One of the central tenets of structural chemistry is the “octet rule” that is obeyed by first row elements. This was first formulated by G. N. Lewis based on experimental data in the early 20th century, and even though it is simple, it is extraordinarily powerful and allows a quick, rational determination of molecular structures. The lowest-energy electron configuration for first-row elements is an 8-electron configuration, and molecular structures will naturally reflect that. This is what leads to the concept that carbon can have no more than 4 bonds, and why ammonium compounds are ionic rather than pentavalent molecular structures. Of course, the octet rule is just a theory, and in science, if a single experiment can disprove a long-standing theory or law, then the theory needs to be reassessed. Many experiments have therefore been carried out over the years designed to test the limits of bonding in first-row elements and whether the “octet rule” still holds under extreme experimental conditions.
One of the first experiments was conducted by Georg Wittig at the University of Heidelberg in Germany. He added phenyllithium to tetramethylammonium bromide, hoping to make a pentavalent nitrogen compound, which is not completely crazy, since organolithium reagents are very strong nucleophiles. However, he instead obtained a nitrogen ylide, from deprotonation of one of the methyl groups, since organolithium reagents are also strong bases!
These nitrogen ylides were not very stable, but did add in a nucleophilic manner to aldehydes and ketones. Wittig then extended this chemistry to the next element below nitrogen, phosphorous, and discovered that phosphonium ylides were exceedingly stable. They also had enormous synthetic value since upon addition to aldehydes or ketones, they yielded the alkene upon extrusion of phosphine oxide. This reaction is now known as the Wittig reaction, and is one of the workhorse reactions in synthetic organic chemistry for the synthesis of olefins. Wittig ended up sharing the Nobel Prize in chemistry in 1979 with H. C. Brown for his work in the development of synthetic methods. More details about his work can be read in his Nobel Lecture.
This is only the beginning of the story of hypervalence, however. While first-row elements cannot take on more than 8 electrons (due to a combination of sterics and orbital constraints), second row and lower elements can (just like in the above example). In fact, one of the long-standing mysteries in chemistry is the stability and lack of reactivity of sulfur hexafluoride. This has also spurred recent interest in the organofluorine community on the synthesis of organic derivatives of SF6, that is, organo-SF5 compounds. For a long time, the only higher-valent derivatives of the main group elements that were known were all main group fluorides (e.g. SF6, ClF3, ClF5, BrF3, BrF5, PF5, IF3, IF5, IF7, and others). These were (and to a degree, still are) considered rather exotic compounds to the traditional practicing synthetic organic chemist. But thanks to the work of J. C. Martin, some of these reagents have been “tamed” for organic synthesis.
J. C. Martin was at the University of Illinois, Urbana-Champaign for most of his career, and built up his reputation in physical organic chemistry and main group chemistry. Over the course of his career, he investigated the hypervalent chemistry of several elements, including sulfur, phosphorous, bromine, and especially iodine. His work on iodine chemistry led to the discovery of the reagent now known as the Dess-Martin periodinane. This is an I(V) reagent, and can be thought of as a “tamed” version of IF5. The reagent is widely used in organic synthesis as a mild and chemoselective oxidant, and the 1983 JOC communication detailing the synthesis and use of the reagent remains J. C. Martin’s most highly cited paper, with over 2,000 citations to date. J. C. Martin also synthesized an organic Br(III) compound, analogous to BrF3.
This reagent is also a strong oxidant, just like BrF3, but it is not widely used in organic synthesis. The study of Br(III) and Br(V) chemistry remains a niche area of research; to the best of my knowledge, there are only 1 or 2 labs worldwide doing research in this area. It is inherently dangerous, but there are some interesting results emerging from this area. Last year, a Japanese group published a paper showing how they could do metal-free amination of alkanes using a Br(III)-nitrenoid reagent! This chemistry is generally dominated by the use of precious metals (see Justin Du Bois’ recent publication in Acc. Chem. Res.). This is due to the fact that the oxidized forms of Br, such as Br(III) or Br(V) have enormous chemical potential energy; there is a very strong thermodynamic driving force for Br to come back to it’s normal valency.
J. C. Martin also did interesting research attempting to synthesize compounds containing pentavalent carbon (analogous to the SN2 transition state) and compounds containing pentavalent and hexavalent boron. These were pioneering studies, and even though they were unsuccessful, revealed a lot of information about structure and bonding. The carbon compound does not really rest in the pentavalent form in the ground state, but undergoes a “bell-clapper” rearrangement. Similarly, although pentavalency and hexavalency were claimed in the case of the boron compound based on boron NMR data, convincing evidence could not be obtained (in the form of an X-ray crystal structure).
Some of his most interesting work (in my opinion) was done towards the end of his career. He disclosed the concept of “σ-aromaticity” and synthesized some model systems that demonstrated this behavior. σ-aromaticity is a theoretically very interesting concept, and studies like this are at the heart of physical organic chemistry. The prototypical compound is the hexaiodobenzene dication, which can accessed by 2-electron oxidation of hexaiodobenzene.
Oxidizing hexaiodobenzene does not remove electrons from the aromatic π system; it removes electrons from the iodine atoms. Due to their size and relatively close proximity, they can delocalize the charge through bonding interactions. Theroretical calculations show that the HOMO (highest occupied molecular orbital) of hexaiodobenzene is antibonding between the iodine atoms; removing those electrons allows weak bonding interactions to take place. This is borne out by the 13C NMR shift observed upon oxidation, and the fact that the species is diamagnetic (has no unpaired electrons). Unfortunately, since J. C. Martin’s demise in 1999, this field has languished. More studies on σ-aromaticity would definitely be a welcome addition to the theoretical framework of organic chemistry and more groundbreaking results could serve to revive physical organic chemistry today.