CH226.15
•Compounds containing nitrogen
•Summary of Group III trends
Molecular species containing B-N bonds
Aminoboranes, R3B–NR3, are isoelectronic with hydrocarbons, R3C–CR3.
•H3N-BH3 is a solid at room temperature, polar molecule
•H3C-CH3 is a gas that condenses at –89 °C, nonpolar
The striking difference in the properties is due to hydrogen bonding between H3BNH3 molecules (recall that strongly hydrogen bonded H2O is a liquid under normal
conditions, whereas H2S is a gas, because of weaker H-bonding)
2
Borazine
We now extend the discussion to include compounds with boron-nitrogen multiple
bonds. Borazine, B3N3H6 is a colorless liquid with an aromatic odor and physical
properties that resemble those of benzene, C6H6.
Borazine can be prepared by different methods, for example:
3H3N–BH3 (heating) → (HBNH)3 + 6H2
3
Borazine
Borazine, (HBNH)3, is isoelectronic and isostructural with benzene, C6H6.
δ +0.19
δ -0.19
δ +0.03
δ -0.82
δ -0.03
δ +0.82
B–N 1.42 Å
B–H 1.20 Å
N–H 1.01 Å
C–C 1.39 Å
C–H 1.08 Å
In borazine, the difference in electronegativities of boron (2.0) and nitrogen (3.0)
leads to a charge distribution which makes the B and N atoms susceptible to a
nucleophilic and an electrophilic attack, respectively.
4
π-Molecular orbitals of borazine
The π-MO’s of borazine are very similar to those of benzene. Bonding MO’s have
more electron density on N, whereas the antibonding MO’s are more concentrated
on the less electronegative B atom, as expected.
Energy
Unoccupied π-antibonding orbitals
Occupied π-bonding orbitals
5
Borazine
Borazine is much more reactive than benzene. The following are representative
reactions of borazine:
1. (HBNH)3 + 3HCl → (ClHBNH2)3
addition reaction
2. 2(ClHBNH2)3 + 6NaBH4 → 2(H2BNH2)3 + 6NaCl + 3B2H6
‘reduction’
cyclotriborazane
B–N 1.57 Å
6
Compounds with B=N and B≡N bonds
Compounds with double and triple B–N bonds are known and have been structurally
characterized. They are more reactive than the corresponding alkenes and alkynes,
and are typically stabilized by bulky groups on the atoms of B and N.
t-Butyl-(t-butylimino)borane
1.26 Å
Di-isopropylamino-bis(trifluoromethyl)borane
1.37Å
7
B-N bonds
Covalent radii: 0.82 Å (B) + 0.75 Å (N) = 1.57 Å
B-N bonds in selected species:
Species
B-N distance / Å
Me3N–BBr3
1.60
Single σ-bond
Me3N–BC13
1.58
Single σ-bond
Hexagonal-(BN)n
1.45
σ + some π-bonding
B(NMe2) 3
1.44
σ + some π-bonding
Mes2B=NH2
1.38
Double bond
Mes2B=N=BMes2
1.35
Double bond
tBuB≡NtBu
1.26
Triple bond
Mes = 2,4,6-Me3C6H2
8
Group III: Some trends
1. The chemistry of boron is quite different from that of the heavier Group III
elements. It differs from aluminum in the following ways.
a. Boron oxides and hydroxides are acidic whereas those of aluminum are
amphoteric.
b. Boron is a non-metal and semi-conductor, whereas aluminum is a metal.
Boron is very inert and only attacked by hot concentrated oxidizing acids.
c. No simple salts of B3+ are known, whereas those of Al3+ are numerous.
d. Boron forms a wide range of molecular hydrides. AlH3 has a polymeric
structure which resembles that of AlF3.
e. The stereochemistries of many boron compounds are based on trigonal sp2
and tetrahedral sp3 geometries. Aluminum forms many compounds with
tetrahedral, trigonal bipyramidal, and octahedral geometries.
f. π-Bonding in B–N, B–O and B–F compounds is much more significant than that
for the corresponding aluminum compounds.
9
Group III: Some trends
2. Aluminum, gallium, indium, and thallium all form a range of compounds in the +3
oxidation state. However, compounds in the +1 oxidation state become
progressively more stable down Group III.
3. The oxides of aluminum and gallium are amphoteric and indium and thallium
oxides are more basic.
4. The octahedral aqua-ions [M(OH2)6]3+ are acidic. The pKa values for the
equilibria: [M(OH2)6]3+  [M(OH2)5(OH)]2+ + H+ are Al, ~5; Ga, 3; In, ~4; Tl, 1.
5. The MX3 compounds are Lewis acids and the Lewis acid strengths decrease in
the order: Al > Ga > In.
6. The stability of the hydrides decreases down the Group and there are no stable
In-H and Tl-H compounds. Ga2H6 is extremely unstable.
7. Aluminum is resistant to corrosion because of an impermeable oxide layer, but is
soluble in non-oxidizing mineral acids (e.g. HCl). Ga and In dissolve readily in
acids, but Tl dissolves only slowly in H2SO4 and HCl.
10