HYPERVALENT MOLECULES

Many of the heavier atoms appear to exceed the octet rule. How do we identify and draw molecules with these atoms?

If you don’t initially recognize that a species is hypervalent (exceeds the octet rule), you will find that the sum of formal charges exceeds the actual charge. For example, six coordinate SF6 would give a charge on the S atom of +4 rather than the actual zero charge expected for the molecule. We could break three bonds to form [SF3]+ and three F– ions (with three ionic bonds), and some prefer this representation along with the other resonance forms, however, that is not a very convenient representation, even though it obeys the octet rule and could be considered to be more accurate.

Hypervalent molecules: Project

Determining the Lewis structure of species such as SF6 and [XeOF5]– is conventionally done by counting the total number of electrons and then distributing them until you have none left. This total electron count is unnecessary. Each terminal halogen atom requires one more electron and thus forms a single covalent bond with one electron from the central atom. Each O atom requires two more electrons from the central atom, either by formation of a double bond or a dative bond (or a zwitterionic bond) with the central atom. Any valence electrons left over on the central atom form lone pairs. For SF6, there are no electrons left over after forming 6 covalent bonds, thus there are no lone pairs and an octahedral structure is formed. In the case of [XeOF5]–, of the eight valence electrons on Xe, five form covalent bonds to F atoms and two are used to bind the O atom, thus, two electrons (one from the Xe atom and one from the charge) are left over to form a lone pair, and the molecule forms a pentagonal pyramid.

Hypervalent molecules: Project

There are a number of important species that are commonly drawn as hypervalent when it is not necessary to do so. The sulfate dianion, [SO4]2– shown below is a good example. The one on the left is more correct chemically whereas the one on the right is easier to draw and is more commonly seen.

Other main group oxides in this category include [PO4]3–, [ClO4]–, SO2 and [SO3]2–.

Hypervalent molecules: Project

HYPERVALENT MOLECULES

There are a number of important species that are commonly drawn as hypervalent when it is not necessary to do so. The sulfate dianion, [SO4]2– shown below is a good example. The one on the left is more correct chemically whereas the one on the right is easier to draw and is more commonly seen.

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Other main group oxides in this category include [PO4]3–, [ClO4]–, SO2 and [SO3]2–.