))(4) and Zr(NMe(2))(4) results in transamination with 2 dimethylamides providing Ti(NMe(2))(2)(dpma) and Zr(NMe(2))(2)(NHMe(2))(dpma), respectively. Addition of 2 equiv of H(2)dpma to Zr(NMe(2))(4) and Hf(NMe(2))(4) results in production of the homoleptic complexes Zr(dpma)(2) and Hf(dpma)(2). Conversely, treatment of Ti(NMe(2))(4) with 2 equiv of H(2)dpma does not provide Ti(dpma)(2), which was available by addition of 2 Li(2)dpma to TiCl(4). The properties of the isostructural series M(dpma)(2) were investigated by single crystal X-ray diffraction, cyclic voltammetry, (14)N NMR, and other techniques. By (14)N NMR, it was found that the pyrrolyl resonance chemical shift changes approximately linearly with the electronegativity of the metal center, which was attributed to pi-interaction between the pyrrolyl nitrogen lone pair and the metal. Other complexes produced during this study include Ti(CH(2)SiMe(3))(NMe(2))(dpma), TiCl(2)(THF)(dpma), and Ti(OCH(2)CF(3))(2)(THF)(dpma). Two isomers for Ti(CH(2)SiMe(3))(NMe(2))(dpma) were isolated and characterized.     

Crystal and electronic structure and magnetic properties of divalent europium perovskite oxides EuMO3 (M = Ti, Zr, and Hf): experimental and first-principles approaches

A comparative study of the crystal and electronic structure and magnetism of divalent europium perovskite oxides EuMO(3) (M = Ti, Zr, and Hf) has been performed on the basis of both experimental and theoretical approaches playing complementary roles. The compounds were synthesized via solid-state reactions. EuZrO(3) and EuHfO(3) have an orthorhombic structure with a space group Pbnm at room temperature contrary to EuTiO(3), which is cubic at room temperature. The optical band gaps of EuZrO(3) and EuHfO(3) are found to be about 2.4 and 2.7 eV, respectively, much larger than that of EuTiO(3) (0.8 eV). On the other hand, the present compounds exhibit similar magnetic properties characterized by paramagnetic-antiferromagnetic transitions at around 5 K, spin flop at moderate magnetic fields lower than 1 T, and the antiferromagnetic nearest-neighbor and ferromagnetic next-nearest-neighbor exchange interactions. First-principles calculations based on a hybrid Hartree-Fock density functional approach yield lattice constants, band gaps, and magnetic interactions in good agreement with those obtained experimentally. The band gap excitations are assigned to electronic transitions from the Eu 4f to Mnd states for EuMO(3) (M = Ti, Zr, and Hf and n = 3, 4, and 5, respectively).     

Gravimetric determination of the abundance ratio of zirconium and hafnium, based on the in situ thermal decomposition of K(4)[(Zr,Hf)(C(2)O(4))(4)].5H(2)O

Careful heating of K(4)[(Zr,Hf)(C(2)O(4))(4)].5H(2)O results in a two-step thermal decomposition which can be written as: K(4)[(Zr,Hf(C(2)O(4))(4)].5H(2)O --> K(4)[(Zr,Hf)(C(2)O(4))(4)] --> {2K(2)CO(3)+(Zr,Hf)O(2)}. The weight-ratio of the successive decomposition products depends on the abundance ratio of Zr and Hf, and forms the basis for the present method of gravimetric determination.     

Mono- and dinuclear complexes of sulfones with the tetrachlorides of group 4

The reactions of dialkyl sulfones [R(2)SO(2): R = Me, Et, Ph, R(2)=-(CH(2))(4)-] with the metal tetrachlorides of Group 4 [MCl(4): M = Ti, Zr, Hf] give different products mainly depending on the sulfone/M molar ratio. Compounds of formula [M(2)Cl(8)(R(2)SO(2))(2)][M = Ti, R(2)=-(CH(2))(4)-; M = Zr, R = Et, R = Ph] and [MCl(4)(R(2)SO(2))(2)](sulfone/M = 2)[M = Ti, R = Me; M = Zr, R = Me, R = Ph, R(2)=-(CH(2))(4)-; M = Hf, R = Me, R(2)=-(CH(2))(4)-] have been obtained. By X-ray diffraction methods the dinuclear titanium and zirconium adducts, [Ti(2)Cl(8)(mu-sulfolane-O,O')(2)] and [Zr(2)Cl(8)(mu-Ph(2)SO(2)-O,O')(2)] have been established to contain bridging sulfone and hexacoordinated metal centres, while the mononuclear zirconium complex [ZrCl(4)(Me(2)SO(2))(2)] has cis-monodentate sulfones in a slightly distorted octahedral geometry. The reaction between TiCl(4) and sulfolane (tetrahydrothiophene 1,1-dioxide) in SOCl(2) affords the 1:1 adduct independent of the sulfone/Ti molar ratio. Ligand-exchange and inter-conversion between mononuclear and dinuclear species have been observed by NMR, while the spectral features of the SO(2) moiety have been assigned by IR- and Raman spectroscopies.     

