Hypercoordinated XH_n+1radicals for first-and second-row atoms. a valence bond analysis

A theory of hvnercoordination is developed using the valence bond (VB) curve-crossing diagram model and applied to XH_n+1 radicals that are generated by hydrogen atom attachment to a normal-valent XH_n molecule. Hypercoordinated XH_n+1 radicals fall into two broad classes of valence species: those that can be described by a correlation and avoided crossing of their two Lewis curves, e.g., SiH₅, and those that require at least one additional curve—termed the intermediate curve—such as PH₄. The Lewis curves correspond to the electron-pairing schemes of the normal-valent constituents in the exchange process H• + XH_n → [XH_n+l] → H_nX + H•. The intermediate curve possesses an (n → <r*) excited character and mixes into the Lewis curves, mainly at the hypercoordinated region. This mixing endows XH_n+1 with additional stability and a new electronic character (Figure 2, 18, 19). A third class of XH_n+1 radicals exists, in which the two Lewis curves are crossed by an intermediate Rydberg curve (n → R excitation) which provides an energy well to house a Rydberg XH_n+I radical (Figure 4). The hypercoordination capability of an atom X depends on the X-H bond of the normal-valent XH_n and on the presence of a lone pair on X. The weaker the X-H bond, the more stable the XH_n+1 species relative to its normal-valent constituents XH_n + H•. The XH_n+1 species gains additional stability that is proportional to the ease of ionization of the lone-pair electrons (of H_nX) when such electrons are available. The stability of the Rydberg XH_n+1 radicals is proportional to the proton affinity of XH_n. In accord, the stability of a hypercoordinated radical (relative to XH_n + H•) is predicted to increase down a column of the periodic table and to peak at the Vth family of each period (26). These principles are applied to systems that have been investigated experimentally and computationally: SiH₅, PH₄, SH₃, CH₅, NH₄, and OH₃. UHF-SCF/6-31G* calculations are performed and VB weight analysis is carried out to assess the stability and electronic structure of SiH₅, PH₄, and SH₃. It is concluded that SiH₅, CH₅, and SH₃ are transition states typified by delocalized three-electron three-center (3e, 3c) bonding. On the other hand, PH₄ is either a metastable intermediate (2) or a stable species (1) that is described by roughly equal contributions from (3e, 3c) and (4e, 3c) bonding. Finally, NH₄ and OH₃ are metastable intermediates with a decreasing stability and a Rydberg character. Prediction are made about the chances of finding other stable hypercoordinated XH_n+1 radicals.