P.A. Cox - Inorganic chemistry (793955), страница 37
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Metal-carbon bonds occur inorganometallic compounds.Introduction to nonmetalsGeochemistry (J2)(F1)Organometallic compounds(H10)The elementsWith the valence electron configuration s2p2 the nonmetallic elements of group 14 can form compounds with fourtetrahedrally directed covalent bonds.
Only carbon forms strong multiple bonds, and its compounds show manydifferences in structure and properties from those of Si and Ge. Like the metallic elements of the group (Sn and Pb),germanium has some stable divalent compounds.The abundances of the elements by mass in the crust are: C about 480 p.p.m., Si 27% (second only to oxygen), andGe 2 p.p.m. Carbon is present as carbonate minerals and in smaller amounts as the element and in hydrocarbondeposits. It is important in the atmosphere (as the greenhouse gas CO2; see Topic J6) and is the major element of life.Silicate minerals are the dominant chemical compounds of the crust and of the underlying mantle (see Topic J2).Germanium is widely but thinly distributed in silicate and sulfide minerals.F4—CARBON, SILICON AND GERMANIUM161All three elements can crystallize in the tetrahedrally bonded diamond structure (see Topic D2). Si and Ge aresemiconductors (see Topic D7).
Carbon has other allotropes. Graphite is the thermodynamically stable form atordinary pressures, diamond at high pressures. More recently discovered forms include buckminsterfullerene C60,higher fullerenes such as C70, and nanotubes composed of graphite sheets rolled into cylinders. In these structurescarbon forms three σ bonds, the remaining valence electron being in delocalized π orbitals analogous to those inbenzene (see Topic C7).The elements can be produced by reduction of oxides or halides. Highly divided carbon black is used as a catalyst andblack pigment, and impure carbon (coke) for reducing some metal oxides (e.g. in the manufacture of iron; seeTopic B4). Pure silicon prepared by reduction of SiCl4 with Mg is used in electronics (‘silicon chips’) although muchlarger quantities of impure Si are used in steels.Hydrides and organic compoundsCompounds of carbon with hydrogen and other elements form the vast area of organic chemistry.
Silanes andgermanes are Si and Ge analogs of methane and short-chain saturated hydrocarbons, and can be prepared by variousmethods, such as reduction of halides with LiAlH4:They are much more reactive than corresponding carbon compounds and will inflame spontaneously in air. Stabilitydecreases with chain length in series such asMany derivatives can be made where H is replaced by monofunctional groups such as halide, alkyl, −NH2. Many Si andGe compounds are similar in structure to those of carbon, but trisilylamine (SiH3)3N and its germanium analog differfrom (CH3)3N in being nonbasic and having a geometry that is planar rather than pyramidal about N. This suggests theinvolvement of the N lone-pair electrons in partial multiple bonding through the valence expansion of Si or Ge (seeTopic C2, Structure 8).Si and Ge analogs of compounds where carbon forms double bonds are much harder to make.
(CH3)2SiO is not likepropanone (CH3)3C=O, but forms silicone polymers with rings or chains having single Si-O bonds (1). Attempts tomake alkene analogs R2Si=SiR2 (where R is an organic group) generally result in single-bonded oligomers, except withvery bulky R− groups such as mesityl (2,4,6(CH3)3C6H2−), which prevent polymerization.HalidesAll halides EX4 form tetrahedral molecules (point group Td). Mixed halides are known, as well as fully or partiallyhalogen-substituted catenated alkanes, silanes and germanes (e.g.
Ge2Cl6). Unlike the carbon compounds, halides of Siand Ge are Lewis acids and readily form complexes such as [SiF6]2−. Attack by Lewis bases often leads todecomposition, and thus rapid hydrolysis in water, unlike carbon halides, which are kinetically more inert.162SECTION F—CHEMISTRY OF NONMETALSDivalent halides EX2 can be made as reactive gas-phase species, but only for Ge are stable noncatenated GeIIcompounds formed. They have polymeric structures with pyramidal coordination as with SnII (see Topic G6).