Probing Lewis acidity of Y(BH4)3 via its reactions with MBH4 (M = Li, Na, K, NMe4)

High-energy milling of Y(BH(4))(3) (containing LiCl as a by-product, which has not been removed) with MBH(4) (M = Li, Na, K, (CH(3))(4)N) leads to the first two examples of quasi-ternary yttrium borohydrides: KY(BH(4))(4) and (CH(3))(4)NY(BH(4))(4), while no chemical reaction is observed for LiBH(4) and NaBH(4). KY(BH(4))(4) is isostructural to NaSc(BH(4))(4) (Cmcm, a = 8.5157(4) ?, b = 12.4979(6) ?, c = 9.6368(5) ?, V = 1025.62(9) ?(3), Z = 4), while (CH(3))(4)NY(BH(4))(4) crystallises in primitive orthorhombic cell, similarly to KSc(BH(4))(4) (Pnma, a = 15.0290(10) ?, b = 8.5164(6) ?, c = 12.0811(7) ?, V = 1546.29(17) ?(3), Z = 4). The thermal decomposition of hydrogen-rich KY(BH(4))(4) (8.6 wt.% H) involves the formation of an unidentified intermediate at 200 °C and recovery of KBH(4) at higher temperatures; at 410 °C, KCl and YH(2) are observed. The thermal decomposition of (CH(3))(4)NY(BH(4))(4) occurs via two partly overlapping endothermic steps with concomitant emission of H(2) and organic compounds. Heating of a NaBH(4)/Y(BH(4))(3) mixture above 165 °C results in a mixed-cation mixed-anion borohydride, NaY(BH(4))(2)Cl(2), but not NaY(BH(4))(4). The reduced reactivity of Y(BH(4))(3) towards borohydride Lewis bases when compared to hypothetical scandium borohydride can be explained by the lower Lewis acidity of Y(BH(4))(3) than Sc(BH(4))(3).     

Catalytic dehydrogenation of dimethylamine borane by group 4 metallocene alkyne complexes and homoleptic amido compounds

Dehydrogenation of Me(2)NH·BH(3) (1) by group 4 metallocene alkyne complexes of the type Cp(2)M(L)(η(2)-Me(3)SiC(2)SiMe(3)) [Cp = η(5)-cyclopentadienyl; M = Ti, no L (2Ti); M = Zr, L = pyridine (2Zr)] and group 4 metal amido complexes of the type M(NMe(2))(4) [M = Ti (8Ti), Zr (8Zr)] is presented.     

Predicted group 4 tetra-azides M(N3)(4) (M = Ti-Hf,Th): the first examples of linear M-NNN coordination

Quantum chemical calculations suggest that group 4 tetra-azides M(N(3))(4), where M = Ti, Zr, Hf, and Th, are stable species. They present a unique structural feature; namely, the M-N-N-N fragments are linear. These species are energetically more stable than the corresponding isomers with general formula eta(5)-N(5) -M-eta(7)-N(7), and the Th species, Th(N(3))(4), is the most stable of all. Possible mixed nitride azides NMN(3) were also investigated.     

Contribution of disulfide S2(2-) anions to the crystal and electronic structures in ternary sulfides, Ba12In4S19, Ba4M2S8 (M=Ga, In)

