One of the most important topics of chemistry that is absolutely hammered home to beginning students is the VSEPR theory. It’s literally the first chapter of introductory chemistry texts, because it’s fundamental to all future discussions. VSEPR, or “valence shell electron pair repulsion”, lays out the ground rules for how atoms come together to form molecules. Take the carbon atom, for example. Carbon atoms are the foundation of organic chemistry, and are the primary component of most molecules. A carbon atom is “tetravalent” – it supports four bonds to other atoms. Each bond consists of two electrons, which are negatively charged. So, if you were to just use common sense and sketch out on paper what you thought would be the most stable arrangement of the bonds around the carbon atom, you would most likely sketch a picture of a pyramid, with the carbon atom in the middle of the pyramid.
The reason for this sort of common-sense approach is that you probably already know that similar charges repel each other, just like opposite charges attract. So if you have a small sphere (the carbon nucleus) and have to glue four straight lines (bonds) onto it, and the lines repel each other, the lowest energy arrangement – the bond placement that allows them to be as far apart from each other as possible – is when the four bonds point towards the corners of a pyramid, as I’ve shown in the image that I’ve attached to this article. It’s a three-dimensional structure, and this common-sense approach turns out to be precisely correct in the real world. Carbons with four single bonds attached to them form this pyramid-like arrangement. All of the bonds are repelling each other to their maximum extent, and the structure is therefore as stable as it can be.
This VSEPR theory is drilled into students from day one, and is a cornerstone of organic chemistry. Most students will tell you that it’s never wrong. However, I would slightly alter that blanket statement: VSEPR is almost never wrong. A new result from researchers in the UK, published in the top European science journal Angewandte Chemie (“Applied Chemistry”) has revealed an example of a four bond carbon that is not tetrahedral, at all. It’s flat, or square planar, in the parlance of chemists.
Consisting of a carbon atom bonded to two lithium atoms and two bulky organic groups (ligands), it appears that the substitution by an alkali metal (by lithium) can stabilise the square planar arrangement quite nicely. It’s not a new idea, in concept – it was predicted to be theoretically possible all the way back in the 1970s, and several groups have taken a stab at it. However, all attempts to date have resulted in complicated clusters of atoms known as aggregates, which can be confusing to try and decipher. This new eample has a central carbon bonded to two lithium atoms (located opposite from each other, or “trans”) and is monomeric, meaning it’s a well-defined single molecule and not a cluster of atoms.
The key to the success of this approach – the surprising success of this approach – is the use of the ligand named “N,N,N’,N’-tetramethylethylenediamine”, or TMEDA. The N’s in front of the name are part of the molecules nomenclature, and designate that it is the nitrogens that are substituted with the methyl groups. TMEDA helps to boost the strength of their lithium base – which allows the carbon to be substituted with two lithiums, instead of just one – and it also discourages the final complex from aggregating. This breakthrough should really encourage scientists to not accept a commonly-accepted theory to be an actual law; it’s a theory, and helps to explain some behavior, but certainly doesn’t explain all behavior.
Source: http://www3.interscience.wiley.com/journal/123567747/abstract
