Ab initio MO study on the periodic trends in structures and energies of hypervalent compounds: five-, six-, and seven-coordinated XF 5, XH −6, XF −6, XH 2−7 and XF 2−7 species containing a group 15 central atom (where X is P, As, Sb, Bi)

Abstract

A systematic ab initio study employing effective core potentials (ECP) on the central atoms has been carried out to predict structures, stabilities and the resultant periodic trends for the five hypervalent series XF 5, XH − 6, XF − 6, XH 2− 7 and XF 2− 7, where X is P, As, Sb and Bi. The D 3h and C 4v structures of XF 5 correspond to minima and pseudorotation transition states, respectively. The pseudorotation barriers are found to decrease with increasing size of X, being 4.3 (X=P), 3.0 (X=As), 1.8 (X=Sb) and 1.3 kcal mol −1 (X=Bi). This trend parallels the trend in the differences in axial and equatorial XF distances in XF 5 ( D 3h). XH − 6 ( O h) and XF − 6 ( O h) are found to be minima for all X, whereas their D 3h counterparts correspond to transition states for trigonal twist distortion. In contrast to the fluxional five-coordinated neutrals XH 5 and XF 5, the six-coordinated anions XH − 6 and XF − 6 are predicted to be rigid with the barriers to the O h− D 3h distortion ranging from 25 to 65 kcal mol −1. These barriers also decrease monotonically down the column following the trend in the differences [X-L( D 3h)]−[X-L( O h)], where [X-L()] is the distance between X and L=H or F in structure D 3h or O h. All XH − 6 and XF − 6 are indicated to be thermodynamically stable with respect to loss of H − and F −, respectively. On the other hand, XH − 6 appear to be unstable toward the loss of H 2 molecule, a feature also shown by the four-coordinated XH − 4 and five-coordinated XH 5 hypervalent hydrides studied by us earlier. The periodic trends revealed in the thermodynamic stabilities of XH − 6 and XF − 6 are irregular. No stable structure is found for the XH 2− 7 heptahydrides. By contrast, all XF 2− 7 heptafluorides are predicted to be structurally stable at the RHF level (zero imaginary frequencies), exhibiting pentagonal bipyramidal D 5h geometries for X=P, As, Sb, but a distorted pentagonal bipyramid of C 1 symmetry for X=Bi. XF 2− 7 species are unknown and their synthesis is expected to be very difficult to achieve, mostly due to the significant thermodynamic instabilities of these anions toward the loss of F −.