Metal distances

Now the gradual decline in intensity for h — 4, 8,12 (Table I) requires that uy = -J-, and hence % = -J-. This puts the two sets of metal atoms in the same place, and is hence ruled out. It may also be mentioned that structure 1 would place eight metal atoms on a cube diagonal, giving a maximum metal-metal distance of 2.03 A, which is considerably smaller than metal-metal distances observed in other crystals. Structure 2, dependent on one parameter u, has structure factors... 

Only covalent bonds between Li and Na and transition metals are known, there being none with the lower group-IA metals or with inner transition elements. Bonding is inferred from metal-metal distances as well as calculations. ... 

In addition to the standard constraints introduced previously, structural constraints obtainable from the effects of the paramagnetic center(s) on the NMR properties of the nuclei of the protein can be used (24, 103). In iron-sulfur proteins, both nuclear relaxation rates and hyperfine shifts can be employed for this purpose. The paramagnetic enhancement of nuclear relaxation rates [Eqs. (1) and (2)] depends on the sixth power of the nucleus-metal distance (note that this is analogous to the case of NOEs, where there is a dependence on the sixth power of the nucleus-nucleus distance). It is thus possible to estimate such distances from nuclear relaxation rate measurements, which can be converted into upper (and lower) distance limits. When there is more than one metal ion, the individual contributions of all metal ions must be summed up (101, 104-108). If all the metal ions are equivalent (as in reduced HiPIPs), the global paramagnetic contribution to the 7th nuclear relaxation rate is given by... 

Table 1 shows that the physicochemical properties of the support material were modified by the pre-treatment process. The particle sizes. Dp, which are summarized in the Table 1 were calculated from the X-ray diffraction patterns of prepared catalysts and a commercial catalyst(30 wt% Pt-Ru/C E-TEK) by using Scherrer s equation. To avoid the interference from other peaks, (220) peak was used. All the prepared catalysts show the particle sizes of the range from 2.0 to 2.8nm. It can be thought that these values are in the acceptable range for the proper electrode performance[7]. For the prepared catalysts, notable differences are inter-metal distances(X[nm]) compared to commercial one. Due to their larger surface areas of support materials, active metals are apart from each other more than 2 3 times distance than commercial catalyst. Pt-Ru/SRaw has the longest inter-metal distances. 

Design parameters of the anode catalyst for the polymer electrolyte membrane fiiel cells were investigated in the aspect of active metal size and inter-metal distances. Various kinds of catalysts were prepared by using pretreated Ketjenblacks as support materials. The prepared electro-catalysts have the morphology such as the sizes of active metal are in the range from 2.0 to 2.8nm and the inter-metal distances are 5.0 to 14.2nm. The electro-catalysts were evaluated as an electrode of PEMFC. In Fig. 1, it looked as if there was a correlation between inter-metal distances and cell performance, i.e. the larger inter-metal distances are related to the inferior cell performance. 

But when the contents of Nafion ionomer was increased from 30 to 45 % to find out the better electrode structures, the Pt-Ru/SRaw, which had showed the lowest single cell performance, became the best electro-catalyst. By this result one can conclude that as long as the structure of the electrode can be optimized for the each of new electro-catalysts, the active metal size is a more important design parameter rather than inter-metal distances. Furthermore, when the electro-catalysts are designed, the principal parameters should be determined in the consideration of the electrode structures which affect on the electron conduction, gas permeability, proton conductivity, and so on. 

Metal-metal distances in supported metal clusters (e.g., It4, nearly 2.70 A) determined by EXAFS spectroscopy essentially match those in coordinatively saturated clusters of the same metal (e.g., Ir4(CO)i2). These distances are about O.2-O.3A greater than the metal-metal distances in the free (gas-phase) clusters (e.g., It4, 2.44 A) [32]. Similar results have been determined for supported OS5C [33] and Rhe [28,29]. 

Fenske, D., Baum, G., Zinn, A. and Dehnicke, K. (1990) Ag2[Ph-C(NsiMe3)2]2 and Au2[Ph-C(NsiMe3)2]2 - amidinato complexes with short metal-metal distances. Zeitschrifi fur Natuiforschung B, 45, 1273-1278. 

The first copper(I) complex of tris(hydroxymethyl)phosphine ((760) tetrahedral) has been reported by Samuelson and co-workers. This group addressed the question of anion-controlled nuclearity and metal-metal distances in copper(I)-bis(diphenylphosphino)methane complexes, and in this endeavor they reported the structures of complexes (761) (Cu-Cu separation 3.005-3.128 A), (762) (Cu-Cu separation 3.165 A) and (763) (tetrahedral Cu-Cu 3.293 A). 6 They synthesized and provided structural evidence of oxy anion- encapsulated copper(I) complexes of this ligand. The complexes (764) (distorted tetrahedral Cu-Cu 3.143 A), (765) (distorted tetrahedral Cu-Cu 3.424A), (766) (distorted trigonal Cu-Cu 3.170A), and (767) (Cu-Cu 3.032-3.077A) were reported. They studied solid-state emission spectra of these complexes.567 During this pursuit they... 

After several years of effort it has been possible to reproduce Schmidbaur fs addition of methyliodide W The problem appears to be that the methyliodide product is extremely photosensitive in solution. It decomposes with the production of methyl radicals and the ultimate formation of a mixture of the Au(I), and Au(II) diiodide products. The X-ray crystal structure of the methyliodide product is presented in Figure 5. Two features are to be noted. Firstly, the metal-metal distance lengthens only a small amount, 0.04A, but the Au iodide distance lengthens by approximately 0.20. The structural trans effect caused by the methyl group is not attenuated by the Au-Au bond. 

---