Topological characterization of three-electron-bonded radical anions

The three-electron bond in radical anions of the HnXYHm- type, with X, Y = Cl, S, P, Si, F, O, N, C and n, m = 0-2, has been investigated from the topological analysis of the electron localization function (ELF) at the BH&HLYP level. It is shown that the topological modifications arising within the bonding region upon vertical electron attachment are of three different types, according to the vertical electron affinity (vEA) of the neutral compound: for vEA smaller than -16 kcal mol(-1) the bonding population remains unchanged, as in H4P2, for negative vEA greater than -16 kcal mol(-1) the bonding population decreases, as in H2S2, and for positive vEA the bonding population disappears, as in Cl-2. However, after relaxation of the geometry, the formation of the three-electron bond is accompanied in all cases by the disappearance of the X-Y bonding basin (which is the signature of the covalent bond in the neutral parent molecule). From a quantitative point of view, the topological approach also allows us to characterize the transfer of charge and spin densities that arises upon these processes toward the lone pairs basins of the X and Y atoms. Finally, to quantify the electron fluctuation between the two moieties, an index of delocalization has been defined from the analysis of the variance of the lone pairs population. This index increases approximately linearly with the dissociation energy De of the radical anions, provided that they are separated into a group of weakly bonded ones (D-e < 18 kcal mol(-1)) and a group of strongly 3e-bonded ones (D-e > 18 kcal mol(-1)).