(μ-Di-tert-butylsilanediolato)bis[bis(η5-cyclopentadienyl)methylzirconium]

The reaction of t-Bu2Si(OH)2 with two equivalents of Cp2Zr(CH3)2 produces the title t-Bu2SiO2-siloxide bridged dimer, [Zr2(CH3)2(C5H5)4(C8H18O2Si)] or [Cp2Zr(CH3)]2[μ-t-Bu2SiO2] (1), where one methyl group is retained per zirconium atom. The same product is obtained at room temperature even when equimolar ratios of the silanediol and Cp2Zr(CH3)2 are used. Attempts to thermally eliminate methane and produce a bridging methylene complex resulted in decomposition. The crystal structure of 1 displays typical Zr—CH3 and Zr—O distances but the Si—O distance [1.628 (2) Å] and O—Si—O angle [110.86 (15)°] are among the largest observed in this family of compounds suggesting steric crowding between the t-Bu substituents of the silicon atom and the cyclopentadienyl groups. The silicon atom lies on a crystallographic twofold axis and both Cp rings are disordered over two orientations of equal occupancy.

Pyrrolyl complexes of the early transition metals. 3. Preparation and crystal structure of (η5-C5H5)2Zr(η1-NC4H2Me2)2 and Zr(η1-NC4H2Me2)4

The title compounds were synthesized by the reactions of NaNC4H2Me2 with the appropriate complex metal chloride in THF. The crystal structures have been determined from X-ray data measured by counter methods. (η5-C5H5)2Zr(η1-NC4H2Me2)2 crystallizes in the orthorhombic space group Pbca with cell dimensions a = 15.797(6), b = 14.327(5), c = 16.417(6) Å, and ρcalcd = 1.46 g cm−3 for Z = 8. Full-matrix least-squares refinement led to a final R factor of 0.041 based on 536 observed reflections. Zr(η1-NC4H2Me2)4 belongs to the monoclinic space group P21/n with a = 14.065(5), b = 10.717(4), c = 15.733(6) Å, β = 90.61(4)°, and ρcalcd = 1.31 g cm−3 for Z = 4. A final R value of 0.029 resulted from the refinement on the basis of 2649 observed reflections. The dicyclopentadienyl derivative exhibits two features of importance. The Zr—N—centroid angles are 159 and 168°, and the Zr—N bond lengths are 2.24(2) Å. In the homoleptic complex the corresponding values are 164–169° and 2.069(3)–2.090(3) Å. In both cases the bonding parameters are indicative of a substantial amount of π overlap between the zirconium atom and the pyrrolyl nitrogen atom.

Evidence for a shear mechanism for the precipitation of γ-zirconium hydride in zirconium

In zirconium-hydrogen alloys, rapid cooling from an elevated temperature causes precipitation of the face-centred tetragonal (fct) phase, γZrH, in the form of needles, parallel to the close-packed <1120>zr directions (1). With low hydrogen concentrations, the hydride solvus is sufficiently low that zirconium atom diffusion cannot occur. For example, with 6 μg/g hydrogen, the solvus temperature is approximately 370 K (2), at which only the hydrogen diffuses readily. Shears are therefore necessary to produce the crystallographic transformation from hexagonal close-packed (hep) zirconium to fct hydride.The simplest mechanism for the transformation is the passage of Shockley partial dislocations having Burgers vectors (b) of the type 1/3<0110> on every second (0001)Zr plane. If the partial dislocations are in the form of loops with the same b, the crosssection of a hydride precipitate will be as shown in fig.1. A consequence of this type of transformation is that a cumulative shear, S, is produced that leads to a strain field in the surrounding zirconium matrix, as illustrated in fig.2a.

Refinement of the crystal structure of zirconyl chloride octahydrate

The crystal structure of zirconyl chloride octahydrate, ZrOCl2•8H2O, has been refined by the least-squares method with new three-dimensional data. Existence of the [Zr4(OH)8(H2O)16]8+ tetranuclear complex has been confirmed. However, the coordination polyhedron about each zirconium atom differs considerably from the D4d antiprismatic geometry reported previously. It is, in fact, more closely related to the D2d dodecahedron, and has twofold axial symmetry within the limits of experimental error. Mean bond lengths in the [Zr4(OH)8(H2O)16]8+ complex, which approximates closely to D2d point-group symmetry, are: Zr—OH (bridging) = 2.142 ± 0.019 Å and Zr—OH2 (terminal) = 2.272 ± 0.032 Å.