Bonding with hydride bridges

Bonding of this type and other boron hydrides that have three-center two-electron bonds with hydrogen bridges is discussed in Chapter 13. 

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

Alkylaluminum hydrides react with metal hydrides to eliminate RH, and if the metal fragment is bonded to a cyclopentadienyl ring, H2 may also be eliminated to produce a new aluminum-carbon bond, as illustrated in Equations (54)—(57).11,84,83 The hydride-bridged Al-Mo bond in 52 is considerably longer than the direct Al-Mo interaction (294.4(2) vs. 263.6(2) pm).83 A similar reaction with (C5H4R)Co(G2H4)2 yields a mixture of 5382... 

Amongst the hydro-closo-polyborates B H 2- members with n = 6 to 12 are known. All hydro-closo- but also hydro-nido-polyborates act as bases they are, however, not typical Lewis-bases as they miss free electron pairs. However, the negative charge at the hydrogen atoms allows for an interaction with Lewis acids A by formation of hydride bridge bonds (3c2e bonds) ... 

Figure 10.8. Interaction of a C—H bond with a strong Lewis acid (a) initial interaction (b) bonding MOs of a hydride bridge.

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