Chemistry of Group 13 Elements

(Prepared by Prof. S. Bhattacharya as a study material for B.Sc. Sem II students)

GENERAL FEATURES

Element (symbol) Electronic Configuration Oxidation State Atomic / Metallic Radius (Å) B [He] 2s22p1 III 0.885 Al [Ne] 3s23p1 (I) III 1.43 Ga [Ar] 4s24p1 I III 1.225 In [Kr] 5s25p1 I III 1.67 Tl [Xe] 6s26p1 I III 1.70 • Oxidation state III is common, however, stability of oxidation state (I) increases down the group. (Inert pair Effect) • Anomalous reduction in size from Al to Ga is due to the presence of 10 d electrons in the penultimate shell which shield the nuclear charge less effectively • Boron is a typical non-metal obtained by reduction of B2O3 as an amorphous black solid. All other elements are metals. • Aluminum is the most abundant element in the earth’s crust. Gallium has a very low M.P. (30 oC) though its density (at room temp) more than twice of that of Al. • Oxidation state of (I) is more stable in case of Tl. Tl(I) salts are poisonous.

OXIDES OF BORON AND RELATED COMPOUNDS 1. Orthoboric Acid Structure 2 B(OH)3 is trigonal planar (sp hybridization of orbitals of central boron). In solid state B(OH)3 units are hydrogen bonded together into 2D sheets with almost hexagonal symmetry. Distance between any two layers is quite large (3.18 Å) as a result crystals easily break into fine powders

H H

O O H H H B H O O O B H B H H H O O O O

H H H H

O O O O H H H B H B O O O H H H B H O O

H H Chemical Nature - H3BO3 [or B(OH)3] is a weak acid. It does not give protons but accepts OH ion (Lewis acid) and only partially reacts with water. 1

H O+ + [B(OH) ]- B(OH)3 + 2H2O 3 4 (pK = 9.25) (eq-- i) At higher concentrations polymeric metaborate species are formed. H O+ + [B O (OH) ]- 3H3BO3 3 3 3 4 pk = 6.84 (eq-- ii)

B O OH O B OH B O HO Thus, B(OH)3 cannot be titrated properly with NaOH as a sharp end point is not observed. In presence of cis-diols the titration can be performed satisfactorily because such compounds shift the above written equilibrium (eq-- i) to the right hand side completely.

HO OH HO HO O O B O B + HO O OH B HO OH HO O O

When heated, orthoboric acid is dehydrated to give metaboric acid and finally to boron sesquioxide

2. Boron Sesquioxide (B2O3) and Borates Boron and all other elements of the group when heated in oxygen give respective sesquioxide. As mentioned already, dehydration of orthoboric acid also gives boron sesquioxide o 100 C red heat B O B(OH)3 HBO2 2 3 orthoboric acid metaboric acid boron sesquioxide Chemical Nature Boron sesquioxide is amphoteric. It reacts with metallic oxides (bases) forming salts called borates or metaborates. In borax bead test boron sesquioxide (or borax) is mixed with a metal oxide and heated on a flame forming a glass like bead. The beads of many transition metals are characterized by their colors. (Borax bead test). This test provided first proof that vitamin B12 contains cobalt.

CoO + B2O3 Co(BO2)2 cobalt metaborate (Blue) B2O3 may also behave like a base if reacted with a strongly acidic compound such as phosphorus pentoxide giving boron phosphate. 2BPO B2O3 + P2O5 4

3- Orthoborates contain discrete BO3 ions. For example Mg3(BO3)2, LnBO3 (Ln is a lanthanide metal ion).

In metaborates simple (BO3 triangles or BO4 tetrahedra) join together to form a variety of polymeric chain and ring structures. Two units join together sharing one corner it is called pyroborate 4- O O

B O B

O O Three units join together sharing corners may form a ring as in the case of sodium and potassium metaborates

O O

B B O O O

Many triangular units may polymerize into an infinite chain

O O

B B

O O O O O

B B

O O

Similarly tetrahedral units may also be present as discrete ions, sharing one corner, sharing two corners forming a ring or a polymeric chain. A few other Compounds: O O B O O B O B O O B B O O O A complex metaborate such as K[B5O6(OH)4] consists of a spiro anion in which there are 4 triangular BO3 units and a central tetrahedral BO4unit.

Borax, Na2[B4O5(OH)4].8H2O This is the most common metaborate used as a primary standard for titrating against acids.

Na2[B4O5(OH)4].8H2O + 2HCl 4H3BO3 + 2NaCl + 5H2O. Two moles of HCl are consumed because when borax is dissolved in water both B(OH)4 and B(OH)3 are formed but only

B(OH)4 reacts with HCl. The end product of this titration is H3BO3 (again an acid but very weak) thus indicator used in this titration should not be affected by it. Methyl orange is one such indicator that changes color in the pH range 3.1-4.4

O O HO B O B OH O O B

OH Structure of borax.