Thecompound CF formed by reaction of fluorine and graphite has one F atom bonded to every C, thus disrupting the πbonding in the graphite layer but retaining the σ bonds and giving tetrahedral geometry about carbon. (Bromine formsintercalation compounds with graphite; see Topic D5.)Oxygen compoundsWhereas carbon forms the molecular oxides CO and CO2 with multiple bonding (see Topics C1 and C5), stable oxidesof Si and Ge are polymeric. Silica SiO2 has many structural forms based on networks of corner-sharing SiO4 tetrahedra(see Topic D3).
GeO2 can crystallize in silica-like structures as well as the rutile structure with six-coordinate Ge. Thisstructure is stable for SiO2 only at very high pressures, the difference being attributable to the greater size of Ge.Thermodynamically unstable solids SiO and GeO can be made but readily disproportionate to the ioxide.CO2 is fairly soluble in water but true carbonic acid is present in only low concentration:The apparent Ka given by the product of these two equilibria is 4.5×10−7 (pKa= 6.3), much smaller than the true valuefor carbonic acid, which is more nearly in accordance with Pauling’s rules (pKa=3.6; see Topic E2).
The hydration ofCO2 and the reverse reaction are slow, and in biological systems are catalyzed by the zinc-containing enzyme carbonicanhydrase (see Topic J3).SiO2 and especially GeO2 are less soluble in water than is CO2, although solubility of SiO2 increases at hightemperatures and pressures. Silicic acid is a complex mixture of polymeric forms and only under very diluteconditions is the monomer Si(OH)4 formed.
SiO2 reacts with aqueous HF to give [SiF6]2−.The structural chemistry of carbonates, silicates and germanates shows parallels with the different oxide structures. Allcarbonates (e.g. CaCO3) have discrete planaranions (see Topic C1, Structure 11). Silicate structures arebased on tetrahedral SiO4 groups, which can be isolated units as in Mg2SiO4, but often polymerize by Si—O—Sicorner-sharing links to give rings, chains, sheets and 3D frameworks (see Topics D3, D5 and J2).
Many germanates arestructurally similar to silicates, but germanium more readily adopts six-coordinate structures.Other compoundsCarbon disulfide CS2 has similar bonding to CO2, but SiS2 differs from silica in having a chain structure based onedge-sharing tetrahedra, and GeS2 adopts the CdI2 layer structure with octahedral Ge (see Topic D3).Nitrogen compounds include the toxic species cyanogen (CN)2 (2) and the cyanide ion CN−, which formsstrong complexes with many transition metals (see Topics H2 and H6). Si3N4 and Si2N2O are polymeric compoundswith single Si—N bonds, both forming refractory, hard and chemically resistant solids of interest in engineeringapplications.Compounds with metals show a great diversity. A few carbides and silicides of electropositive metals, such as Al3C4and Ca2Si, could be formulated with C4− and Si4− ions although the bonding is certainly not very ionic.
Compounds withtransition metals are metallic in character, those of Si and Ge being normally regarded as intermetallic compounds,F4—CARBON, SILICON AND GERMANIUM163those of carbon as interstitial compounds with small carbon atoms occupying holes in the metal lattice. Some such asTaC and WC are remarkably hard, high melting and chemically unreactive, and are used in cutting tools. Fe3C occurs insteel and contributes to the mechanical hardness.Many compounds with E-E bonding are known (see Topic D5). CaC2 has C22− ions (isoelectronic with N2) and reactswith water to give ethyne C2H2. On the other hand, KSi and CaSi2 are Zintl compounds with single-bondedstructures.
Ge (like Sn and Pb) forms some polyanions such as [Ge9]4− (see Topics C7 and G6).Organometallic compounds containing metal-carbon bonds are formed by nearly all metals, and are discussedunder the relevant elements (see especially transition metals, Topic H10). Some analogous Si and Ge compounds areknown.Section F—Chemistry of nonmetalsF5NITROGENKey NotesThe elementAmmonia andderivativesOxygen compoundsOther compoundsRelated topicsNitrogen has a strong tendency to form multiple bonds. Dinitrogen isa major constituent of the atmosphere.
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