Hydrogen-bridged structures

This is known as a hydrogen-bridge structure. There are not enough electrons to make all the dotted-line bonds electron-pairs and hence it is an example of an electron-deficient compound. The structure of diborane may be alternatively shown as drawn in... 

Aluminium tetrahydridoborate is a volatile liquid. It is the most volatile aluminium compound known. It is covalent and does not contain ions but has a hydrogen-bridge structure like that of diborane, i.e. each boron atom is attached to the aluminium by two hydrogen bridges ... 

Localized Bonds. Because boron hydrides have more valence orbitals than valence electrons, they have often been called electron-deficient molecules. This electron deficiency is pardy responsible for the great interest surrounding borane chemistry and molecular structure. The structure of even the simplest boron hydride, diborane(6) [19287-45-7], B2H , was sufficiendy challenging that it was debated for years before finally being resolved (57) in favor of the hydrogen bridged structure shown. 

The existence of singly hydrogen-bridged structures such as shown above has been questioned and an alternative has been proposed 38>. 

While this material has not been isolated, it is stable in ether solution. It boron-11 nmr spectrum consists of a singlet and doublet in the area ratio of 1 1 which is consistent with the proposed structure in general, bridge coupling is not observed for singly hydrogen-bridged structures. 

The first experimental example of a doubly-hydrogen bridged structure, i.e. fraws-[RSn(yU,-H)]2 (R = 2,6-Tip2CsH3, Tip = 2,4,6-triisopropylphenyl), was reported recently303. This compound is stable in the solid state at room temperature, but in hydrocarbon solvents (e.g. hexane, benzene, toluene) and diethyl ether it dissociates to RHSn monomers. 

As early as 1940 the American chemist F. Stitt demonstrated that in B2H0 the barrier hindering the internal rotation about the hypothetical B—B bond is much higher as compared to ethane. Then some more experimental data appeared to promote the hydrogen bridge structure. Dilthey s model was again taken up in the publications by B. V. Nekrasov, Ya. K. Syrkin, M. E. Dyatkina in the USSR, and some foreign chemists. 

Considerable interest has centred on the structures of the boranes. They are all electron-deficient that is they have too few valency electrons to permit every one of the adjacent atoms to be held together by electron-pair bonds. Diborane has been the most studied and much evidence has accumulated for a hydrogen-bridged structure (Fig. 121). 

Although we have included these B subgroup hydrides with the salt-like compounds it is possible that the bonding is at least partially covalent in some or all of these compounds an obvious suggestion for BeH2 is a hydrogen-bridged structure like that of Be(CH3)2. 

Sorensen and coworkers showed that at -130°C the cyclodecyl cation exists as a static 1,6- or 1,5-hydrogen-bridged structure (84c or 84c). Similar behavior was observed for the 1,6-dimethyl analog 85a.The bridging hydrogen in ion 84c was observed at an unusually high field, that is, 5 H = -6.85. 

The hydrogen bridge structure of diborane (B2H6) is shown in Fig. 

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