Molecular orbital theory boron hydrides

Valence theory, boron hydride, diborane and, 124-126 electron deficiency and, 121-122 higher hydrides and, 126-128 molecular orbitals and, 128-131 three-center bond and, 122-124... 

Moffitt and Ballhausen,41 and the application of the molecular orbital theory to electron-deficient compounds, particularly the hydrides of boron, by Eberhardt, Crawford, and Lipscomb has been fully described by these authors in their original papers.14 The present review will, therefore, be devoted entirely to those developments in the molecular orbital theory which have been associated with its application to the electronic spectra of unsaturated hydrocarbons. These developments form in themselves a relatively coherent story, the main lines of which are now clear, and it seems a suitable moment at which to put the history of the subject into perspective. Before doing this, however, it will be convenient to outline the general premises of the molecular orbital theory. 

See, for example, F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistryt Wiley-Interscience, New York, 1962, pp. 200-203 W. N. Lipscomb, Boron Hydrides, Benjamin, New York, 1963, Chap. 2 C. J. Ballhausen and H. B. Gray, Introductory HoUs on Molecular-Orbital Theory, Benjamin, New York, 1965, Chap. 7. 

A combination or resonance hybrid of these four structures produces a more symmetrical molecule where all of the four borons around the base of the square pyramid are equivalent. This is in agreement with the known structure. As boron hydrides get larger or more symmetrical, the numbers of individual structures making up the hybrid get to the point where it is essentially unmanageable. For these molecules the molecular orbital theory, which constructs orbitals for bonding from the atomic orbitals of multiple atoms, is appropriate. 

Ab initio molecular orbital theory at the HF/6-31G level has been used to investigate the structure of Lewis acid/base adducts of boron hydrides with argon and a variety of substrates that may be encountered in the mechanism for the oxidation of diborane. By use of fourth-order Moller-Plesset theory, i.e., MP4SDTQ, correlation effects are calculated at the HF/6-31G geometries. From HF/6-31G calculations, the following distances (in A) and angles for Ar-boron hydride adducts were found [22] ... 

We now outline two approaches to a description of valence in the boron hydrides. The first employs three-center bonds. This particular kind of localized molecular orbital seems most suitable for the smaller, more open hydrides. Its use in the more complex hydrides will require delocalization of the bonding electrons either by a molecular orbital modification or a resonance description. The second approach is simply that of molecular orbitals, which is particularly effective in the more condensed and symmetrical hydrides. These approaches merge as the discussion becomes more complete. It is an important result that filled orbital descriptions are obtainable for the known boron hydrides. Also some remarks about charge distribution in the boron hydrides are possible. But the incompleteness of this valence theory in this nontopological form is indicated by the lack of a large number of unknown hydrides, whose existence would be consistent with these assumptions. 

These and related methods of structure prediction depend upon the allocation of specific electron counts to different framework geometries and these applications have been very successful in assigning structures to cluster carbonyls and their derivatives. However, for the higher polynuclear carbonyls as the molecularity of the compounds increases the predictive power of the theories becomes less decisive in differentiating between alternative structures. In essence the Wade-Mingos approach assumes that the frontier orbitals of the complex primarily involve metal orbitals so that any variation in the electron occupation will be reffected in a structural change in the metal framework. The Wade theory also requires that the structure of the complexes are based on triangulated polyhedra as found for the boron hydrides. 

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