Boron hydrides determination

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

The electrical conductivity of these alloys is metallic i.e., inverse proportional to the temperature between 4 to 300 K and is about 7 x 106 (Qm) 1 at room temperature. The crystal structure determination [7] shows the B atomic arrangement to consist of coplanar 4B clusters (isolated from one another) with three B atoms at the vertices and one B at the center of a triangle as shown in Fig. 5. This isolated 4B atomic arrangement has no precedent either in metal-borides [14] or boron hydrides [15]. 

The diammoniate can be identified by means of its x-ray powder diffraction pattern and by chemical analysis. For analysis a sample of the substance is hydrolyzed with 6 N hydrochloric acid for several hours at 90° in a previously evacuated and sealed tube, which is equipped with a breakable side arm allowing gas transfer into the vacuum system. Hydridic hydrogen is measured volumetrically nitrogen and boron are determined by standard acid-base titrations. Anal. Calcd. for [BH2(NH3)2][BH4] hydridic H, 9.80 B, 35.05 N, 45.35. Found hydridic H, 9.74 B, 34.5 N, 45.7. 

The systematic principles of boron hydride structures ahd chemistry are the principal subjects of the present review. There are several reasons why these principles became clear such a long time after the discovery of these compounds (a) most of the compounds must be handled in grease-free vacuum line systems (b) some of the boron hydrides are unstable at ordinary temperatures, explosive on contact with air, and toxic (c) the structures are based on principles, still incompletely developed, of electron-deficient compounds and (d) location of the hydrogen atoms is a crucial part of the structure determinations, unlike the situation in hydrocarbons, and had to be done for the most part in X-ray diffraction studies of single crystals grown at low temperatures. 

The first piece of information obtained from a structure determination is the connectivity of atoms. This leads to the chemical formula, if previously unknown. Penicillin and vitamin B12 (see Figures 1.12 and 1.13, Chapter 1) are examples of compounds for which X-ray diffraction analyses established the atomic connectivity and, as a result, the chemical formulas and subsequent geometries of these molecules. Other examples include the boron hydrides, which were found to have unexpected arrangements of atoms. For many of these compounds the atomic connectivity was not known before the X-ray diffraction analyses. 

This volume has been written for those chemists and biochemists who may never themselves do X-ray diffraction analyses of crystals, but who need to be able to understand the results of such studies on structures of immediate interest to them. The fields of structural biology and chemistry have blossomed in the years since X-ray diffraction was discovered in 1912. For example, the three-dimensional structures of benzene, graphite, the alkali halides, the boron hydrides, the rare gas halides, penicillin, vitamin Bjj, hemoglobin, lysozyme, transfer RNA, and the common-cold rhinovirus have been determined and, in each case, the results have greatly increased our understanding of fundamental chemistry and biochemistry. 

The TEC model developed by Teo also has been successfully applied to rationalize the geometries of a large number of cluster compounds. The TEC model combines Lauher s rule with Euler s theorem and adds an adjustable parameter This parameter X is equal to the number of electron pairs present in excess of that predicted by the 18-electron rule. " X has also been interpreted in terms of the number of missing antibonding orbitals. Given a value for X, determined by the shape of the cluster, an equation predicts the electron count for a cluster. Theoretical justification of the parameter X is based largely upon the classical molecular orbital calculations performed by Hoffmann and Lipscomb via the extended Hiickel method on the corresponding polyhedral boron hydride clusters The values... 

With the exception of polynuclear boron hydrides, cluster chemistry has developed mainly over the past 40 years. Prior to this a number of compormds that would now be classified as clusters had been but their nature was not recognized. It is perhaps interesting to ask the question of why cluster compounds, that is molecules containing metal-metal bonds, were considered prior to 1940 to be a relatively uncommon feature in inorganic chemistry The problem was in part associated with the dominance of organic chemistry coupled with the difficulties in the rapid determination of the structure of compounds of this type. The determination of the X-ray structure of relatively simple compounds could take months or years to accomplish. As stated above, for the s and p block elements the compounds were also generally found to be unstable to air and moisture, whilst for the d and f block the compounds were often very complex in structure and the stoichiometry difficult to assess. This led to a general lack of appreciation of the occurence and stability of metal-metal or element-element bonds. 

