Chapter 13: The Group 13 Elements
• Group 13 Elements: boron, aluminum, gallium, indium and thallium.
• Boron is the only nonmetal in the group.
• Aluminum is the most abundant Group 13 element.
• There is an increase in metallic character from B to Tl; B is a nonmetal, Al is a
metalloid and Ga, In and Tl are all metals.
• Boron’s diagonal relationship with silicon.
o Boron and silicon form acidic oxides, B O 2n3 SiO ; al2minum forms an
o Boron and silicon form many polymeric oxide structures and glasses.
o Boron and silicon form flammable, gaseous hydrides; aluminum
hydride is a solid.
• Aluminum is an electropositive metal but it is very inert because of the
presence of a passivating surface oxide film, not oxidized by air.
• Gallium is brittle at low temperatures but liquefies at 30°C and has the widest
liquid range of 30-2420°C.
• All of the elements form hydrides oxides and halides in the +3 oxidation state.
o Many of the compounds made from lighter Group 13 elements have
incomplete octets and act as Lewis acids (electron pair acceptor).
o Boranes: binary hydrogen compounds of B.
o Boron trihalides are monomeric in the gas, liquid and solid state unlike
the rest of the halides in Group 13.
o 4 B(OH) (3) + heat 2 B O (2)3+ 6 H O (l2
o B 2 3l) + 2 NH +3heat 2 BN (s) + 3 H O (g)2
BN structure consists of planar sheets of atoms like graphite.
BN and graphite have similar physical properties, both used as
lubricants but BN is a white, nonconducting solid.
• The +1 oxidation state becomes more stable down the group and is the most
stable oxidation state for compounds of thallium (due to inert pair effect).
o Because the F ion is so small, the trifluorides are mechanically hard
ionic solids that have much higher melting points and sublimation
enthalpies than other halides; they are not very soluble and do not act
as a Lewis acid.
o The heavier trihalides of Al, Ga and In are soluble in a wide variety of
polar solvents and are excellent Lewis acids.
• The most important oxosalts of Group 13 are called alums, MAl(SO ) • 124 2O, 2
where M is a univalent cation like Na , K , Rb , Cs , Tl or NH . 4+
o Gallium and indium can also form analogous series of salts but B and Tl
do not because B is too small and a Tl atom is too large.
o Alum is used widely to describe other compounds with the general
formula MM’ (SO ) •4 2 H O, 2here M’ is often a d metal, such as Fe.
• Boron forms an extensive range of polymeric, cage-like compounds which
include the borohydrides, metallaboranes and the carboranes.
o Closo Structure: borohydrides with the formula [B H ]n. n2-
o Nido Structure: when boron clusters have the formula [B H n n+4] formula.
o Arachno Structure: when clusters of boron have the formula [B H n n+6.
o Metallaboranes: borons that form many metal-containing clusters.
o Carboranes: a large family of clusters that contain both B and C atoms. The Detail
• Aluminum is highly abundant; thallium and indium are the least abundant of
the Group 13 elements.
• 2 BBr 3g) + 3 H (g2 2 B (s) + 6 HBr (g)
• The most useful compound of boron is borax (Na B O (OH) 2 4 5); th4 most2
commercially important element is aluminum because it its light and resistant
• Diborane (B H) can be synthesized by metathesis between a boron halide and
a hydride source; many of the higher boranes can be prepared by the partial
pyrolysis of diborane.
o 3 LiBH 4et) + 4 BF (e3) 3 LiBF (et4 + 2 B H (g)2 6
o All the boranes are colorless and diamagnetic.
o They range from gases (B H an2 6 H ) th4o8gh volatile liquids (B H ) to 5 9
sublimable solid (B H10.14
o B2H 6g) + 3 O (2) 2 B(OH) (s) 3
o B2H 6g) + 6 H O2(l) 2 B(OH) (aq3 + 6 H (g) 2
o B2H 6s a Lewis acid and the mechanism of this hydrolysis reaction
involves coordination of H O 2cting as a Lewis base.
• All the boron hydrides are flammable, sometimes explosively, and many of
them are susceptible to hydrolysis.
• Soft and bulky Lewis bases cleave diborane symmetrically; more compact and
hard Lewis bases cleave the hydrogen bridge unsymmetrically.
o Symmetric Cleavage: B H 2 6 is broken symmetrically into two BH 3
fragments; each of which forms a complex with a Lewis base.
o Unsymmetrical Cleavage: generally observed when diborane and a few
other boron hydrides react with strong, sterically uncrowded bases at
low temperatures (NH ). 3
• Although it reacts with hard Lewis bases, diborane is best regarded as a soft
• Hydroboration, the reaction of diborane with alkenes in ether solvent,
produces organoboranes that are useful intermediates in synthetic organic
• The tetrahydridoborate ion is useful intermediate for the preparation of metal
hydride complexes and borane adducts.
o B2H 6 2 LiH 2 LiBH 4