Diboranes Lewis-base adducts

Since 1964, a large number of borane adducts have been prepared through either the direct reaction of diborane(6) with Lewis bases or through less direct procedures which involve displacement of hydride ion from BH4. Table 3 below lists most of these materials and some of their properties. 

Boranes are strong reducing agents and the neutral molecules, inflame spontaneously in air, although the anions [BnHn]2- have remarkable kinetic stability. Diborane itself reacts with Lewis bases to give donor-acceptor complexes with BH3, which is a soft Lewis acid and forms adducts with soft bases such as CO (1). More complex products often result from unsymmetrical cleavage of B2H6, for example,... 

Although the bond angles are known to vary considerably depending on the metal and halogen, the arrangement is approximately tetrahedral around each metal atom. When the trihalides are dissolved in solvents that are Lewis bases, the dimers separate and complexes containing the monomer and the solvent are formed as a result of Lewis acid-base interactions. Such behavior is similar to that of borane and diborane in that the monomer, BH3, is not stable but adducts of it are. This type of behavior is illustrated in the following equations ... 

Although BH, exists in the form of Lewis acid-base adducts and as a presumable intermediate in reactions of diborane. only trace quantities of the free molecule have been detected. The equilibrium constant for dimerization is approximately lO and the enthalpy of dissociation of the dimer to the monomer is about +150 kJ mol or slightly more. 120... 

As a conclusion to this section it should he mentioned that reactions between diborane and several other nitrogen Lewis bases besides those mentioned above have been investigated. These nitrogen donors include hydrazine and its methyl derivatives (130), hydroxylamine and its. V-metliyl derivatives (45), and O-methylhydroxylamine and its A-methyl derivatives (11). In general, borane adducts are formed (Table II) which are thermally unstable with respect, to release of hydrogen. 

Carbon monoxide is an extremely weak Lewis base towards conventional Lewis acids. It does not complex with the boron trihalides, although it does yield a weak adduct H3B CO with diborane. On the other hand it forms numerous complexes with transition elements. The source of this difference is that in the latter complexes not only is the weakly donating a-orbital of CO involved, but also the 7C orbitals which can function as acceptors. The conventional explanation is that a synergic effect exists in which the n interaction removes electron density from the metal, allowing a donation from the ligand to be enhanced. 

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] ... 

Borane, BH3, is an avid electron pair acceptor because only six valence electrons are present at boron. Pure borane exists as a dimer in which two hydrogens bridge the borons. In aprotic solvents that can act as electron donors, such as ethers, tertiary amines, and sulfides, diborane forms Lewis acid-base adducts. 

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