Three semiconducting ternary sulfides have been synthesized from the mixture of elements with about 20% excess of sulfur (to establish oxidant rich conditions) by solid-state reactions at high temperature. Ba(12)In(4)S(19) ≡ (Ba(2+))(12)(In(3+))(4)(S(2-))(17)(S(2))(2-), 1, crystallizes in the trigonal space group R ?3 with a = 9.6182(5) ?, b = 9.6182(5) ?, c = 75.393(7) ?, and Z = 6, with a unique long period-stacking structure of a combination of monometallic InS(4) tetrahedra, linear dimeric In(2)S(7) tetrahedra, disulfide S(2)(2-) anions, and isolated sulfide S(2-) anions that is further enveloped by Ba(2+) cations. Ba(4)In(2)S(8) ≡ (Ba(2+))(4)(In(3+))(2)(S(2-))(6)(S(2))(2-), 2, crystallizes in the triclinic space group P ?1? with a = 6.236(2) ?, b = 10.014(4) ?, c = 13.033(5) ?, α = 104.236(6)°, β = 90.412(4)°, γ = 91.052(6)°, and Z = 2. Ba(4)Ga(2)S(8) ≡ (Ba(2+))(4)(Ga(3+))(2)(S(2-))(6)(S(2))(2-), 3, crystallizes in the monoclinic P2(1)/c with a = 12.739(5) ?, b = 6.201(2) ?, c = 19.830(8) ?, β = 104.254(6)° and Z = 4. Compounds 2 and 3 represent the first one-dimensional (1D) chain structure in ternary Ba/M/S (M = In, Ga) systems. The optical band gaps of 1 and 3 are measured to be around 2.55 eV, which agrees with their yellow color and the calculation results. The CASTEP calculations also reveal that the disulfide S(2)(2-) anions in 1-3 contribute mainly to the bottom of the conduction bands and the top of valence bands, and thus determine the band gaps.     

Simplest homoleptic metal-centered tetrahedrons, [M(OH2)4]2+, in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid (M = Co, Ni, Cu)

Dissolution of a tetrafluoroborate or perchlorate salt of [M(OH(2))(6)](2+) (M = Co, Ni, Cu) in 1-ethyl-3-methylimidazolium tetraluforoborate ionic liquid ([emim]BF(4)) results in significant solvatochromism and increasing intensity of color. These observations arise from partial dehydration from the octahedral [M(OH(2))(6)](2+) and formation of the tetrahedral [M(OH(2))(4)](2+). This reaction was monitored by the intense absorption band due to the d-d transition in the UV-vis absorption spectrum. The EXAFS investigation clarified the coordination structures around M(2+) {[Co(OH(2))(4)](2+), R(Co-O) = 2.17 ?, N = 4.2; [Cu(OH(2))(4)](2+), R(Cu-O) = 2.09 ?, N = 3.8}. (1)H and (19)F NMR study suggested that both [emim](+) and BF(4)(-) are randomly arranged in the second-coordination sphere of [M(OH(2))(4)](2+).     

Molecular structures of two metal tetrakis(tetrahydroborates), Zr(BH4)(4) and U(BH4)(4): equilibrium conformations and barriers to internal rotation of the triply bridging BH4 groups

The molecular structures of Zr[(mu-H)(3)BH](4) and U[(mu-H)(3)BH](4) have been investigated by density functional theory (DFT) calculations and gas electron diffraction (GED). The triply bridged bonding mode of the tetrahydroborate groups in the former is confirmed, but both DFT calculations and GED structure refinements indicate that the BH(4) groups are rotated some 12 degrees away from the orientation in which the three bridging B-H bonds are staggered with respect to the opposing ZrB(3) fragment. As a result the symmetry of the equilibrium conformation is reduced from T(d) to T. Bond distances and valence angles are as follows (DFT/GED): Zr-B = 232.2/232.4(5) pm; Zr-H(b) = 214.8/214.4(6) pm; B-H(b) = 125.3/127.8(8) pm; B-H(t) = 119.4/118.8(17) pm; angle ZrBH(b) = 66.2/65.6(3) degrees; the smallest dihedral angle of type tau(BZrBH(b)) = 48/45(2) degrees. DFT calculations on Hf(BH(4))(4) indicate that the structure of this molecule is very similar to that of the Zr analogue. Matrix-isolation IR spectroscopy and DFT calculations on U(BH(4))(4) show that while the polymeric solid-state structure is characterized by terminal triply bridging and metal-metal bridging bidentate BH(4) groups, all BH(4) groups are triply bridging in the gaseous monomer. Calculations with one of the two nonbonding 5f electrons on U occupying an a(1) and the other distributed equally among the three t(2) orbitals indicate that the equilibrium conformation has T(d) symmetry, i.e. that the three B-H(b) bonds of each tetrahydroborate group are exactly staggered with respect to the opposing UB(3) fragment with tau(BUBH(b)) = 60 degrees. Calculations including spin-orbit interactions indicate that Jahn-Teller distortions from T(d) symmetry are either absent or very small. The best agreement between observed and calculated GED intensity data was obtained for a model of T(d) symmetry, but models of T symmetry with dihedral angles tau(BUBH(b)) > 42 degrees cannot be ruled out. Bond distances and valence angles are as follows (DFT/GED): U-B = 248.8/251.2(4) pm; U-H(b) = 227.7/231.5(6) pm; B-H(b) = 126.0/131.6(5) pm, B-H(t) = 119.5/117.8(11) pm; angle UBH(b) = 65.6/63.1(3) degrees. It is suggested that the different equilibrium conformations of the three molecules are determined primarily by repulsion between bridging H atoms in different tetrahydroborate groups.     