Sodium Peroxoborate is a brightener in washing powders. 2- HO O O OH B B HO O O OH Peroxoborate ion.

Oxides of other elements

Alumina (Al2O3) There are 2 crystalline forms and 1 fibrous form: 1. Corundum (- alumina)- Very inert material . Consists of hcp lattice (formed by Oxygen atoms) and two thirds of octahedral holes are occupied by Al3+ ions. Extremely hard material (9 on Moh’s scale); used for cutting glasses. When impurity of iron oxide and silica are present the material is called emery. 2. -alumina is used in chromatography. It dissolves in acids 3. Fibrous alumina: fine silky fibers are commercially made which find application in chemically inert paper, rope and fabric.

Al(OH)3, Ga2O3,Ga(OH)3 are amphoteric while Tl2O3 and In2O3 are basic and do not form hydroxides or hydrates. TlOH resembles alkali metal hydoroxides

Tetrahydroborates (borohydrides) - Thetrahydroborate (BH4 ) ion is tetrahedral. The alkali metal salts are white ionic solid. The sodium salt being the most common, used as reducing agents in organic reactions.

LiBH4 reacts violently with water

NaBH4 can be recrystallized from cold water

KBH4is quite stable. (Correlate the reactivity with size of cations)

Beryllium, aluminum and transition metal borohydrides become increasingly covalent and volatile.

The structure of Be(BH4)2 given in J.D Lee’s book is the following

H H

H H B B H H

However, the structure reported by Marynic and Lipscomb is helical.

Al(BH4)3 H H B H H H H Al B H H H H B H H Zr(BH4)4 2- 1. Closo-boranes BnHn H H

H H H H H H B Zr B H H H H H H

H H

Hydrogen bridges are present in all these compounds. These are (electron deficient) 2-electron-3- center bonds.

AlH3 is also an electron deficient compound. It is extensively polymerized by 3-center bonds. Li[AlH4] is a useful reducing agent.

Boron Hydrides

Diborane B2H6 is a covalent colorless gas which is highly reactive. It reacts with oxygen to give B2O3, with water to give H3BO3 and with ammonia to give products as shown below:

B2H6.2NH3 ow H /L ess N 3 Exc e ratur empe (BN)x B2H6 t Excess NH3/Higher Temp NH 3 in 2:1 mol ar rat o/ Hi B N H gher 3 3 6 Temp

Other than diborane there are a number of higher Boron hydrides or higher boranes which are broadly classified into three categories:

Type Formula Example

2- BnHn H Closo-boranes B H B (Closed structure) B H

H B B H B 2 - B6H6 H

Nido-boranes BnHn+4 H B H H H B H H B B (Nest like structure) H H B B5H9 H H Arachno-boranes BnHn+6 H B H H H (Spider’s web like structure) B H H H B H B

B4H10 H Besides normal 2-electron-2-centre bonds these molecules may also have electron deficient BBB and BHB bonds.

Diborane: Structure Bonding H 1. 33 H H Ao

o o B 96 B 122 1 .1 o 9 A H H H

Important Features: 1. Molecule has a centre of symmetry. 2. Presence of Bridging hydrogen atoms (Note: valency of H is 1 in most of its compounds but the bridging hydrogens here are bonded to 2 different B atoms). 3. Four terminal Hydrogens are different from the two bridging ones as confirmed by Raman

spectroscopy and only the terminal hydrogens can be substituted (resulting in B2H2Me4)

without breaking the molecule into BMe3.

Diborane: Bonding

Consider any of the two boron atoms. It has 3 electrons in its valence shell (2s22p1). Classically, these three electrons can participate in three covalent bond formations only, however it is forming 4 bonds. Compounds like BF3 are Lewis acids and can accept an electron pair

(lone pair) from a suitable donor like NH3 to form adducts such as BF3NH3. In these adducts B is tetrahedral and has coordination no. 4. However, in diborane all the 4 bonds are with hydrogen atoms only. None of the hydrogens (each having only one electron) can act as a lone pair donor in this molecule.

Valence Bond Treatment: A. Resonating structures Initially it was attempted to explain the bonding on the basis of resonance. The following resonating structures (canonical forms) can be written:

H H BH BH2 H2B 2 H2B H H

A large number of canonical forms can be written if all the hydrogens are involved in resonance, thus imparting very high stability to the molecule. As mentioned already the two hydrogens involved in bridging are different from the terminal ones. Which means only two hydrogens are involved resonance. Lack of formal bonds in the resonance forms shown above seems somewhat artificial.

