Main group III elements

1. Main group III elements Chapter 12,13

2. The Periodic Table of the Elements Most Probable Oxidation State +1 0 +2 +3 +_4 - 3 - 2 - 1 H He Li Be B C N O F Ne +3 +4 +5 +1 + 2 Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cu Zn Ga Ge As Se Br Cr Mn Fe Co Ni Kr Rb Sr Zr Nb Ag I Mo Tc Ru Rh Pd Xe Y Cd In Sn Sb Te Ba Au Hg W Re Os Ir Pt Rn Cs La Hf Ta Tl Pb Bi Po At Fr Ra Ac Rf Du Sg Bo Ha Me +3 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr +3

3. 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.

4. 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.

5. 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.

6. Boron • Isolation of the element: • Boron is made in 95-98% purity as an amorphous powder by reduction of the oxide B203with Mg • Or Zn

7. Borosilicate glass-pyrex Detergents Flame retardants Ceramics Pyrotechnics Used in production of impact resistant steels Control rods in nuclear reactors Uses of Boron

8. Common Bonds in Boranes • 2c-2e- B-H • 3c-2e- B-H-B • 2c-2e- B-B • 3c-2e- B-B-B

9. 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. 790.

10. Tetraborane B4H10 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.

11. Dodecaborane [B12H12]2-

12. Elemental Forms of Boron b-rhombohedral, B12(B12)12, (B12)(B12)(B60) a- rhombohdral Housecroft, C. E.; Sharpe, A. G. inorganic Chemistry. New York: Pearson Education Limited, 2001. 251-2.

13. Boron Deltahedra – Parent Clusters Housecroft, C. E.; Sharpe, A. G. inorganic Chemistry. New York: Pearson Education Limited, 2001. 275.

14. Boron • The structure of Boranes:

15. Boron • The hydrides of Boron: • Diborane: • Lab quantities: • Industrial Quantities:

16. Boron

17. Boron • Reactions of Boranes:

18. Boron • Deca boranes:

19. Boron • Borohydrides of many metals have been made and some representative syntheses are:

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

21. Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: principles of structure and reactivity, 4th ed. New York: HarperCollins College Publishing, 1993. 799.

22. 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

23. 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

24. 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.

25. 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

26. 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 .

27. 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

28. 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)xa slippery white solid with a layer structure resembling that of graphite, but with hexagonal rings of alternating B and N atoms.

29. Boron • Hydrated borates contain polyoxo anions in the crystal, with the following important structural features: 1. Both B03and tetrahedral B04groups are present, the number of B04units being equal to the charge on the anion. 2. Anions that do not have B04 groups, such as metaborate, B3063-, 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.

30. 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 B303rings are joined by hydrogen bonding. On continued heating of form-III of HB02, between 130 and 150°C, HB02-II is formed.

31. Boron

32. 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

33. 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)

34. 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.

35. 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+)

36. 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

37. Aqua ions

38. Hydroxides

39. 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

40. 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.

41. Summary of group IIIa trends • (e) Frequently forms polyhedral structures: boranes and borates. • (f) Forms an oxide, B203, and a hydroxide, B(OH)3both 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-.

42. 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-.

43. 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) Increasinglyformweakercovalentbondsondeseentofthegroup,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-.