Bridge positions

Boron haUdes have also been shown to insert into B—B bonds to give initial products with the new boryl moiety in a bridge position (80). 

Boranes also form derivatives ia which main group elements occupy a bridging position between two boron atoms, rather than a polyhedral vertex. An extensively studied system is -R MB Hg, where R = H, CH, C2H3, halogen, and M = Si, Ge, Sn, Pb (185). The stmcture of l-Br- J.-[(CH3)3Si]-B3H2... 

Interpretation of tiie ratio of capture of competing nucleophiles has led to the estimate that bromonium ions have lifetimes on the order of 10 s in methanol. This lifetime is about 100 times longer than fliat for secondary caibocations. There is also direct evidence for the existence of bromonium ions. The bromonium ion related to propene can be observed by NMR spectroscopy when l-bromo-2-fluoropropane is subjected to superacid conditions. The terminal bromine adopts a bridging position in the resulting cation. 

Photolysis of 159 leads to a migration of the SMe group to the bridging position and formation of 162. This product can be further decarbonylated to 163, which is also the product of photolysis of 159 and 161. 

In contrast, porphyrinogens 16 (Fig. 5) possess only 16 Ti-electrons and as a consequence the delocalization over the whole macrocycle is absent. The conformations of porphyrinogens are not planar any more and can approximate to the conformations known for calix[4]arenes (vide supra). Compounds 16 (Fig. 5) may, therefore, be considered as heteroatomic calixarene derivatives, of which some have additional heteroatoms in the bridging positions [15, 28-30]. 

Notice that there are three curved arrows here. For some reason, students drawing this mechanism commonly forget to draw the third curved arrow (the one that shows the expulsion of Br ). The product of this hrst step is a bridged, positively charged intermediate, called a bromonium ion ( onium because there is a positive charge). In the second step of our mechanism, the bromonium ion gets attacked by Br (formed in the hrst step) ... 

Fig.6 Binding energies of Cu (full lines) and Ag (broken lines) on a Si(lll) surface. The perpendicular distance between the adsorbate atoms and the plane of the surface silicon atoms is denoted by h. Hollow, top, and bridge positions of the adsorbate atoms are indicated by the labels A, B, etc. as shown in the insert, u corresponds to an unrelaxed and r to a relaxed geometry of the neighboring surface Si atoms (after Ref.47)...

The precise structural role played by the water molecules in these cements is not clear. In the zinc oxychloride cement, water is known to be thermally labile. The 1 1 2 phase will lose half of its constituent water at about 230 °C, and the 4 1 5 phase will lose water at approximately 160 C to yield a mixture of zinc oxide and the 1 1 2 phase. Water clearly occurs in these cements as discrete molecules, which presumably coordinate to the metal ions in the cements in the way described previously. However, the possible complexities of structure for these systems, which may include chlorine atoms in bridging positions between pairs of metal atoms, make it impossible to suggest with any degree of confidence which chemical species or what structural units are likely to be present in such cements. One is left with the rather inadequate chemical descriptions of the phases used in even the relatively recent original literature on these materials, from which no clear information on the role of water can be deduced. 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

The first published crystal structure of the full length HHR [126] in which there was no solvent or ions resolved showed A9 and the scissile phosphate in close proximity, consistent with the interpretation of thio effect measurements [130], and the G8 02 and G12 Ni poised to act as a general acid and base, respectively, as proved in previous photocrosslinking [131] and mutation experiments [132], Given the strong evidence that Mg2+ participates directly in the catalytic process together with the spatial proximity of the A9 and scissile phosphate, made the placement of an Mg2+ ion in bridging position a reasonable assumption. 

Causes the Migration of the Mg2+ Ion from the C-Site to the Bridging Position... 

Figure 14-10. A schematic view of the possible migration of the Mg2+ ion from the C-site to the bridging position. Spontaneous migration was predicted from the simulation for the transition states and the deprotonated reactant state, with the Mg2+ ion initially placed at the C-site...

Some carbonyl hydrides, such as Cr2(CO)ml l, have the hydrogen atom in bridging positions as shown in the structure... 

Even the Cannizzaro reaction might have a similar transition state. Although the coordinating tendency of alkali metals is less than that of aluminum or magnesium, it is not negligible. An alternative structure is possible in this case in which a proton occupies the bridging position. 

The chemisorption of species occurs at specific sites on the electrode, for example on top of certain atoms, or in the bridge position between two atoms. Therefore, most adsorption studies are performed on well-defined surfaces, which means either on the surface of a liquid electrode or on a particular surface plane of a single crystal. Only fairly recently have electrochemists learned to prepare clean single crystal electrode surfaces, and much of the older work was done on mercury or on amalgams. 

When the smaller P(Me)3 ligand was used, not only the halogens but also a hydride were found in the bridging position, as was concluded from the fact that the phosphorus atom connected to the rhodium (I) center was now coupled to a hydride. 

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