From a chemist’s point of view, one of the most fascinating aspects of boron is that it forms a very complex series of hydrides. Most people with some scientific education are familiar with the hydrides of carbon. Carbon is the only other element to form a complex and extended series of hydrides. One of the most interesting things here is the profound differences between the boron hydrides and the carbon hydrides.

Of these differences, one of the most obvious is that the structures of the boron hydrides are quite different from those of carbon.

The skeletons of the carbon hydrides and their relatives are typified by chains and rings, and many of us learn about these in school: compounds such as benzene C₆H₆, propane C₃H₈, and so on.

Other elements can be joined to carbon in these types of molecules, for example oxygen to give us the alcohols, such as ethanol CH₃CH₂OH, and to give us the organic acids, such as acetic acid or ethanoic acid, CH₃COOH, and so on. Nitrogen and oxygen can give us the amino acids. These, for example, can get together, eliminate H₂0, and so string together to make protein chains. In the diagram, note that chemical scientists use the letter 'R' for any chemical grouping they can't be bothered to write down. In proteins R varies a lot, which gives proteins all their different biological properties.

Rings and chains can be joined together, for example two benzene type rings to form naphthalene, the traditional component of ‘moth-balls’ , traditionally put in clothes cupboards to prevent the larvae of the clothes moth from eating our woollens. Anthracene has three such rings.

The combinations and permutations are infinite, and the chemistry of these compounds of carbon is what is called organic chemistry. On this planet, all life forms have evolved using the structural possibilities of these carbon chains and rings, and so many of these organic compounds are found in the biosphere. This of course is why they are called ‘organic’.

The boron hydrides are called boranes. The boranes and their relatives have quite different structures from those of organic compounds.

Instead of rings and chains, they form cages and clusters. These cages and clusters can be quite small, as in the gaseous compound tetraborane, B₄H₁₀, which has four boron atoms, but can also get much bigger. 

These two molecules illustrate an important feature of the architecture of the basic boron hydride skeletons - they consist of triangular units joined together at their edges. The resulting geometries are called ‘triangulated polyhedra’ or ‘deltahedra’. They are quite different from typical carbon hydride skeletons.

For those who haven’t studied chemistry, these structures might not seem all that strange. For example, the Atomium in Brussels represents what many may think is a typical ‘molecule’. However, this type of architecture is not typical for atoms in a molecule. The Brussels Atomium is in fact a representation of the type of atomic arrangement that we see in a lump of metal, quite different from a typical molecule.To make the shapes of the boron frameworks clearer, we haven’t drawn the hydrogen atoms in on the above sketches. The skeleton on the extreme left is that of B₄H₁₀ and that on the extreme right [B₁₂H₁₂]^2-. The ones in the middle are of B₆H₁₀ and the [B₉H₁₄]^- anion. Here’s a picture of the B₆H₁₀ molecule with hydrogen atoms included. Next to it is a molecule of B₁₀H₁₄, a very useful starting material for a lot of borane chemistry:

For those who haven’t studied chemistry, these structures might not seem all that strange. For example, the Atomium in Brussels (overleaf) represents what many may think is a typical ‘molecule’. However, this type of cluster architecture is not typical for atoms in a molecule. The Brussels Atomium is in fact a representation of the type of atomic arrangement that we see in a lump of metal (in this case an iron crystal), and not in a molecule: 

In fact, most molecular compounds examined by scientists so far have chain or ring structures related to the organic types of structures above, or consist of several elements or groups of elements joined to a central atom. Often this central atom is an atom of a metallic element, such as iron, gold, platinum, and so on. These are called ‘metal complexes.’ One example is the ferricyanide anion [Fe(CN)6]2- , used to stabilise table salt (interesting: most people when they see ‘cyanide’ think of a deadly poison). Related to this is the structure about the iron atom in a molecule of haem, a component of the haemoglobin in blood, which has ring-like structures bound to the central iron atom.

So the cluster structures of the boranes and related species are quite different from the structures of organic chains and rings, and from the structures of metal complexes based on a single central metal atom. Here are the skeletons of two; two molecules both of constitution S₂B₁₆H₁₆.

These have sulphur in the cluster as well as boron. We haven’t drawn the hydrogen atoms in these sketches. Compounds that have the same formula but differ in their atomic positions, like these two, are called isomers.

Here’s a picture of another sulphur-containing boron hydride, [S₂B₁₈H₁₉]^-, now showing where all the atoms are. Like [B₁₂H₁₂]^2-, mentioned above, this molecule is also an anion, but has only one negative charge on it.

These show that these boron hydride clusters can get very complicated, and that other atoms besides boron can be joined to boron in these cages and clusters. Carbon is another good example, and a famous compound here is C₂B₁₀H₁₂. Like the [B₁₂H₁₂]^2- anion, a molecule of C₂B₁₀H₁₂ has an icosahedral skeletal structure, but, unlike [B₁₂H₁₂]^2- , it is neutral molecule - it bears no charge. 

The carbons can be in different positions relative to each other, so this compound has different isomers too. The C₂B₁₀H₁₂ isomers are incredibly stable.

Not only carbon and sulphur, but most other elements, can be joined to boron in the clusters. Additionally, the types of chain and ring building blocks that we see in the carbon hydrides can also be used to join the boron hydride cages and clusters together. These links can involve carbon, or boron, or other elements or combinations of elements.

Unfortunately (or perhaps fortunately) for the borane scientist, nature didn’t exploit boron in the same way that it used carbon. This means that, on this planet, it is up to human beings to find out precisely what kind of compounds boron hydride chemistry has available. For carbon, nature has done that kind of work over billions of years of evolution.

One consequence of this is that we can easily get at carbon hydrides for study by heating a piece of timber, or extracting a coffee bean, evaporating urine, etc. (whatever turns you on). This means we now know a lot about the carbon hydrides and other organic compounds. In turn, this means we can predict their properties, and can therefore think of ways of using them - new polymers, new drugs, new detergents, etc.  On the other hand, we have no idea what behaviour new boron hydrides and other borane-based compounds will have. We have to discover what compounds are available, and then find out what properties they do have, and then think of how we can use them. Also, we have to buid them up from quite simple compounds, because Nature hasn't done the work for us already. In such a wide-open area for new discovery science, who knows what may be revealed? Look at the way this boron compound stacks up in the solid state. Can we exploit this kind of property?

And look at the way this new ‘pentapus’ borane anion grabs onto and wraps around other kinds of molecule. Can we exploit this kind of property too ? –perhaps for grabbing noxious species from the environment or from other places we don’t want them to be. 

Because the polyhedral structures of the boron skeletons in borane molecules are so weird, and quite different from the familiar organic chemical structures, this is obviously a fascinating area for new discovery science. A relatively small group of laboratories throughout the world is dedicated to finding out about this new type of chemistry. Tap into their various links (see The Leeds Boron Group and BoronWeb for a listing of links) to find out exactly what they are doing.