Polyhalide Anions in Crystals. 3. The Br₈²⁻ anion in diquinuclidinium octabromide, the crystal structures of Me₄PBr₃ and quinuclidinium tribromide, and Ab initio calculations on polybromide anions

The crystal structure of diquinuclidinium octabromide contains the hitherto unreported centrosymmetric planar, roughly Z‐shaped Br 8 2− anion. The Br 3 − anion in Me 4 PBr 3 is only slightly distorted, whereas the Br 3 − distortion in quinuclidinium tribromide is considerable. Detailed examination of the geometry of asymmetric Br 3 − anions in crystals shows that the two Br–Br bond lengths are correlated and that the correlation is representable by a simple power function, leading to an estimate of the BrBr bond length in the symmetric Br 3 − anion of 2.541 {24} Å. A relationship is also shown to exist between the N …︁ Br and H(N) …︁ Br distances in NH(N) …︁ Br hydrogen bonds in organoammonium and related bromides. The high linear correlation of the XX bond lengths in the X 8 2− (X = Br, I) anions provides a useful means of estimating Br–Br bond lengths from the corresponding II bond lengths and vice versa. Ab initio RHF/6–311 G* optimizations of the equilibrium geometries of several Br n m − anions reveal that linear Br n − are to be formulated as Br − (Br 2 ) 2k when n = 1, 5, 9 = 4 k + 1, and as Br 3 − (Br 2 ) 2k when n = 3, 7 = 4 k + 3. The V‐shaped Br 5 − anion has an only slightly lower total electronic 6–311 G* energy than the linear Br 5 − . For Br 7 the energy is lowest for the C 3v (trihedral) configuration and increases in the order C 3 v < D 3 h (planar) D ∞ h (linear) for the other configurations examined.