Borane complexes electronic structure

In addition to these limited procedures a number of experimental methods (vibrational spectroscopy, dipole moment measurements, electron diffraction, NMR, etc.) have been employed to determine the relative stabilities of these complexes.11,23 Intense effort has been directed towards establishing some kind of correlation between NMR parameters and stability of the borane complexes. The chemical shifts alone rarely show good correlation. However, complexation shifts (the chemical shift difference between the free and complexed borane or ligand) and various spin-spin coupling constants correlate better with calorimetric data, especially for ligands or boranes belonging to structurally similar series (Table 2).10,24... 

Upon reaction with basic transition metal complexes the general tendency of electronically unsaturated boranes to form Lewis acid-base adducts should result in the formation of borane complexes (I). [CpzWILfBFs)] (1), which is known from many textbooks represents the best known example for such a compound. Recently, however, the proposed constitution of 1 was disproved, and it was shown that reactions of [Cp2WH2] with boranes yield salt like compounds 2 and zwitterionic species 3, respectively. For the formation of 2 one equivalent of HF is required, and in this reaction the borane acts as a fluoride source, while the WH2 moiety supplies a proton (Figure 3). Until now no structurally authentic borane complex was reported. 

Scheme 19 The continuum of electronic structure between limiting descriptions of rhodium complexes as boryl hydrides or as a boranes...

Abstract A variety of structural types of transition metal complexes containing a-borane ligands are reviewed. Their structure and bonding are discussed. Compared with other types of a-complexes, a-borane complexes display quite different structural and bonding properties because of the electron unsaturated boron center in the a-borane ligands in the precoordination state. The availability of an sp3-hybridized orbital at the boron center allows stronger back-donation interaction without breaking the coordinated B-H bond. The role played by a-borane complexes in hydroboration and borylation reactions has also been reviewed. 

Borane and Carbaborane Complexes.—The chemistry of borane and carba-borane complexes incorporating a CpM residue has been reviewed and several new such borane and carbaborane derivatives have been prepared. The nature of the bonding and electronic structures of CpM(carbaborane) complexes has been discussed. ... 

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