Pentaborane 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 structure of decaborane(16) 200> reveals that it should be considered as an apically substituted derivative of pentaborane(9). The bond energy of the unique boron-boron bond which connects the two B5H8 cages has been determined to be 3.2 0.2 e.v. from mass spectrometry 201> the standard heat of formation is 34.8 kcal. per mole 202>. An analysis of the vibrational spectrum of decaborane(16) has been made by Pinson and Lin 203> who note the absence of the strong absorption at 901 cm-1 which was previously reported 204>. 

The structure of m cfo-pentaborane(9) is obtained by the removal of a single vertex from [B6H6]2 and is shown in Fig. 11. Since electron- and X-ray diffraction, and microwave spectroscopy have all been used to determine structural parameters, the data are all summarized in Table 3. 

Structural data are available for several Ayp/to-pentaboranes which are BsHg2L adduct species77 . Shore and coworkers have determined the crystal and molecular structure of BjHg 2 P(CH3)2 77a. The structure is a shallow pyramid (C3 point symmetry) with basal borons distorted from a square pyramid. The two trimethyl-... 

The purity of two samples of pentaborane was also determined with this instrument. Sample 1 was 99.99 mole-% pure, while Sample II analyzed as 99.91 mole-%. When the two samples were mixed together to obtain a sample purity of 99.94 mole-%, the experimental calorimetric purity was 99.949 mole-%. 

Other known acid-catalyzed pentaborane substitution reactions also result in apically substituted pentaboranes (251, 261-263). This is in agreement with the predicted site of electrophilic substitution based on charge distributions, as determined from valence bond and molecular orbital treatments (17,264,265). Furthermore, the position of observed attachment of boron on the carbon skeleton (i.e., the most substituted carbon when unsymmetrical alkenes are used) is consistent with this predicated mechanism (249). The position of boron attachment is kinetically rather than thermodynamically controlled, for basal substituted alkylpentaboranes are found to be about 3 kcal/mole more stable than the corresponding apically substituted pentaboranes (266). 

Although the structure of the dialkyl derivative of pentaborane(ll) has not been unambiguously determined, it is believed that the two alkyl groups are attached to different boron atoms. This assignment is partially supported... 

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