CHAPTER 1 Stereochemistry and Bonding in Main Group Compounds: 21 iii) XeF 5 : The central atom in the XeF 5 molecule is Xe which has eight valence electrons (2, 8, 18, 18, 8). Now because four fluorine atom needs one electron to complete its octet (2, 7) while the fifth group F needs zero electrons, the Xe atom uses its four valence electrons to create five bond-pair-domains, while the four electrons are left as two lone pair domains. Now though the geometry for seven electron pair domains is perfectly pentagonal bipyramidal; but in this case, the non-bonding domain would require more space than the bonding domains.
The question arises here is what position these non-bonding electron pair domains should occupy in a pentagonal bipyramidal frame. Out of the axial and equatorial sites, the axial site is more suitable for the placement of lone pair as explained earlier. Now owing to the fact that lone-pair–lone-pair repulsion is highest, the second non-bonding domains should be placed trans to the first one in a pentagonal bipyramidal frame to give a perfect pentagonal-planar geometry. Figure 19. Structure of XeF 5 molecule from the VSEPR model. The AXE Method The commonly used electron counting scheme when applying the VSEPR theory is generally called as the "AXE method”.
The A represents the central atom while the X represents each of the ligands attached. The latter E symbolizes the number of lone electron pair domains around the central atom. The value of X and E is collectively labeled as the steric number. The VSEPR uses this steric number to predict the molecular geometries according to the atomic positions only. For instance, the AX 2 E 1 infers a bent molecule that has three atoms AX 2 which are not in one straight line because of the presence of a lone pair. The deviations from idealized geometries can be rationalized from the lone pairs, multi-electron-electron-pair domains and the presence of electronegative groups.
Steric Number Molecular geometry Molecular geometry Molecular geometry Molecular geometry 0 lone pairs 1 lone pair 2 lone pairs 3 lone pairs 2 Linear (CO 2 ) Figure 20. Continued on the next page… Buy the complete book with TOC navigation, high resolution images and no watermark. Copyright © Mandeep Dalal