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Group 13 and 14 • Most dramatic variation in a period from top to bottom (non-metals to metals) – Oxidation states +3 and +4 – Metals +1,+2 • Light members B,C, Si are important cationic nonmetals • Typically oxophiles and fluorophiles – borates, silicates, alumina, carbonates • Readily form alkyl and hydrogen covalent bonds – boranes, silanes, hydrocarbons Group 14: Carbon • Carbon has vast organic chemistry – Forms bonds with H and electronegative elements O, S, N, Halogens • Lack of C-O extended structures • Carbon cluster chemistry – Fullerenes – Nanotubes • Carbide chemistry 1 C-OH Condensation • B, Si, Al and many metallic hydroxides condense readily to form extended MO-M polymers and networks – Siloxanes, alumina, borates • One explanation: – Condensation typically requires a coordination+1 intermediate • protonated or aquated – i.e. Si(OH)4 forms Si(OH)4(H2O) to form SiO-Si bonds Carbon Cluster Chemistry • Relies on C-C covalent bond formation • Simple examples: network structures graphite and diamond • Fullerenes: C2n ranging from n=30-48 – Exciting (overhyped) new allotrope of C – Relatively easily synthesized – C60 I point group • E, 12 C5, 20 C3, 15 C2 2 Reactivity: Halogens • C60 reacts according to localized double and single bonds, I.e. it isn’t aromatic Organometallic Fullerenes • Endohedral Fullerenes – Caged metal ions Mx@Cn • Synthesized by laser ablation of C/M surfaces • M includes Sc, Y, La typically in C80, C82, C84 • Fullerene Ligands – Most transition metals have been bonded to fullerene in varying fashion, – typically as a η2 ligand I.e. similar to double bond not aromatic ring 3 Pd(η2 –C60)(PPh3)2 Nanotubes: the Useful Fullerenes • Discovered shortly after fullerenes • Structuraly can be thought of fullerenes with excess C6 rings stretching out the sphere in one dimension • Very high aspect ratio materials 4 Applications • • • • Nanowires Nanocomposites Nanoswitches Nanocatalysts • Macro Problem: – too stable, won’t form bonds with matrices – Requires reflux in conc. HNO3/H2SO4 to “functionalize” nanotube surfaces SWNTs and MWNTs • “crude” synthetic methods result diverse products mostly multi-wall nanotubes 5 Amorphous Carbons • Non Crystalline Carbon – carbon black – activated carbon – carbon fibers – charcoal – soot • All are mixtures of graphite and fullerenes, properties almost completely determined by surface area, impurities and oxidation Carbides: C anion chemistry • Compounds of C with electropositive elements • Three types – Saline Carbides: Ionic Carbides • Grps 1,2, and Al – Metallic Carbides • Carbides with the transition metals, most common – Metalloid Carbides • Carbides with B and Si, exceedingly hard materials 6 Saline Carbides: Ionic • Three types – Intercalation: graphite with Grp 1 metal vapor – Dicarbides: contain the C22aceylide anion in the holes in a metal ion lattice: CaC2 – Methides: very rare, C4anions from Na, Be, Al carbides • Na4C, Be2C, Al4C3 Intercalated Graphites • Two types of intercalated graphite – Buckled layers – pi system destroyed • Colourless non-conductor • Carbon monofluoride (CF)n : lubricious material – Retained pi systems • • • • Coloured, electrical conductors Properties vary with intercalant Alkali metals with varying degrees of inclusion Better conductors than graphite itself Æ addition of electrons to the pi system • KC8 Æ KC24 Æ KC36 Æ KC48Æ KC60 7 Metallic Carbides • Formed by combination of C with the transition metals • Formally are interstitial alloys of C anions in the octahedral holes of metal crystal structure • However, some evidence for directional bonding: i.e. the hard sphere model of ionic lattices doesn’t always apply Metalloid Carbides: Si, B • Small electronegativity differences give covalent C-Si or C-B bonding • Silicon carbide: SiC carborundum • Boron carbide: B4C • extremely hard, inert materials, v. high melting • Industrial uses, particularly SiC – Made by high temperature reduction of Si or B oxides in the presence of graphite 8 Grp 14: Silicon • “If we weren’t made out of C, we would be made of Si” • Some very similar behaviour to C • Significant differences – Catenation vs Condensation Bond C-C Si-Si S-S Energy 356 226 226 Bond C-O Si-O S-O Energy 336 368 330 Bond Stability vs Polarity • Note: bond stability is not the last word in reactivity, – depends on reaction mechanism – S-Cl (391 kJ mol-1) vs Si-C (275 kJ mol-1) – Charge separation is important, – Si-Cl is very polar, thus Si undergoes nucleophilic attack by OH- to form Si-OH – Si-C is stable to nucleophilic attack 9 Silicates • Useful, very common, stable • Based on tetrahedral Si: SiO4 units, attached at vertices to other SiO4 units Si HO OH OH OH OH Si O Si O OH OH OH SiO4 Structures • SiO44-, Si2O72-, Si6O1812- 10 Cages as Well Silicate Glasses: • amorphous structure of SiO42- units, a random assembly of units • Random assembly: – Coordination of 3 or 4 for non O elements (eg. B, Al, Si) – Only one O atom shared between any two non O atoms • Inclusion or Al, B, alkali metals alters the degree of crystallization in the glass, alters physical and chemical characteristics 11 Siloxanes (silicones) • Formed by the condensation of (R)xSi(OH)4-x species • Result is Si-O-Si bonds with other Si-R functionality present HO Si O CH3 Si CH3 CH3 CH3 CH3 O CH3 Si O CH3 Si O CH3 CH3 n Aluminosilicates • Silicates which contain aluminum ions are aluminosilicates • Al may combine with silicate matrix in two ways – Occupying vacant Oh holes – Occupying Td holes, displacing Si atom • Al3+ and Si4+ are similar in size so replacement is common • Requires charge compensation, one additional alkali cation 12 Layered Aluminosilicates • Common minerals – talc, clay, mica • Individual layers are charge neutral, weakly bound to each other • Minerals exhibit cleavage planes and water storage Molecular Sieves & Zeolites • Aluminosilicate crystalline materials with cavities on a molecular scale • Na12[(AlO2)12(SiO2)12]-xH2O A Type 13 Group 14: Carbon and Silicon • Carbon – catenation – cluster chemistry • fullerenes and tubes, reactivity, uses – carbides • saline, metallic, metalloid • Silicon – reactivity, condensation vs catenation – silicates, siloxanes, aluminosilicates 14