B. Three centered bonding Let us consider that each B atom is sp3 hybridized. Out of the four orbitals, three are singly occupied while the fourth one is vacant (See the figure A below showing the orbitals involved in bonding of all the atoms separately). Two of these orbitals form two normal covalent bonds with two hydrogen atoms (terminal B-H bonds). While the third one overlaps with the empty orbital of the other boron and the singly occupied 1s orbital of a hydrogen atom. Thus forming a three centered (B-H-B) bond using only two electrons (one from a B and other form the H). See figure B

H H H

B B B B

H H H

Figure A

[Now imagine the atoms are coming closer so as to form the B2H6 structure (as shown above) each unpaired electron will be shared so that an electron pair bond is formed. In case of terminal bonds two orbitals will overlap while in case of bridging bonds three orbitals will be involved]

H H H

B B H H H Figure B Thus the bridging B-H-B bonds are 2-electron-3-center bonds. The finally obtained molecular orbitals are bent and accordingly named as banana bonds. [Note: HBH terminal planes are perpendicular to the central BHBH unit]

H H H

B B

H H H

Molecular Orbital Treatment Now let us concentrate on the orbitals forming the bridge. One sp3 orbital from each of the two B atoms and one 1s atomic orbital of hydrogen. A linear combination of these three orbitals will give rise to three molecular orbitals One of these, the Ψb is a bonding molecular while the Ψa is 3 anitbonding molecular orbital. The third one, Ψn is non-bonding, in which the signs of the two sp 8

orbitals are out of phase and cannot have nay net overlap with the 1s orbital of H present in between.

3 3 Ψa = sp (B) – 1s(H) + sp (B)

3 3 Ψn = sp (B) - sp (B)

3 3 Ψb = sp (B) + 1s(H) + sp (B)

M.O. Energy diagram:

a

n

sp3 Borbitals Energy 1s H orbital b

Halides

1. Trihalides

Trihalides of boron are covalent compounds. M.P. and B.P. increases in the order BF3 < BCl3 < 2 BBr3 < BI3. BF3 is a gas, it has a trigonal planar structure. Central B uses its sp hybrid orbitals to form three bonds with three F atoms. The B-F bonds (1.30 Å are significantly shorter than the expected B- F single bond (on the basis of their covalent radii) and the bond energy 646 kJ/mol is very high. These facts indicate for a higher B-F bond order. The empty p orbital is present perpendicular to the plane of the molecule which accepts an electron pair from any one of the F atoms, thus forming a p-d bond. Now the octet of B is complete. Since all the three B-F bonds are identical, it is obvious that the  bond is delocalized as shown below: 9

F F F F F F

B B B

F F F Donor molecules form coordinate bond with the empty orbital of BF3, as a result such p-d bonding is not possible and the BF bond length increases (B-F bond length is 1.39 Å in Me3N BF3) The fluorides of Al, Ga, In and Tl are ionic and have high melting points. The other halides are largely covalent when anhydrous (in water these form aqua complexes). AlCl3, GaCl3 and AlBr3 are dimers. - 3+ AlCl3 crystal at low temperature has a close packed lattice of Cl ions in which Al ions occupy the octahedral holes. At higher temperatures Al2Cl6 is formed and the volume of the solid increases significantly. This illustrates how close the bonding in this compound is to the ionic/covalent border.

2. Other halides

Boron forms monohalides and dihalides . The dihalide is dimeric B2X4 while the monohalides are oligomeric. Compounds such as Ga2Cl4 and In2Cl4 are actually complexes of the type + M [MCl]4.̅ [One metal ion in +1 while the other one in +3 state].

Cl Cl Cl Cl B B Cl Cl Cl B B B B Cl B B Cl B B Cl Cl B Cl B B B Cl Cl Cl Cl

B2Cl4 B Cl B8Cl8 4 4

Cl Cl Cl Al Al Cl Cl Cl

Al2Cl6 Complexes

Smaller size and higher charge (+3) lead to increased complexation tendencies of the ions as 3+ compared to the cations of group 1 and 2. Aqua complexes e.g. [M(H2O)6] , complexes of halide, 3- - hydride ligands such as [MCl6] and [MH4] are well known. Organic anions like, acetylacetonate, carboxylate, oxalate, are amongst the common ligands forming complexes of group 13 elements.

H C 3 H3C

H C 3 CH H3C O 3 O CH3 O O O O Al Al O O O CH 3 O CH3 O O H C 3 H3C

H C 3 H3C Al(acac)3 complex

Further Reading: 1. Concise Inorganic Chemistry, J.D. Lee 2. Basic Inorganic Chemistry, F.A. Cotton, G. Wilkinson and P.L. Gaus