Boron is unique amongst the elements of group 13 in that it is a non-metal. The element has two naturally occurring isotopes, ¹⁰B and ¹¹B, the ratios of which vary depending upon the mineral in which it is found and also on where it is mined but the proportion is ¹⁰B tends to be approximately 20%. The minerals in which boron is principally found are ulexite (NaCa[B₅O₆(OH)₆]·5 H₂O), borax (Na₂ [B₄O₅(OH)₄]·8 H₂O), colemanite (Ca₂ [B₃O₄(OH)₃]₂·2 H₂O)and kernite (Na₂ [B₄O₅(OH)₄]·2 H₂O). Large deposits of borax are found in the Mojave desert in California and there are other large mineral deposits in Turkey.

Preparation, Physical Properties and Uses

The minerals can be treated to form the oxide, B₂O₃, from which elemental boron can be extracted by reaction with magnesium

B₂O₃ + 3 Mg → 2 B + 3 MgO

Other methods for obtaining purer forms of boron involve the reaction of the trihalides of boron, BX₃, with either zinc and high temperatures (~900 °C) or with hydrogen over a hot tantalum catalyst.

2 BCl₃ + 3 Zn → 3 ZnCl₂ + 2 B

2 BX₃ + 3 H₂→ 6 HX + 2 B

There are a few different allotropes (physical forms of the pure element) which are either black amorphous powders or dark red crystalline solids. All the allotropes consist of B₁₂ units where the boron atoms form an icosohedron but differ their packing. They all have very high melting point (around 2200 °C) and have low density and electrical conductivity. Crystalline allotropes of boron are fairly inert and only react with hot solutions of oxidising agents. Amorphous boron is much more reactive however. For example, it reacts with ammonia to produce a white waxy compounds with the overall formula BN. This compounds has a graphite like structure with layers of sheets that are made up of interlocking hexagonal arrangements of alternating B and N atoms.

The chemistry of boron is extremely diverse and complex. The element has extremely high ionisation energies (first three ionisation energies 801, 2427 and 3659 kJ mol^-1) which can not be offset by the energies given out due to enthalpies for lattice formation or hydration. Because of this there are no compounds containing discrete B³⁺ ions. The element has a similar electronegativity to those of C and H and so boron compounds are highly covalent. It is also said to be electron deficient having only three valence electrons. It forms coordinatively unsaturated trigonal planar compounds of the form BX₃ which can become tetravalent and therefore saturated by acting as a Lewis acid, accepting electron from a donating molecule to form what is called an adduct. Boron also copes with it's electron deficiency by forming compounds containing cage or basket like polyhedral arrangements of boron atoms in which the available electrons are shared out.

The element finds many uses. Metal boride compounds, such as TiB₂, are important materials finding applications as turbine blades and rocket motor nozzles due to their highly inert nature and ability to withstand high temperatures. Boron is also used in the nuclear industry because of it's ability to readily absorb neutrons. Metal borides and boron carbides therefore find uses as nuclear shielding materials and are used in reactor control rods. A more familiar use of boron is in the glass Pyrex which is a borosilicate made by dissolving silica, SiO₂, in molten boron oxide, B₂O₃.

Oxygen Containing Compounds of Boron

There are many examples of oxygen containing boron compounds and most of the minerals in which boron is found are borates. These are often comprised of interconnected trigonal planar BO₃ and tetrahedral BO₄ units that form rings and chains of alternating B and O atoms, some examples are shown in figure 1. These arrays are often anionic and many of the minerals also incorporate water molecules into their structure as they crystallise.

Figure 1: Some examples of borate anions present in boron containing minerals.

An important oxygen compound of boron is boric acid, B(OH)₃. It is acidic when dissolved in water as it forms B(OH)₄^- and liberates a proton.

B(OH)₃ + H₂O → B(OH)₄^- + H⁺

The anion B(OH)₄^- occurs in many borate type minerals. Boric acid reacts with alcohols in the presence of sulphuric acid to give alkyl orthoborate compounds, B(OR)₃, which can be reacted with alkali metal hydrides to give borohydride anions of the form [BH(OR)₃]^- which are useful reducing agents. It will also form stable compounds by reacting with dialcohols, eg (HO)H₂C-CH₂(OH), to give cyclic species, eg see figure 2.

Figure 2: The cyclic product from the reaction of boric acid with ethane diol.