The Colonel did not discover boron hydrides, nor was he even the first to determine the structure of a boron hydride. However, he was the first to see the patterns. He taught the world the structural systematics and showed how the... 

Problem L19. Interestingly, suppose you determined that boron hydride was largely dimeric (how might you do this ) with a structure as shown in the figure... 

The alkali metal is estimated by hydrolyzing an aliquot of the reaction mixture with water and titrating the base formed against a standard acid. Boron is determined by GC analysis of the alcohol obtained by oxidation of the alkylbo-ranes. Hydride is estimated by measuring the hydrogen evolved after hydrolysis of the solution. 

Stock separated the above volatile boron hydride product mixture into pure substances and then determined the chemical formula and physical constants of each. These... 

The correct structure of B2H6 was determined by infrared spectroscopy, and this was the only boron hydride structure simple enough to be determined by this means. The actual B2H6 structure is... 

Boron hydrides thus contain four different kinds of bonds ordinary boron-hydrogen bonds, boron-hydrogen-boron three-center bridge bonds, ordinary boron-boron bonds, and boron-boron-boron three-center bonds. W. N. Lipscomb developed a method for determining what combinations of these structural features are possible for a specific boron hydride formula and what are the possible structures. This system is equivalent to the simple bonding rules in organic chemistry that make it possible to translate an empirical formula into possible organic structures. 

Although they are not classified as organoboron compounds, boron hydrides, chloroboranes, boron halides and boric acid esters are discussed in the first part of this chapter as also is the determination of hydridic and active hydrogen in these types of compounds. 

Koski et al have studied the use of pyrolysis followed by the use of thermal conductivity cells for the determination of deuterium in deuterated boron hydrides, their organic derivatives and nitrogen compounds. The volatile boron compounds were first... 

Sawinski and Suffet applied a Melpar flame photometric system to the determination of boron hydrides, the detector was fitted with an optical filter having a maximum transmittance of 546nm. The column packing was OV-1 on Chromosorb W. Down to 0,7ng decaborane could be detected with this equipment,... 

Fairchild, R. G. Kahl, S. B. Laster, B. H. Kalef-Ezra, J. Popenoe, E. A invitro determination of uptake, retention, distribution, biological efficacy, and toxicity of boronated compounds for neutron-capture therapy-a comparison of porphyrins with sulfhydryl boron hydrides. Cancer Res. 1990, 50, 4860-4865. 

Infrared spectroscopy has assisted in the determination of the structure of certain complex inorganic molecules, uch as the metal carbonyls, inter-halogen compounds, boron hydrides, and nitrogen oxides. The practice used has been to compare the observed spectrum with the spectrum expected for an assumed model on the basis... 

INDO-parameterised calculations based on eq. 3 give very good results for some electron-deficient boron hydrides (Table 2) (13). Significant contributions to the shielding are obtained from the two- and three-centre terms. Hence current densities in relatively remote parts of the molecule play a significant role in determining the boron chemical shifts. 

The definitive method for determining static structures is X-ray diffraction. Indeed, the 1976 Nobel Prize in Chemistry was awarded to Professor William N. Lipscomb for his work in determining structures of the boron hydrides by diffraction methods. However, it must be remembered that packing forces and solvation effects may change the preferred structure between solid state and solution. Another technique, which combines theory and experiment, has established a reliability on a par with X-ray diffraction for confirming structures. It is called the ab /n/n o/IGLO/NMR method (see NMR Chemical Shift Computation Structural Applications for an extensive discussion of calculated NMR chemical shifts) and combines calculated chemical shifts for a number of possible structures with the experimentally measured chemical shifts in solution. 

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