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6 4 to 6 5 Main Group 6.4 Group 13: B, Al, Ga, In, Tl READING: Chapter 12, Intro & Sections 12.3 – 12.6, 12.11 – 12.18; Chapter 4, Section 4.3.-4.9 • Trivalent: B, Al, Ga. Mono- and trivalent: In, Tl. Inert Pair effect: Although the ionization energy decreases down the group with increasing atomic radius (heavier elements form cations more readily), the heavier elements also show greater stability of M+ (ns2np0). One possible explanation is that the ns2 electrons are harder to remove due to a relativistic contraction experienced by heavier elements (RECALL!!) • The first element, boron is unique and metalloid; all others are metals. This is an example of the diagonal relationship: The first element of a group often shows properties/reactivity patterns similar to the second element of the next group to the right – this particularly important for the first 4 groups: H Li Be B C Na Mg Al Si Explanation: The charge densities (defined as the ratio charge/radius) of the corresponding cations (Li+/Mg2+, Be2+/Al3+, B3+/Si4+) are comparable. → These pairs show a similar balance of covalent/electrostatic bonding to more electronegative partners and therefore similar reactivity patterns. • Electronegativity: B > Al < Ga > In > Tl The anomaly in the general trend to lower EN down the group is called the alternation effect and results from increased effective nuclear charges of the 4p elements due to the presence of their poorly shielding 3d electrons. (The alternation effect is also seen in Group 14 where Ge is more electronegative than Si.) • Lewis acidity: Neutral compounds of Group 13 elements tend to be Lewis acids. Boron: Typical compounds: • Boranes: B2H6 (diborane is a dimer of BH3); B10H14, B6H10, B4H10, etc… a large variety of cage-like structures). Flammable! 170 • Tetrahydroborate ion: NaBH4 and LiBH4 (drying agents, like LiAlH4; reducing agents) • Trihalides: BF3 (gas), BCl3 (liquid), BI3 (solid). Monomeric. Lewis acids. Trigonal planar. • Oxygen compounds: B2O3 (a glass), B(OH)3 (boric acid), Na2(B4O7)•10 H2O (Borax) • Nitrogen compounds: (BN)n; H3BNH3, B3N3H6 Lewis acid: an electron pair acceptor. I.e., has an energetically low lying LUMO. Lewis base: an electron pair donor. I.e., has an energetically high lying HOMO. Adduct: Reaction of a Lewis acid with a Lewis base results in a coordination compound. Def.: A coordination compound (or complex) is a chemical entity that consists of two or more components that can also exist “by themselves”. For an adduct to form, the symmetries of the HOMO (base) and the LUMO (acid) must match! Note: The shapes and thus the symmetries of the acid and base components may change from their “free” state in order to meet this condition. An example: BH3-CO • During the reaction, the symmetry of diborane (D2h) changes to that of two C3v BH3 fragments, i.e. analogous to NH3 BUT, unlike NH3, the 3a1 orbital is empty forming the LUMO of the acid. • The MOs of CO and NH3 are found in your lecture notes. • The two fragments combine to give the only known main group carbonyl coordination compound. 171 • Just as AO's of individual atoms can be combined to give MO's, the MO's of the fragments BH3(C3v) and CO(C∞v) can be combined to give the adduct H3B-CO: Source: Purcell + Kotz, Inorganic Chemistry, 1977, Holt-Saunders • The important interaction is the new σ-bond between the LUMO of the acid BH3 (a1 in C3v) and + the HOMO of the base CO (Σ = a1 in C∞v). • There is also a much weaker secondary retrodative π-interaction between the HOMO of the acid (e symmetry) and the LUMO of the base (also e symmetry) • The last point leads to a phenomenon called synergistic bonding (give a little, take a little), which is extremely important in transition metal complexes (→ CHEM 365). HOMEWORK: Sketch the MOs and construct an MO energy diagram for the formation of the BH3NH3 adduct. To which hydrocarbon is this analogous? 172 Boron Nitride – inorganic graphite (replace all atoms with carbon and you get graphite). • High temperature lubricant – used in rocket engines and nuclear reactors. • Insulator (graphite is a two-dimensional conductor). • Also exists in a diamond-like cubic allotrope that is formed at high pressure (> 50000 bar). Def.: Allotropes are compounds or elements of the same bulk compositions, but different molecular structure and hence chemical and physical properties. • The structure of the cubic diamond-like allotrope of (BN)n: all atoms have a Td environment. • Replace all atoms with carbon and you get diamond, which is thus isoelectronic and isostructural to cubic boron nitride. • Both materials are very hard, in fact diamond is the hardest material known – a direct consequence of the molecular structure. • The most important application of (synthetic) diamond is in drill bits for technical and geological applications, e.g. this one … … was used here … for the US-DOE geothermal energy program. Diamonds are a drill’s best friend! 