Properties of gaseous closo-[B6X6]2- dianions (X = Cl, Br, I).

ABSTRACT

Electronic structure, collision-induced dissociation (CID) and bond properties of closo-[B6X6]2- (X = Cl-I) are investigated in direct comparison with their closo-[B12X12]2- analogues. Photoelectron spectroscopy (PES) and theoretical investigations reveal that [B6X6]2- dianions are electronically significantly less stable than the corresponding [B12X12]2- species. Although [B6Cl6]2- is slightly electronically unstable, [B6Br6]2- and [B6I6]2- are intrinsically stable dianions. Consistent with the trend in the electron detachment energy, loss of an electron (e- loss) is observed in CID of [B6X6]2- (X = Cl, Br) but not for [B6I6]2-. Halogenide loss (X- loss) is common for [B6X6]2- (X = Br, I) and [B12X12]2- (X = Cl, Br, I). Meanwhile, X˙ loss is only observed for [B12X12]2- (X = Br, I) species. The calculated reaction enthalpies of the three competing dissociation pathways (e-, X- and X˙ loss) indicated a strong influence of kinetic factors on the observed fragmentation patterns. The repulsive Coulomb barrier (RCB) determines the transition state for the e- and X- losses. A significantly lower RCB for X- loss than for e- loss was found in both experimental and theoretical investigations and can be rationalized by the recently introduced concept of electrophilic anions. The positive reaction enthalpies for X- losses are significantly lower for [B6X6]2- than for [B12X12]2-, while enthalpies for X˙ losses are higher. These observations are consistent with a difference in bond character of the B-X bonds in [B6X6]2- and [B12X12]2-. A complementary bonding analysis using QTAIM, NPA and ELI-D based methods suggests that B-X bonds in [B12X12]2- have a stronger covalent character than in [B6X6]2-, in which X has a stronger halide character.