BONDING IN THE TRIHALIDES (X-3(-)), MIXED TRIHALIDES (X2Y-) AND HYDROGEN BIHALIDES (X2H-) - THE CONNECTION BETWEEN HYPERVALENT, ELECTRON-RICH 3-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 i deas from qualitative molecular orbital theory to computational result s from density-functional calculations. A systematic, unified investig ation showed that the bonding in all of these diverse anions can be un derstood in terms of the Rundle-Pimentel scheme for electron-rich thre e-center bonding. It also showed the equivalence of the donor-acceptor and hypervalent bonding views of these molecules. Less symmetrical tr ihalide ions were studied as well, e.g. the reasons why IICl- is favor ed over IClI- were explored. This site preference is considerably less pronounced in the I--IBr system. The donor-acceptor perspective (X- a ttack at the two possible sites of X-Y) was found to be useful. Simila rly formation of XHX- from X- and HX is strongly favored over formatio n of XXH-, and the energy difference between these two geometries incr eases with increasing electronegativity of X.