Series of comparable dinuclear group 4 neo-pentoxide precursors for production of pH dependent group 4 nanoceramic morphologies

A series of similarly structured Group 4 alkoxides was used to explore the cation effect on the final ceramic nanomaterials generated under different pH solvothermal (SOLVO) conditions. The synthesis of [Ti(μ-ONep)(ONep)(3)](2) (1, ONep = OCH(2)C(CH(3))(3)) and {[H][(μ-ONep)(3)M(2)(ONep)(5)(OBu(t))]} where M = Zr (2) and Hf (3, OBu(t) = OC(CH(3))(3)) were realized from the reaction of M(OBu(t))(4) (M = Ti, Zr, Hf) and H-ONep. Crystallization of 1 from py led to the isolation of [Ti(μ-ONep)(ONep)(3)](2)(μ-py) (1a) whereas the dissolution of 2 or 3 in py yielded {(μ(3)-O)(μ(3)-OBu(t))[(μ-ONep)M(ONep)(2)](3)} M = Zr (2a) and Hf (3a). The structurally similar congener set of 1-3 was used to investigate variations of their resultant nanomaterials under solvothermal conditions at high (10 M KOH), low (conc. (aq) HI), and neutral (H(2)O) pH conditions. Reproducible nanodots, -squares, and -rods of varied aspect ratios were isolated based on cation and the reaction pH. The hydrolysis products were reasoned to be the "seed" nucleation sites in these processes, and studying the hydrolysis behavior of 1-3 led to the identification of [Ti(6)(μ(3)-O)(7)(μ-O)(μ-ONep)(2)(ONep)(6)](2) (1b) for 1 but yielded 2a and 3a for 2 and 3, respectively. A correlation was found to exist between these products and the final nanomaterials formed for the acidic and neutral processes. The basic route appears to be further influenced by another property, possibly associated with the solubility of the final nanoceramic material.     

Zirconium, hafnium, and tantalum amide silyl complexes: their preparation and conversion to metallaheterocyclic complexes via gamma-hydrogen abstraction by silyl ligands

New transition metal silyl amide complexes (Me(2)N)(3)Ta[N(SiMe(3))(2)](SiPh(2)Bu(t)) (1) and (Me(2)N)M[N(SiMe(3))(2)](2)(SiPh(2)Bu(t)) (M = Zr, 2a, and Hf, 2b) were found to undergo gamma-H abstraction by the silyl ligands to give metallaheterocyclic complexes (3) and (M = Zr, 4a, and Hf, 4b), respectively. The conversion of 1 to 3 follows first-order kinetics with DeltaH() = 23.6(1.6) kcal/mol and DeltaS() = 3(5) eu between 288 and 313 K. The formation of 4a from (Me(2)N)Zr[N(SiMe(3))(2)](2)Cl (5a) and Li(THF)(2)SiPh(2)Bu(t) (6) involves the formation of the intermediate 2a, followed by gamma-H abstraction. Kinetic studies of these consecutive reactions, a second-order reaction to give 2a and then a first-order gamma-H abstraction to give 4a, were conducted by an analytical method and a numerical method. At 278 K, the rate constants k(1) and k(2) for the two consecutive reactions are 2.17(0.03) x 10(-)(3) M(-)(1) s(-)(1) and 5.80(0.15) x 10(-)(5) s(-)(1) by the analytical method. The current work is a rare kinetic study of the A + B --> C --> D (+ E) consecutive reactions. Kinetic studies of the formation of a metallaheterocyclic moiety have, to our knowledge, not been reported. In addition, gamma-H abstraction by a silyl ligand to give such a metallaheterocyclic moiety is new. Theoretical investigations of the gamma-H abstraction by silyl ligands have been conducted by density functional theory calculations at the Becke3LYP (B3LYP) level, and they revealed that the formation of the metallacyclic complexes through gamma-H abstraction is entropically driven. X-ray crystal structures of (Me(2)N)(3)Ta[N(SiMe(3))(2)](SiPh(2)Bu(t)) (1), (Me(2)N)Zr[N(SiMe(3))(2)](2)Cl (5a), and (M = Zr, 4a, and Hf, 4b) are also reported.