Halides

The best known are the trihalides, BX₃. They are all strong Lewis acids acting as electron pair acceptors. They are quite volatile with BF₃ and BCl₃ being gases at room temperature. They are all water sensitive and undergo hydrolysis to give boric acid and HX though BF₃ is only partially hydrolysed giving a mixture of boric acid and the tetrafluoroborate anion, BF₄^-. For example

BCl₃ + 3 H₂O → B(OH)₃ + 3 HCl

4 BF₃ + 6 H₂O → 3 H₃O⁺ + B(OH)₃ + 3 BF₄^-

Lewis bases, such as amines (R₃N), phosphines (R₃P), ethers (R-O-R) or anions, react with the boron trihalides by donating lone pairs of electrons forming larger associated molecules called adducts. In these reactions, the boron goes from being trigonal planar and hence sp² hybridised to being tetrahedral and therefore sp³.

BX₃ + :N(CH₃)₃→ X₃B-N(CH₃)₃

Since the most electronegative halide is fluorine we would expect BF₃ to form the most stable adducts as the boron would more readily accept a lone pair of electrons, however this is not the case. It is the heavier boron halides such as BBr₃ that form the more favoured Lewis acid/base adducts. This reverse order in the expected reactivity comes down to the nature of the bonding in the BX₃ reactant. There is a partial double bond formed between the B and X atoms. When X is Cl or Br, this partial double bonding involves the 3p and 4p orbitals of the halogens which are larger and differ in energy to the 2p orbitals of the boron atom. Since fluorine is in the same row of the periodic table as boron, the 2p orbitals it uses are of comparable size and energy to those on boron and so there is much better overlap than there is for Cl and Br giving a greater double bond character. This somewhat stabilises the unsaturated BF₃ as a lot more energy is required to overcome this partial double bonding than in BBr₃.

If the Lewis base contains an acidic proton then instead of adduct formation we can get a substitution reaction that results in the elimination of HX and can sequentially lead to complete solvolysis, for example

BCl₃ + C₂H₅OH → Cl₂B-OC₂H₅ + HCl →→ B(OC₂H₅)₃

Another feature of the reactivity of the boron trihalides is halide exchange. If one halide of boron, BX₃ is mixed with a another halide of boron, BY₃ then we form mixed halide boron compounds, eg BX₂Y. The reaction is thought to proceed by the formation of transient dimers where a halide one on boron atom forms a bond to bridge to the boron of another molecule. With the new B-X bond formed, the old B-X bond is broken and so the halide atom is transferred though these transient species have not been detected.

There are also many examples of subhalide compounds of boron where the ratio of halide atom to boron atoms is less than 3. We can get simple molecules such the unstable monohalides BF and BCl which can be generated in the gas phase, and the diboron tetrahalides, B₂X₄ in which there is a B-B bond. We can also get compounds of the form B_nX_n such as B₉Cl₉ which are cluster molecules in which the boron atoms are bonded to each other forming a complex polyhedron.

Nitrogen Compounds of Boron

Compounds containing a B-N bond are interesting chemical species. Since N has one more electron and B has one less electron than carbon, a B-N fragment has the same number of electrons as a C-C fragment (they are said to be isoelectronic) and may be thought of as being analogous. As we have seen before, the reaction of ammonia, NH₃, with elemental boron at high temperatures leads to the formation of compound with the same structure as graphite but with alternating B and N atoms. However, the simplest boron-nitrogen compounds are the amine boranes. These are Lewis acid/base adducts of amines and boranes (see later) but can also be formed from the reaction of ammonium salts with a tetrahydroborate, BH₄, containing compound

[RNH₃]Cl + NaBH₄→ LiCl + H₂ + RH₂N-BH₃

These compounds with a single B-N bond are somewhat analogous to alkanes containing C-C bonds. The strength of the bond depends on the nature of the groups already on N and B. An amine borane that has a weak bond that is essentially a donor-acceptor interaction would have the bond drawn as an arrow, ie N→B. If the electrons from nitrogen are completely shared then we will have a stronger full bond. Since the nitrogen atom has fully donated it's lone pair and forms four bonds we draw it as having a positive charge whilst the boron atom which accepts the electrons is drawn with a negative charge, ie N⁺-B^-.

Just as the reaction of boron trihalides with alcohols results in elimination of HX, amine boranes can also eliminate HX to give aminoboranes. The bonding interaction between the B and N atoms can be imagined to be somewhere between being a single B-N bond and a B^-=N⁺ double bond. These compounds can therefore be thought of as being analogous to alkenes containing C=C double bonds, eg

(CH₃)₂NH + BCl₃→ (CH₃)₂HN→BCl₃→ (CH₃)₂N⁺=B^-Cl₂ + HCl

One of the main reactions of aminoboranes is substitution of the groups on boron. If the product aminoborane in the example above is treated with a Grignard reagent, RMgX (see organometallic compounds of the alkaline earth metals), the Cl atoms will be replaced by organic groups labelled R.

