Boron Hydrides

William N. Lipscomb

• Editorial: Dover Publications
• Año: 2013
• ISBN:
  9780486271989
• Categoría: Ciencias físicas
• Categoría: Química

In this classic monograph, Nobel Prize–winning chemist William N. Lipscomb elucidates his area of expertise: the general structural principles and reactions of boron hydrides and related compounds. Lipscomb's work appeared only a decade after the structures of boron hydrides were first elucidated and their chemistry formulated into a widely applicable framework. His observations led to a major reconsideration of how atoms bond to form stable molecules.
A concise treatment of the many separate parts of the structural theory and its relation to chemistry, this volume begins with an overview of boron hydrides and related structures, progressing to three-center bonds and their applications, molecular orbitals, nuclear magnetic resonance studies of boron hydrides and related compounds, and reactions of the boron hydrides. More than 120 diagrams and figures illustrate a variety of structures.

1

Boron Hydrides and Related Structures

The boron hydrides, first described and characterized by Stock,²⁹⁹ form an unusual set of compounds. A systematic description of their chemistry is now possible, owing to the elucidation of their molecular structures, a valence-theory description, and many new studies of their chemical reactions. Just on the horizon is a detailed mechanistic description of their chemical reactions, and a predictive chemistry approaching that of carbon chemistry. In this monograph an attempt is made to systematize the molecular structure properties, the valence structures in terms of three-center bonds and molecular orbitals, the nuclear magnetic resonance (NMR) spectra, and, finally, the known chemical reactions.

1–1 The Hydrides

Tentative evidence, mostly of a physical type, places the number of presently identifiable distinct hydrides somewhere near thirty, but only ten have been isolated and well characterized. These known hydrides are B₂H₆, B₄H₁₀, B₅H₉, B₅H₁₁, B₆H₁₀, B₉H₁₅, B₁₀H₁₄, B₁₀H₁₆, B₁₈H₂₂, and iso-B₁₈H₂₂. The structural evidence for them is outlined below, and is followed by similar discussions concerning the boranates (boron hydride ions), the amine derivatives, the halides of boron, and the carboranes.

B₂H₆

That the bridge type of structure is the correct one for diborane is generally recognized to have been established by Price^242,243 on the basis of the infrared spectrum. Prior to this study, however, there did exist a general realization^{17,186,203,237,297,298} that the physical evidence was overwhelmingly in favor of this structure. The failure of general acceptance of, for example, the high barrier to internal rotation and the earlier infrared evidence²⁹⁷ as indicative of the bridge structure, must be attributed to the insistence^8,10 that the early electron diffraction evidence favored the ethane type of structure. A later electron-diffraction study,¹⁰⁵ however, confirmed the bridge structure and was in disagreement with the ethane type of structure. The structure is shown in Fig. 1–1.

The correct molecular geometry was first proposed by Dilthey.⁵² However, interpretations in terms of valence bonds or molecular orbitals were made only comparatively recently.^203,237,317 Whether one thinks in terms of a “protonated double bond,” i.e., a four-atom bridge bond, in an ethylenic type of structure, or in terms of the interaction of two BH₃ groups, is, in any complete analysis, merely a matter of taste, but analogy of bonding geometry and electronic structure with these properties of ethylene is very striking.

B₄H₁₀

Both hydrogen and boron positions (Fig. 1–2) were established unambiguously by the X-ray diffraction study^207,208 in which there were 616 observed reflections to determine 14 parameters counting the boron, scale, and temperature parameters. Hydrogen atoms, not included in the counting of parameters, appeared both in the presence of the boron atoms and when the boron atoms were subtracted out in the electron-density maps. This model was independently suggested in the recent electron-diffraction study¹³⁹ on the basis of its plausible relation to the other boron hydrides and of its consistency with the electron-diffraction data, but these results are not unambiguous, for a slight modification of the model decided upon in the earlier diffraction study⁹ is equally consistent with the electron-diffraction data. Tests of other possible models were not made in this most recent electron-diffraction study, but the results of the earlier study were shown to be incorrect.

Figure 1–1 The structure of B₂H₆. The B₁—B₂ distance is 1.77 A, the B—H distance is 1.19 A, the B—H_µ (bridge) distance is 1.33 A, and the H—B—H angle is 121.5°.

Figure 1–2 The structure of B₄H₁₀. The B₁—B₃ distance is 1.71 A, and the other four close B · · · B distances are 1.84 A. The B—H distances average to 1.19 A, while the B—H_µ (bridge) distances are 1.33 A toward B₁ and B₃ and 1.43 A toward B₂ and B₄.

Comparison of the results of the X-ray and electron-diffraction study indicates the molecular parameters, B₁—B₂ = 1.84 A, B₁—B₃ = 1.71 A,

B₂B₁B₄ = 98°, B—H = 1.19 A, B₁—H_µ = 1.33 A, and B₂—H_µ = 1.43 A. This is a model obtained from the X-ray values of boron-boron distances and bond angles, including hydrogen angles, and from the electron-diffraction values of the B—H distances, but with the asymmetry of the hydrogen bridges reversed to agree with the X-ray results. This combination avoids the systematic errors introduced by the lack of complete convergence of the Fourier series in the hydrogen distances of the X-ray study and the very large uncertainties in the bond angles in the electron-diffraction study.

The boron arrangement can be considered as a fragment of either the icosahedron or the octahedron because the B₂—B₁—B₄ bond angle is between 105° and 90°.

B₅H₉

The certainty with which this structure (Fig. 1–3) is established is based upon the crystal-structure study,^58,59 in which, at the outset, molecules of C...