This Lewis structure and VSEPR problem is based on a paper from Inorganic Chemistry in 2010 reporting the crystal structures of a series of salts of the [XeF]+ cation. The [MF6]– and [M2F11]– anions (M = As, Sb, Bi) were used as counterions, and in all cases, the [XeF]+ cation interacts with the anion via a weak bond between the Xe and a fluoride of the anion to form an ion-pair in the crystalline solid. These somewhat unusual ions provide an interesting application of the predictive powers of Lewis structures and VSEPR theory to molecular structure, backed up by experimental data on bond distances and bond angles and can provide an opening to the descriptive chemistry of the heavier pnicogens. These ions also provide interesting examples to discuss the model and utility of assigning formal charge in Lewis structures and how to describe the bonding in molecules with expanded octets. Although not specifically addressed in the attached problem, which focuses on the bismuth salts, one could extend the discussion to periodic trends in these salts with different counter ions from the pnicogen group. The reference to the original paper is Inorg. Chem. 2010, 49, 8504-8523.
- Practice drawing Lewis structures for molecules with expanded octets and assigning (sometimes unusual) formal charges
- Gain experience applying VSEPR to predict the molecular structure and approximate bond angles in different molecules
- Relate the predictions of relatively simple theories to actual experimental results obtained from X-ray crystallography
- Learn the names of the heavier pnicogen elements
This was assigned as a problem to be worked at the board by small groups during a conference session for a second-year inorganic chemistry course.