Boron—carbon bonds metal hydrides

Such complexes possess sharply reduced chemical reactivity and consequently they often tend to stabilize the valence state of the acceptor metallic atom. Lithium tetraphenylboronate requires heating in acid solution in order to effect cleavage of the boron-carbon bonds and is quite stable in air toward oxidation. The acceptance of the phenyl anion has satisfied the electronic demands of boron. A direct preparation of analogous alkyl complexes has been realized by heating lithium aluminum hydride with ethylene under pressure 139) ... 

These trends agree with the frontier orbital analysis. In particular, the delivery of hydride from carbon breaks a relatively unpolarised bond, making the hydride notably soft, as we saw earlier in its capacity to attack pyridinium salts preferentially at the 4-position. The metal hydrogen bond will be more polarised, and metal hydrides should therefore be harder. Similarly, the delivery of hydride from boron will make it softer than when it is delivered from the more electropositive metal, aluminium. It also seems that, among a,(3-unsaturated carbonyl compounds, the susceptibility to conjugate reduction increases in the sequence ketones < esters < acids < amides but there are too few examples to be sure. 

Hydride abstraction of a hydrogen directly bonded to a metal atom has been used to synthesize cationic metal carbonyls. This may be accomplished by protonation, as outlined in Section C,l,f, or with a Lewis acid, such as boron trifluoride, in the presence of carbon monoxide (98). 

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