The total ionisation potential for the three valence electrons for boron is very high. So the bonding is via covalent bond. The normal electronic configuration will be;

This will give the compound a planar structure. Example: BCl₃.

Bonding of the sp� electrons of boron with the valence electron of chlorine will provide boron with a valence shell of only six electrons. We would expect some form of coordination similar to beryllium to provide the other two electrons to achieve the needed octet.

This is provided by the planer structure and the correct orientation of the p-orbitals of boron. These allow valence electrons in the filled p-orbitals of the chlorine atoms to move into the empty p-orbital of the boron atom. This stabilization by the resonance of the electrons in the molecule would explain why all the three B-Cl bonds lengths are equal and much shorter (1.75�) than the expected 1.87�.

LEWIS ACID

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When there is a source of rich electron present, like a nitrogen or oxygen atom, boron chloride would opt for an inter-molecular coordination instead of the intra-molecular resonance shown above. For example when an ethyl ether is present.

This reaction;	Et2O + BCl3	�	[Et2O]+ [BCl3]‾
is recognised as resembling,	H2O + HCl	�	[H3O]+ Cl‾

So boron chloride is looked upon as a type of acid. Acid such as this that does not involve a proton is known as Lewis acid.

INTRA-MOLECULAR RESONANCE vs INTER-MOLECULAR COORDINATION

In all situations the intra-molecular resonance and the inter-molecular coordination are in operation. The dynamics are determined by the stability of the complex formed.

The relative strengths the boron halides as Lewis acids are in the order;

Based on the electronegativities of the halides we would expect a reverse order. So we can only conclude that the order reflects the decreasing importance of the intra-molecular resonance from BBr₃ to BF₃.

I have deliberately chosen boron halides to introduce the chemistry of boron. This already presented some interesting problems. The truth is the chemistry of boron is rather complex. This arises from the fact that we have reach a situation where it is extremely difficult to keep up with the octet rule.

Its reluctance to give away its electron is so great that we now see the change from a metallic element to than of a non-metallic element. Boron is a semiconductor.

Boron will form borides with metallic elements like phosphorus, arsenic, etc, in the same manner chlorine will form chlorides with metallic elements. The only trouble is the molecular formula and structure of boron compounds often took various forms. So it is difficult for example to be certain of the molecular formula for phosphorus borides (for example).

OXO COMPOUNDS

The title "oxo compounds", instead of boron oxide, is to emphasise the fact that boron oxide is not a metallic oxide. Occasionally the term borate is used. As discussed above it is difficult to present the molecular formula or structure for boron oxide.

The principal oxide is often represented as B₂O₃ yet in the solid they are group in units of BO₃. To get the solid just take the unit shown and keep on attaching the exposed boron to another exposed oxygen like a jigsaw puzzle.

In the 3-dimensional solid the whole arrangement is so meddled up that boron oxide could not form a crystalline state. Alternatively we say it is in an amorphous (or random) state. The advantage of a random state is the solid allows light to pass through (transparent). So the oxide is often fused with other oxides to make glass (or borates).

The other forms of oxide are pictured as (BO)_x or (BO₂)_x.

Boron oxide reacts with water to give an acid hydroxide, known as boric acid, B(OH)₃. Boric acid is moderately soluble in water to give a very weak acid. It accepts an OH unit to release the proton from the water.

So it is not a protonic acid but a Lewis acid.

As expected of boron, the chemistry is not simple. At higher concentration the pK changes to 6.84, meaning the acidity increases. The reaction suggested was:

Boric acid will decompose to boron oxide on heating.

Tutorial 1

Are all hydroxides basic?      Answer

BORATES

Many borates in hydrated forms occur naturally. Borax, sodium borate (picture), deposits are found in substantial quantity in India and California. The unit - [O₂BOBO₂]‾⁴ (coloured yellow) - is known as the pyroborate anion. Anhydrous borates, or glass, are made by fusing boric acid with metal oxides.

When hydrogen peroxide is mixed with borate it would form a complex, with the hydrogen peroxide coordinated to the borate. The molecular formula and structure of the complex is uncertain. This is used in powder laundry detergent. The hydrogen peroxide, a bleaching agent, is then released during the wash to break down the dirt.

BORON HYDRIDE

As boron participates in covalent bonding the "salts" like sulphate, nitrate, and carbonate are non-existent. One interesting class of compound is the boron hydrides. Boron compounds, like boron methoxide, easily form boronhydrides with metals.

Sodium boronhydride, (Na⁺[BH₄]‾), is a white crystalline salt, stable in dry air. It must be noted that BH₃ (borane or borine) is not stable enough to be isolated.

Reactions of sodium boronhydride with Lewis acids (like BCl₃ or AlCl₃) produce a diborane.

DIBORANE

Looking at the structure of the molecule you can see that the octet principle was observed, but it would be extremely difficult to explain how one hydrogen atom could form two bonds. For this we will have to come up with new theories.

Science explains what exist. Any principle that can explain similar phenomena is considered acceptable truth. Any case that does no fit the established principle commits no crime, it only cries out for more research. The bigger picture is out there.

The more comprehensive theory is the Molecular Orbital (MO) Theory. Of course this is more complex than the octet principle. To make our life a little easier we used the Valence Bond Theory to explain the BH₂ bonds (colour blue) and invoked the MOT on the four atoms that formed the bridge in the molecule (the yellow portion). The MO Theory would give us four bonding molecular orbitals which would be occupied by two electrons from the boron atoms and two electrons from the hydrogen atoms. These four electrons can now move throughout the molecule (the yellow section) bonding the molecule.

The diborane can react with molecules with nitrogen and oxygen, which are willing to share their lone-pair of electrons. It would behave like a BH₃ entity towards these molecules.

Borazine

Benzene

It could even form some interesting compounds like borazine
(or barozole). Do not worry about this until we come to carbon chemistry and the discussion on benzene. Borazine can be prepared by heating ammonium chloride and BCl₃ to give the
B-chloroborazine, [NH-BCl-NH-BCl-NH-BCl]. The latter can react with sodium boronhydride, in ether, to yield borazine.

Tutorial 2

Assuming that you know what borazine is, why is [NH-BCl-NH-BCl-NH-BCl] known as B-chloroborazine?      Answer