MAIN GROUP III ELEMENTS

Chapter 12,13

HeNeArKrXeRn

The Periodic Table of the Elements

CrMn Fe Co NiMoW

TcRe

RuOs

RhIr

PdPt

Most Probable Oxidation State

+1

+2

+3 +4

+3 +_4 - 3 - 2 - 1

0

HLiNaKRb

CsFr

Sc

Y

BeMgCaSr

Ba

RaLaAc

BAlGa

InTl

Ti

RfHf

Zr

CSi

Ge

SnPb

FClBrI

At

OSSe

TePo

NP

As

SbBi

Zn

CdHg

+ 2+1

CuAgAu

+5

VNb

Ta

CeTh

Pr Nd PmSmEu Gd Tb Dy Ho Er TmYb LuPa U Np Pu AmCmBk Cf Es FmMd No Lr

+3

+3

Du Sg Bo Ha Me

Boron Boron: In nature it is found as Borates:

Ulexite : {NaCa[B5O6(OH)6].5 H20} Borax : {Na2[B405(OH)4]. 8 H 20} Colemanite: {Ca2[B304(OH)3]2.2 H 20)} Kernite: {Na2[B4O5(OH)4].2 H20}

Borates do have complex structures, but common to all is that Boron is contained as trigonal BO3 or tetragonal BO4 units.

Boron The cations in these minerals are

typically alkali or alkaline earth cations. The largest source of Boron is in the form

of Borax found in the mojave desert in california

No ionic compounds involving simple B3+ cations are formed because the ionization enthalpies for boron are so high that lattice energies or hydration enthalpies cannot offset the energy required for formation of a cation.

Boron Boron is sp2 hybridized in trigonal planes. All BX3 planes compounds are strong

lewis acids interaction with Lewis bases (molecules

or ions) gives tetrahedral adducts such as BF3.O(C2H5)2 ,BF4

-, and B(C6H5)-4. The

formation of such Lewis acid-base adducts requires a change to Sp3 hybridization for boron.

Boron Isolation of the element: Boron is made in 95-98% purity as an

amorphous powder by reduction of the oxide B203

with Mg

Or Zn

Uses of Boron Borosilicate glass-pyrex Detergents Flame retardants Ceramics

Pyrotechnics Used in production of impact resistant

steels Control rods in nuclear reactors

Common Bonds in Boranes 2c-2e- B-H 3c-2e- B-H-B 2c-2e- B-B 3c-2e- B-B-B

Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: principles of structure and reactivity, 4th ed. New York: HarperCollins College Publisher, 1993. 790.

Diborane B2H6

Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: principles of structure and reactivity, 4th ed. New York: HarperCollins College Publisher, 1993. 794.

Tetraborane B4H10

Dodecaborane [B12H12]2-

Housecroft, C. E.; Sharpe, A. G. inorganic Chemistry. New York: Pearson Education Limited, 2001. 251-2.

Elemental Forms of Boron

a- rhombohdral b-rhombohedral, B12(B12)12, (B12)(B12)(B60)

Housecroft, C. E.; Sharpe, A. G. inorganic Chemistry. New York: Pearson Education Limited, 2001. 275.

Boron Deltahedra – Parent Clusters

Boron The structure of Boranes:

Boron The hydrides of Boron: Diborane:

Lab quantities:

Industrial Quantities:

Boron

Boron Reactions of Boranes:

Boron Deca boranes:

Boron Borohydrides of many metals have been

made and some representative syntheses are:

Housecroft, C. E.; Sharpe, A. G. inorganic Chemistry. New York: Pearson Education Limited, 2001. 272.

Boron Hydrides

Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chem

istry: principles of structure and reactivity, 4th

ed. New York: HarperCollins College Publishing, 1993. 799.

Wade’s Rules n = number of B atoms in parent closo-

deltahedron Always n+1 bonding e- pairs and n+1

bonding MOs nido has n-1 vertices arachno has n-2 vertices hypho has n-3 vertices

Using Wade’s Rules Find total available bonding e-s:

Each B-H unit gives 2 e-s Each additional H gives 1 e-

Overall charge Find parent closo-deltahedron

n+1 bonding e- pairs Is it closo, nido, arachno, hypho? Lose highest connectivity B first then lose adjacent sites

Determine number of remaining hydrogen atoms Each vertex has a H “sew up” hole with H atoms

Bridging H atoms Low connectivity B atoms can get another 2c-2e- B-H bond Try to keep it as symmetrical as possible

Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: principles of structure and reactivity, 4th ed. New York: HarperCollins College Publisher, 1993. 798.

Boron Neutron Capture Therapy (BNCT)

10B has large cross-section for neutron capture

10B + a + 7Li Products can kill cells Cancer treatment Cages - need high [10B] in cell

Boron The main resemblances to silicon and

differences from the more metallic aluminum are as follows:

1. The oxide B20 3 and B(OHh are acidic. The compound Al(OH)3 is a basic hydroxide, although it shows weak amphoteric properties by dissolving in strong NaOH.

