Coordination and Hyperbonding

Students sometimes assume (mistakenly) that chemical bonding is completed once the electrons are maximally paired up in a closed-shell species of valid Lewis structural form. The error of this assumption was recognized nearly a century ago with discovery of numerous complexes that defied Lewis structural formulation, unless written as two (or more) distinct species. Such complexes therefore appear to violate the valence rules that usually govern chemical stmcture and reactivity, apparently involving some type of extra-valence Nebenvalenz, in the phrase of German inorganic chemist Alfred Werner) that demands significant extension of Lewis structural concepts. Nowadays, the term hypervalency is commonly used to describe species that have too many bonds for conventional Lewis structural depiction, or seem to require chemical association mechanisms beyond those of closed-shell Lewis structure formation. 

A simple and provocative example of such strange association complexes is provided by the bifluoride ion (FHF ). This species can be formulated perfectly well as the Lewis-compliant HF molecule and F fluoride anion. 

Discovering Chemistry With Natural Bond Orbitals, First Edition. Frank Weinhold and Clark R. Landis. 2012 John Wiley Sons, Inc. Published 2012 by John Wiley Sons, Inc. 

Of course, it is initially tempting to characterize FHF as some type of ion-dipole complex of classical electrostatic origin. However, numerous lines of chemical evidence indicate the superficiality and inaccuracy of such description. These include the following  

In this chapter, we wish to explore how the NBO program detects and characterizes such distinctive bond types. This includes how the fundamental Lewis 

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Figure 5.56 illustrates the variation of NRT bond orders bef for reactant and product C—F bonds along the reaction coordinate, and Fig. 5.57 illustrates the corresponding variations of natural atomic charge on the two F atoms. Despite the numerical scatter, one can see in Fig. 5.56 that the reactant (6a0 and product (6cp) bond orders respectively diminish and increase while preserving approximately constant total bond order in the shifting toF Cp hyperbond,... 

Common coordination motifs of hyperbonded complexes and the trans influence ... 

Whereas d8 Ni selects the rectangular di-allylic hyperbonding pattern in (4.121), d6 Fe of ferrocene offers an additional vacant d orbital and hence opens up new geometrical possibilities of an additional cu bond. In concert with the three cu bonds and nominal sd2 (90°) hybridization (Table 4.52), the two Cp ligands are naturally expected to coordinate in r 5 (L2X) fashion to occupy the six octahedrally arrayed coordination sites of the metal. Visualization of this coordination mode is aided by considering the possible patterns of L-type (filled circles 7tcc) and X-type (half-filled circles radical) sites of L2X Cp... 

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