20 A Textbook of Inorganic Chemistry – Volume I 6. Seven electron-pair domains: i) IF 7 : The central atom in IF 7 molecule is I which has seven valence electrons (2, 8, 18, 18, 7). Now because each fluorine atom needs one electron to complete its octet (2, 7), the I atom uses its all seven valence electrons in sharing to create seven bond pair domains only. Hence the geometry for minimum repulsion will be pentagonal bipyramidal; and the normal bond angle between equatorial groups will be 72°, while the normal bond angle between axial fluorine will be 180°.
Now because each axial position has nearest neighbors at 90° while every equatorial position has nearest neighbors at 72°, we can conclude that the less crowding at the axial position would lead to shorter axial bonds comparatively. Figure 17. Structure of IF 7 molecule from the VSEPR model. ii) XeOF 5 : The central atom in XeOF 5 molecule is Xe which has eight valence electrons (2, 8, 18, 18, 8). Now because each fluorine atom, as well as O , needs one electron to complete its octet (2, 7), the Xe atom uses its six valence electrons to create six bond pair domains, while two electrons are left as a lone pair domain.
Now though the geometry for seven electron pair domains is pentagonal bipyramidal; but in this case, a non- bonding domain is present which would require more space than the bonding domains. Now because each axial position has the nearest neighbors at 90° while every equatorial position has nearest neighbors at 72°, we can conclude that the axial position is more suitable for the placement of lone pair. This, in turn, would result in greater lone-pair–bond-pair repulsion yielding the distortion of perfect pentagonal pyramidal-shaped geometry with equatorial bond angles slightly less than their normal of 72°. Figure 18. Structure of XeOF 5 molecule from VSEPR model.
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