The Role of Recoupled Pair Bonding in Hypervalent Molecules

The Role of Recoupled Pair Bonding in Hypervalent Molecules 

Many elements possess the ability to form more bonds than expected based upon the nrunber of unpaired electrons that are present in their atomic ground states. The elements P, S, Cl, Kr in the p-block and the elements below them in the periodic table behave in this marmer. Compoimds involving these elements that exceed the nominal valence for a given colrurm are said to be hypervalent or hypercoordinated, terms coined by Musher [1] and Schleyer [2], respectively. 

Typical examples of hypervalent compounds include SF4 and SF, , where sulfur forms four and six bonds, respectively, in spite of the fact that the ground state of 

S has only two unpaired electrons. By contrast, oxygen is almost exclusively mono-or divalent OH and H2O are stable but OF4 and OFg are not. Other examples of small stable (or metastable) hypervalent fluoride compounds include PFj, CIF3, CIF5, and XeF,j, (n=2, 4, 6). Radicals such as SFj, SF5, and PF4 have also been observed but are difficult to study in the laboratory because of their high reactivity. Nonetheless, they represent hypervalent or hypercoordinated species. 

Explanations for the existence of hypervalent species started with Pauling s proposal of d orbital hybridization [3] where, for example, a set of sp d hybrid orbitals on sulfur was put forward to account for the hexavalence of SFg. Chemical computations [4] ruled out the participation of d atomic orbitals. A later model, the Rundle-Pimentel three-center, four-electron (3c-4e) bonding model [5], does not require d orbital participation and, so, is consistent with chemical computations. Other hypervalency models include the diabatic state model of Dixon and coworkers [6] and the democracy principle of Cooper and coworkers [7]. These various models provide useful insights into some aspects of the electronic structure of hypervalent molecules, but they don t provide an overarching description of these molecules that enables connections to be drawn between hypervalency and related molecular phenomena. 

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I 3 The Role of Recoupled Pair Bonding in Hypervalent Molecules 3.4... 

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