Hypovalency

Exceptions to the Octet Rule Hypervalent and Hypovalent Molecules 21... 

We can alternatively begin the recursion (3.218) with S3+F3 (which is isoelectronic to hypovalent AIF3), leading thereby to... 

The NBO donor-acceptor theory of hypovalency three-center bridge bonds... 

Soon after the quantum revolution of the mid 1920s, Linus Pauling and John C. Slater expanded Lewis s localized electronic-structural concepts with the introduction of directed covalency in which bond directionality was achieved by the hybridization of atomic orbitals.1 For normal and hypovalent molecules, Pauling and Slater proposed that sp" hybrid orbitals are involved in forming shared-electron-pair bonds. Time has proven this proposal to be remarkably robust, as has been demonstrated by many examples in Chapter 3. 

Thus, in each series the chemistry of early transition metals (e.g., hypovalent Sc, Ti, V) is expected to differ appreciably from that of mid to late members (e.g., normal-valent Cr, Mn, Fe, Co, Ni, Cu) and the terminal closed-shell element (e.g., Zn). 

Although orbital hybridizations and molecular shapes for hypovalent metal hydrides of the early transition metals and the normal-valent later transition metals are similar, the M—H bonds of the early metals are distinctly more polar. For example, metal-atom natural charges for YH3 (+1.70), HfH4 (+1.75), and TaHs (+1.23) are all significantly more positive than those (ranging from +0.352 to —0.178) for the homoleptic hydrides from groups 6-10. Indeed, the empirical chemistry of early transition-metal hydrides commonly reveals greater hydricity than does that of the later transition-metal hydrides. 

Figure 4.11 Optimized structures and symmetries of the hypovalent group 3-5 hydrides LaH3, HfII4, and Tails, showing the general resemblance to idealized sdM geometries (Figs. 4.2(b), 4.3(a), and 4.4(b)).

Figure 4.12 Metal hydride bond (ctMh) and antibond (ctMh+) NBOs for hypovalent M = Hf, Ta.

Table 4.15 summarizes optimized bond lengths and NBO Lewis-like structures for 15 saturated normal-valent H MMH compounds (M = W-Pt) as well as corresponding hydrides of hypovalent Ta for comparison. The accuracy of the localized Lewis-like description (as measured by %pf) is found to be reasonably high both for normal-valent and for hypovalent species, typically >98% of the valence-shell density and >99.5% of the total density. 

As in main-group chemistry, hypovalent hydrides of the transition series have pronounced tendencies to form bridging tau bonds. In addition to H4TaTaH4, the multiply bonded species HTaTaH features two symmetrically bridging hydrides. Despite the complexity introduced by such tau bonds, the Ta-Ta interaction clearly has high bond order, as simple Lewis-like structures prescribe. 

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

---