Hypovalency, agostic interactions, and related aspects of catalytic activation at metal centers

7 Hypovalency, agostic interactions, and related aspects of catalytic activation at metal centers 

It should be noted that the hypovalent Lewis acid-base interactions of the present section are in principle closely related to the general metal-ligand coordination interactions discussed in Section 4.5. However, in normal-valent transition-metal 

Owing to the inherent reactivity of species containing one or more formally vacant valence orbitals, few such transition-metal species are known as isolable compounds. 

As indicated in Table 4.33, the optimal Lewis-like description is in good agreement with the expected idealized sd hybridization of the sigma framework (sd2 for YH3, sd3 for ZrH4, sd4 for NbFL). Each structure also has the expected number of vacant nM valence orbitals (three for YH3, two for ZrH4, one for NbHs), with occupancies so small ( 0.002 ) as to be negligible. Thus, these species conform well to the idealized hypovalent Lewis-like description. 

The characteristic tendency of hypovalent transition metals to interact in a side-on (T-shaped) manner with nearby alkyl C—H bonds has been designated as the agostic effect.45 Agostic M- C—H interactions can also be identified with general 3c/2e donor-acceptor interactions of 2c— lc type. Availability of suitable lc acceptor orbitals is a signature of hypovalent early transition metals. 

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