(CH₃)₂N⁺=B^-Cl₂ + 2 RMgCl → (CH₃)₂N⁺=B^-R₂ + 2 MgCl₂

Aminoboranes can also condense to form cyclic dimeric compounds, see figure 3. If this condensation also occurs with elimination of HX then we can get formation of six-membered cyclic molecules called borazines.

Figure 3: Structures of cyclic aminoborane dimers, borazine and it's carbon based structural analogue, benzene.

Borazine is structurally similar and is isoelectronic with it's carbon based analogue, benzene. The two have similar physical properties, however borazine is much more reactive than the fairly inert benzene. This is due to the polarity in the B-N bonds of the ring as there are significant positive and negative charges on the nitrogen and boron atoms respectively.

Boron Hydrides - Boranes

Boron has an extensive chemistry with hydrogen forming a multitude of compounds called boranes. These have varying ratios of B to H atoms and form polyhedral structures with complex bonding between the boron atoms and can also incorporate atoms of ther elements (for pictures of typical boranes follow this link, for interactive structures of other boranes and their dervatives follow this link). In naming boranes, we first give the name with the prefix for the number of boron atoms and then give the number of hydrogen atoms in brackets, for example B₄H₁₀tetraborane(10). The hydrogen atoms in these compounds can be terminal, bonding to just one boron atom, or can occupy bridging positions where it is bound to two B atoms.

The bonding in boranes can't be described conventionally as being wholly in terms of two electron between two atoms. The polyhedral cluster formation in boranes is due to their electron deficiency, there are not enough valence electrons to go round to form such two centre two electron (2c-2e) bonds between all the atoms. Instead many atoms come together and share their electrons engaging in so called multicentre bonding, ie one chemical bond can be formed between a number of atoms using two electrons. In the polyhedral boranes (and the previously mentioned polyhedral monohalides), several multicentre bonds hold the structures together.

The simplest borane is diborane(6), B₂H₆. This molecules exist as a dimer of two BH₃ units in which two H atoms bridge the two B atoms which are tetrahedral (see figure 4). Each B-H-B fragment forms a three centre two electron (3c-2e) bond whilst the terminal hydrogen atoms form conventional 2c-2e bonds.

Figure 4: Structure of diborane(6)

Diborane(6) is a gas that is spontaneously flammable on contact with air and instantly hydrolyses on addition of water to boric acid and hydrogen gas. It can be formed by reacting sodium borohydride, NaBH₄ with either BF₃ or by reduction with iodine. On industrial scales it can be made by reducing BF₃ with sodium hydride.

3 NaBH₄ + 4 BF₃→ 2 B₂H₆ + 3 NaBF₄

2 NaBH₄ + I₂→ B₂H₆ + 2 NaI + H₂

2 BF₃ + 6 NaH → B₂H₆ + 6 NaF

Diborane(6) is a useful reagent for making organoboron compounds which are employed in organic synthesis and can be used to form higher boranes by controlled heating and loss of hydrogen. The heavier boranes are mostly liquids and have generally have greater stability to air and water with increasing molecular weight but their thermal stability is variable, see table 1.

Table 1: Chemical and physical properties of some boranes.

There are also examples of anionic boron hydrides, the simplest being borohydrides containing the BH₄^- ion. The most common example is sodium borohydride, NaBH₄ which is an important reagent in both organic and inorganic chemistry as a source of H^- and as a reducing agent. There also a range of polyhedral anionic boranes which have the general formula B_nH_n^2-. Their structure and bonding follows the same rules as for the neutral boranes. There are also a range of neutral compounds called carboranes in which two BH^- fragment are replaced with CH or C-alkyl group. These are called carboranes and have the general formula B_n-2C₂H_n. These can be produced by reacting a borane with a thioether (R-S-R) to displace H₂, followed by an alkyne, for example

B₁₀H₁₄ + 2 R₂S → B₁₀H₁₂(R₂S)₂ + H₂

B₁₀H₁₂(R₂S)₂ + RC≡CR → B₁₀H₁₀(CR)₂ + 2 R₂S + H₂

The chemistry of boranes, borane anions and carboranes is truly vast and we are only able to have the briefest glimpse here. There is an enormous range of compounds of these types that have been produced and there are even more interesting examples in which fragments of transition metal complexes can be introduced into the structure in place of BH units. There are also examples of anionic carboranes which can act as ligands for transition metal ions.