Nonclassical H-Bonds

Nonclassical H-bonding has an important role in the intramolecular heterolytic cleavage of H2 in these complexes (as illustrated in Scheme 4.15). H-bonding between the internal base and the / -H2 ligand stabiHzes the complex... 

From the previous discussion, we can conclude that when the observed value of /(H-Si) is relatively large, say, in the range 70-160 Hz, it is safe to think that the Si H bond is involved in nonclassical bonding with metal. In contrast, any conclusion on the presence or absence of a Si-H interaction on the basis of a small value of silicon-proton coupling constant in the absence of an independent evidence can be erroneous. In this case, the measurement or calculation of the sign of /(Si-H) can help to identify a direct Si H interaction. 

It is obvious that studying interligand Si-H interactions has reached a great extent of sophistication. At least three classes of nonclassical Si-H bonding can be identified. These are the electron-deficient residual Si H interactions in silane a-complexes and agostic complexes, electron-rich IHI MH SiX, and the more recent multicenter H Si interactions, which are the subject of current debate and have features common to both IHI and a-complexes. This surprising diversity stems from the special role the substituent at silicon can play in tuning the extent of Si H interaction, and from the propensity of silicon to be hypervalent. 

The data in Table 7.2 show that the H- H distances are determined in solutions between 1.40 and 1.96 A, strongly supporting formulation of the interactions as dihydrogen bonds. Finally, systems 32 to 34 in Table 7.2 are shown for comparison, and it is clear that dihydrogen bonds and nonclassical M- H bonds are energetically very similar. 

Complexes which contain tbe ligand are now referred to as nonclassical. while those in which the H—H bond has been severed are called classical. 

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