Boron-hydrogen bonds determination

In closo boranes, the total number of valence electron pairs is equal to the sum of the number of vertices in the polyhedron (each vertex has a boron-hydrogen bonding pair) and the number of framework bond pairs. For example, in 65115 there are 26 valence electrons, or 13 pairs (= 2n + 1, as mentioned previously). Six of these pairs are involved in bonding to the hydrogens (one per boron), and seven pairs are involved in framework bonding. The polyhedron of the closo structure is the parent polyhedron for the other structural types. Table 15.8 summarizes electron counts and classifications for several boranes. The determination of the number of framework electron pairs for the other boranes is a bit counterintuitive on the basis of how the electrons are classified (see example). 

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. 

Lysyj and Greenough applied their procedure to the determination of boron-hydrogen bonds in trimethylaminomonoborane, ((CH3)... 

Boronic acid-derived fluorescent chemosensors are unique in that the inter-molecular interaction is a covalent bond, and not hydrogen bonding as is the case for most conventional fluorescent molecular sensors used for the selective reeognition of hydroxyl carboxylic acids. This chapter summarizes the development of the boronic acid-based chiral fluorescent chemosensors over recent years and the enantioselective fluorescent reeognition of chiral a-hydroxyl carboxylic acids analytes in aqueous solutions. The fundamental scaffolds of these chiral sensors include a fluorophore, an arylboronie aeid binding site, and linker between the two units. The systems usually consist of a bis-boronic acid unit, which is required for enantioselective recognition of the chiral a-hydroxyl carboxylic acid analytes. However, mono-boronic acid fluorescent chemosensors have also been developed. All three components of the chiral boronic acid sensors play an important role in determining the... 

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. 

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