173 Isoelectronic and Isostructural Compounds: Def.: Two molcules are isoelectronic, if they possess the same number and types of bonds and the same total electron count, even though they consist of different atoms. • By necessity, isoelectronic molecules are also isostructural. • Not surprisingly, isoelectronic compounds are often (but not always!) similar in their reactivity patterns and bulk material properties. Examples: • The graphite- and diamond-like modifications of boron nitride are isoelectronic to carbon based graphite and diamond, respectively – see above. • Borazine and benzene: • Methane and aluminum tetrahydride: Aluminum, Gallium, Indium, Thallium: Typical compounds: • Halides: mono- and tri-valent halogenides AlX3, GaX3, TlX. AlX3 is dimeric: Lewis acidity reflects hardness. Acidity toward hard bases: BCl3 > AlCl3 > GaCl3 Acidity toward soft bases: GaX3 > AlX3 > BX3 • Oxides: Al2O3 (alumina), K2SO4Al2(SO4)3 = potash alum 174 The elements: • Aluminum: use in the automotive and aerospace industry as DURALUMINIUM alloyed with Mg and Cu; in ship building as HYDRONALIUM, alloyed with 3-12 % Mg – with disastrous consequences in the BC SeaCat Ferry building program and the Falkland War: Al/Mg + n O2(g) → Al2O3 + MgO + lots of heat ! in water: Al/Mg + n H2O(l) → Al2O3 + MgO + n H2(g) ! …, i.e. Mg and Al burn underwater once ignited ! This reaction has a high activation energy, because the metal passivates itself with an oxide layer a few µm thick – a very common phenomenon with reactive metals. Aluminum forms an amalgam with mercury. Although mercury will not react with the aluminum oxide layer, even a small scratch exposing the metal aluminum can start the formation of the amalgam. For this reason, mercury is banned from transport on aircrafts. Thermite is a solid mixture of metal and metal oxide, often aluminum metal and iron oxide. The aluminum metal reduces the iron oxide in an extremely exothermic reaction used for railway welding. Fe2O3 + 2Al → 2Fe + Al2O3 + heat • Gallium has the largest liquid range of all elements. It melts at 29.76 °C and boils at 2204 °C. It is therefore used in high- temperature thermometers. • Thallium is extremely toxic causing liver and kidney failure, deafness and blindness. Tl2SO4 has been used as rat poison in the past. (See: thallium poisoning of ex-KGB spy Alexander Litvinenko.) 175 Amphoterism and Isopolyoxo Ions Amphoterism: Being able to do two opposite things at the same time. In chemistry: Acid/Base amphoterism, Redox amphoterism Definitions: Acidic oxide: Generates an acidic species upon dissolution in water: Based on these equilibria, what could be a potential effect of an increased CO2 concentration in atmosphere on the world's oceans? Basic oxide: Generates a basic species upon dissolution in water: … if you combine the last three reactions with this one CaCO3 + heat → CaO + CO2(g) … you can build castles, bridges, houses, etc. - why ? Amphoteric Oxide: Can react with both acids and bases, acting either as an acidic or basic oxide, depending on conditions: 176 • Amphoteric behaviour of oxides is primarily observed for BeO, Al2O3, Ga2O3, SnO2, and Sb2O3. • In general the elements on the borderline metallic/non-metallic show amphoterism: Q. Why do some elements form acidic oxides, some basic oxides and some amphoteric oxides? Element oxides and their reaction in water • For all elements except fluorine (why not?) and the noble gases (with the exception of xenon) the following reactions are possible: 1) x E + y/2 O2(g) → ExOy 2) ExOy + y H2O → E(OH)2y 3) E(OH)2y → E(O)(OH)2y-2 + H2O • But for which elements do the hydroxides react as bases and for which as acids? • Are there any trends and can we rationalized them? 177 Periodic trends in oxo-acids and oxo-bases: Metals Semi-metals Non-metals weak acid B(OH)3 NaOH Mg(OH)2 Al(OH)3 Si(OH)4 PO(OH)3 SO2(OH)2 ClO3(OH) Ga(OH)3 In(OH)3 Tl(OH)3 strong base weak base strong acid • The more electro-positive the oxygen binding partner, the more ionic the element-oxygen bond and the easier OH- ions are formed → basic reaction in water. • The more electronegative the oxygen binding partner, the more covalent the element-oxygen + bond and the easier H3O ions are formed → acidic reaction in water. • With increasing valence, the more OH functional groups can be accommodated at the central atom…however the tendency to eliminate water also increases. E(OH)n → E(O)(OH)n-1 + H2O This introduces E=O bonds, which due to their electron withdrawing effect increase the - + polarization of the Eδ -O-Hδ bond system further → acidic reaction in water. A. Correlates with the nature of the element oxygen bond (E-O): Basic oxides: Ionic bond Amphoteric oxides: Both, ionic and covalent contributions Acidic oxides: Covalent bond 178 • For the transition metals (cf.
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