Bonding in the trihalides (X3−), mixed trihalides (X2Y−) and hydrogen bihalides (X2H−). The connection between hypervalent, electron-rich three-center, donor–acceptor and strong hydrogen bonding ‡

Abstract

The nature of the bonding in the trihalides (X₃ ^–), mixed trihalides (X₂Y^–), and hydrogen bihalides (X₂H^–) has been analysed by applying ideas from qualitative molecular orbital theory to computational results from density-functional calculations. A systematic, unified investigation showed that the bonding in all of these diverse anions can be understood in terms of the Rundle–Pimentel scheme for electron-rich three-center bonding. It also showed the equivalence of the donor–acceptor and hypervalent bonding views of these molecules. Less symmetrical trihalide ions were studied as well, e.g. the reasons why IICl^– is favored over IClI^– were explored. This site preference is considerably less pronounced in the I^––IBr system. The donor–acceptor perspective (X^– attack at the two possible sites of X–Y) was found to be useful. Similarly, formation of XHX^– from X^– and HX is strongly favored over formation of XXH^–, and the energy difference between these two geometries increases with increasing electronegativity of X.