Bonding in the trihalides (X-3(-)), mixed trihalides (X2Y-) and hydrogen bihalides (X2H-). The connection between hypervalent, electron-rich three-center, donor-acceptor and strong hydrogen bonding

The nature of the bonding in the trihalides (X-3(-)), 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.