Crystallographic studies, transition metal

Neutron diffraction studies have shown that in both systems Pd-H (17) and Ni-H (18) the hydrogen atoms during the process of hydride phase formation occupy octahedral positions inside the metal lattice. It is a process of ordering of the dissolved hydrogen in the a-solid solution leading to a hydride precipitation. In common with all other transition metal hydrides these also are of nonstoichiometric composition. As the respective atomic ratios of the components amount to approximately H/Me = 0.6, the hydrogen atoms thus occupy only some of the crystallographic positions available to them. 

Teller R, Bau RG (1981) Crystallographic Studies of Transition Metal Hydride Complexes. 44 1-82... 

Before our work [39], only one catalytic mechanism for zinc dependent HDACs has been proposed in the literature, which was originated from the crystallographic study of HDLP [47], a histone-deacetylase-like protein that is widely used as a model for class-I HDACs. In the enzyme active site, the catalytic metal zinc is penta-coordinated by two asp residues, one histidine residues as well as the inhibitor [47], Based on their crystal structures, Finnin et al. [47] postulated a catalytic mechanism for HDACs in which the first reaction step is analogous to the hydroxide mechanism for zinc proteases zinc-bound water is a nucleophile and Zn2+ is five-fold coordinated during the reaction process. However, recent experimental studies by Kapustin et al. suggested that the transition state of HDACs may not be analogous to zinc-proteases [48], which cast some doubts on this mechanism. 

The X-ray structure of lithium l-(dimethylamido)boratabenzene, reported in 1993, provided the first crystallographic characterization of a transition metal-free boratabenzene (Scheme 13).18a The observed bond lengths are consistent with a delocalized anion and with significant B—N double-bond character. In a separate study, the B—N rotational barrier of [C5H5B—NMeBnjLi has been determined to be 10.1 kcal/mol, and it has been shown that TT-complexation to a transition metal can increase this barrier (e.g., 17.5 kcal/mol for (C5H5B-N(i-Pr)2)Mn(CO)3).24... 

Extensive research on crystallographic shear (CS) planes has been reported in the literature as described earlier. Based on this research, including beam-heating studies in electron microscopy, it has been concluded that slight reduction of certain transition-metal oxides (e.g. simple model oxides, WO3, M0O3, V2O5,... 

There has been considerable work in the literature on the structure of very small particles and clusters. Interest in this field has been primarily due to Ino s (1966) early experimental studies of normally fee metals prepared by vapour condensation which showed that a sizable portion of the particles exhibited non-crystallographic structures. These non-crystallographic atomic clusters or polycrystalline nuclei have been observed to consist of pentagonal bi-pyramid or icosahedra form of twinned structures and are known as multiply twinned particles (MTPs). EM studies of supported transition metal catalyst systems have indicated that MTPs sinter faster in catalytic reactions leading to the loss of surface area and are not beneficial to catalysis (Gai 1992). We describe the structure and the role of MTPs in catalysis in the following sections. 

Block-shaped black crystals with a distinct metallic luster are formed which upon crystallographic study are found to crystallize in the triclinic system with lattice constants as follows a = 7.786 (1) A, fe = 13.033 (3) A, c = 18.590 (4) A, a = 110.09 (2)°, 0 = 90.21 (2)°, y = 105.27 (2)°, and Vc = 1699.8 (6) A3. These lattice parameters indicate that the crystals contain 0.5 molecules of TCE in the formula unit as is the case for isostructural (BEDT-TTF)2ClO4(TCE)0 5 in which Vc = 1689.8 A3.10 The electrical properties of this material indicate that they are metallic from room temperature to 90 K at which point a metal-insulator transition occurs. 

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