# What allows sulfur and phosphorus to expand their octet?

There are many compounds in which the stability of a molecule is not governed by the presence of octet configuration in central atom. In most of the cases the central atom is generally sulfur or phosphorus. In these cases either the central atom has 6 electrons, or sometimes the electron count exceeds 8 and 10 or 12 electrons in its valence electron shell.

This makes the concept of octet somewhat incorrect. My question is how can they increase their octet to such an extent?

Could someone explain this in relation with sulfur trioxide?

## 1 Answer

Whether sulfur or phosphorous actually expand their octet is contested within the chemistry community. Another term for this octet expansion is "hypervalency." You can find many works of research regarding hypervalency. The consensus, according to Wikipedia, is that both can expand their octets, but not to a significant extent. In other words, d-orbital utilization is low at best.

Regarding sulfur trioxide, this article may be of interest.

We find no evidence to support notions of p π –d π back-donation from oxygen to sulfur.

Regarding the octet concept, it's only a guideline at best, and nothing more. I wouldn't put too much stock into the concept. If you insist on using it, then only apply it to the following elements: $\ce{C, N, O}$, and $\ce{F}$. And note that for carbon, there are well known exceptions in organic chemistry - carbocations (which have less than an octet of electrons). Carbocations however tend to exist mainly as reactive intermediates. Therefore, a more nuanced statement might be $\ce{C, N, O}$, and $\ce{F}$, when involved in neutral and thermodynamically stable compounds, generally have 8 valence electrons.

this makes the concept of octet somewhat incorrect.

• >If anyone can think of exceptions, please share || $C_7H_7^+$ , $C_8H_8^{2-}$ are well-known examples =). – permeakra Jun 26 '14 at 18:50
• Are you sure those are exceptions? I don't see how they could be exceptions. – Dissenter Jun 27 '14 at 15:29
• chemtube3d.com/gallery/structurepages/C7H7_.html – Dissenter Jun 27 '14 at 15:31
• The examples given above (carbocations, carbanions) are exceptions to the octet rule. However they also belong to a class of compounds known as reactive intermediates. It is the strong "desire" of these non-octet compounds to regain their octet that makes them so reactive. The list of these "offenders" is endless. If you put enough energy into methane you would generate methyl radicals, they too "violate" the octet rule, but are extremely reactive species. I think your statement, "These elements can be expected to follow the octet rule throughout most of general and even organic – ron Jun 27 '14 at 15:58
• chemistry is as accurate as most general statements in chemistry. Perhaps modifying it to read, These elements, when involved in neutral, thermodynamically stable compounds, can be expected to follow the octet rule throughout most of general and even organic chemistry, would make it a bit more "correct". – ron Jun 27 '14 at 16:03