2. Borates and silicates are built on similar structural principles with sharing of oxygen atoms so that complicated chain, ring, or other structures result .

Boron

3. The halides of Band Si (except BF3) are readily hydrolyzed. The AI halides are solids and only partly hydrolyzed by water. All act as Lewis acids.

4. The hydrides of B and Si are volatile, spontaneously flammable, and readily hydrolyzed. Aluminum hydride is a polymer, (AlH3)n

Boron Crystalline boron is very inert and is

attacked only by hot concentrated oxidizing agents. Amorphous boron is more reactive. With ammonia for instance, amorphous boron at white heat gives (BN)x a slippery white solid with a layer structure resembling that of graphite, but with hexagonal rings of alternating B and N atoms.

Boron Hydrated borates contain polyoxo anions in the

crystal, with the following important structural features:

1. Both B03 and tetrahedral B04

groups are present, the number of B04

units being equal to the charge on the anion.

2. Anions that do not have B04 groups, such as metaborate, B306

3-, or metaboric acid, B303(OH)3, hydrate rapidly and lose their original structures.

3. Certain discrete as well as chain-polymer borate anions can be formed by the linking of two or more rings by shared tetrahedral boron atoms.

Boron Boric acid: The acid B(OH)3 can be obtained as white

needles either from borates, or by hydrolysis of boron trihalides.

When heated, boric acid loses water stepwise to form one of three forms of metaboric acid, HB02. If B(OH)3 is heated below 130°C, the so-called form-III is obtained, which has a layer structure in which B303

rings are joined by hydrogen bonding. On continued heating of form-III of HB02, between 130 and 150°C, HB02-II is formed.

Boron

Boron Halides: Boron trihalide is a gas (bp -101 deg C) Boron trihalides are the strongest lewis

acids. They react with Lewis bases B-X bonds are somewhat shorter than is

expected from the sum of the single-bond covalent radii. This suggests a delocalized π-bond system

Al, Ga, In, Tl Al is the most common of the elements It is produced in pure form by

electrolysis, and is the most dirty of the industrial processes.

Costs a lot of energy. Main source is Bauxite, a hydrous Al –

oxide Al is attacked by diluted acids, but

passivated by strong acids. Al oxides are used to protect metals

(anodized)

Ga,In,Tl They are made from their salts by

electrolysis. Ga is used mainly in semiconductors with

Group V elements. (GaAs). Tl is a trace element and is very toxic.

Main use to get rid of spies.

Oxides Al has only one oxide formed Al2O3 There is an alpha and a gamma oxide. Difference is the process and the

temperature to get alpha or gamma oxide.

Mixed Al oxides are ruby (Cr3+)and sapphire

(Fe2+,Fe3+, Ti4+)

Halides Halides are formed of all elements, the

only one that is special is TlI3. Tl and I2 form rather a Tl1+ and I3-

compound All halides readily dissolve in benzene

Aqua ions

Hydroxides

Hydrides The most important hydride is LiAlH4 It is a strong reducing agent and is

mainly used in organic chemistry It is used e.g. to hydrate double bonds

Summary of group IIIa trends

1. Boron

(a) Forms no simple B3 +cation.

(b) Forms covalent compounds almost exclusively, and all polyatomic ions have covalent bonds.

(c) Obeys the octet rule, the maximum covalence being four.

(d) Forms trivalent compounds that readily serve as Lewis acids.

Summary of group IIIa trends

(e) Frequently forms polyhedral structures: boranes and borates.

(f) Forms an oxide, B203, and a hydroxide, B(OH)3 both of which are acidic.

(g) Forms covalent halides that are readily hydrolyzed.

(h) Forms numerous covalent hydrides, all of which are volatile, flammable, and readily hydrolyzed.

(i) Forms a stable and important hydride anion, BH4-.

Summary of group IIIa trends

2. Aluminum

(a) Readily forms an important 3+ ion, because it is electropositive.

(b) Is much more metallic than boron, and forms a greater number and variety of ionic substances.

(c) Forms both molecular and ionic substances, with coordination numbers of six and higher.

(d) Forms two oxides, only one of which is acidic. (e) Forms a hydroxide that is weakly amphoteric, although

mostly basic. (f) Forms solid halides that are only partially hydrolyzable. (g) Forms a polymeric hydride. (h) Forms an anionic hydride (AlR-) that is more reactive

than BH4-.

Summary of Group IIIa trends

3. Gallium, Indium, and Thallium

(a) Readily give the M3 + ion in solution, and have a rich coordination chemistry typical of metals.

(b) Form increasingly stable lower valent compounds, especially TI+.

(c) Increasinglyformweakercovalent bondsondeseentofthegroup,enhancing the formation of monovalent compounds.

(d) Form MX3 halides that are increasingly aggregated in the solid state (through halide ion bridges) to give coordination numbers of four, six, and higher.

(e) Do not form important EH4- anions, except perhaps GaH4

-.