Chemistry 242 - Inorganic Chemistry II
Chapter 15 - Silicon, Germanium, Tin and Lead
- Silicon is th most abundant element after oxygen in the earth's crust occuring mostly as the silicates and quartz (SiO2).
- The other elements are acutally quite rare (~10-3%) but tin an dlead are found in concetrated pockets of their ores (cassiterite, SnO2 and galena, PbS2) from which they are easily obtained, so they have been known and used since antiquity.
- The existence of germanium ('ekasilicon") was predicted by Mendleev when he constructed the first reasobably complete periodic table. It was isolated in 1886 from coal and zinc ores.
- The main uses of tin an dlead are as the metals and as synthetic reagents as their organo compounds. Silicon and germanium are the basis of the semicondutor industry.
Because of the great versatility of carbon, it is interesting to compare the other members of group 14.
- Carbon dioxide is molecular gas whereas silicon dioxide is a network solid (in all its forms several
- Dehydration of alcohols, ROH yields both alkenes and ROR depending on the conditions, but dehydrating R3SiOH yields only R3SiOSiR3.
- Multiple pp - pp bonding between silicon atoms and other first row elements was thought not to exist until the '60's when the first Si=C bonds were observed as transient species, and then isolated in heavily sterically hindered compounds:
Replacement of the hydrogen with tertiary butyl groups allows the isolation of similar compounds of silicon, germanium and tin.
- Silicon can use empty d-orbitals for pp - dp bonding. Examples are planar (H3Si)3N vs pyramidal (H3C)3N and linear H3SiNCO vs H3CNCO which is bent at the nitrogen. In both cases the nitrogen lone pair is delocalized by p-bonding with the silicon(s).
In CR3OCR3 the Si-O-Si angle is around 109 o whereas in R3SiOSiR3 the angles range from 140 to 180 o indicative of Si=O dp - pp bonding.
(CH3)3COH is a very weak acid but (CH3)3SiOH is stronger because the ion (CH3)3SiO- is stabilized by dp - pp bonding.
See Table 15-1 for summary information about the tetravalent state:
See table 15-2 for summary information about divalent states.
- All show tetrahedral coordination.
- Five coordinate complexes e.g. MX5- or MnX5-n- can be trigonal bipyramids or rarely square pyramidal if a constraining chelating ligand is used, e.g.[XSi(O2C2H4)2]-.
- 6-coordinatate complexes are normally octahedral.
- Very often, but not always, the lone pair is stereochemically active and influences the molcular shape. F2Pb is bent and so is SiCl3- .
Isolation and Properties of the Elements
- Silicon and germanium can be made by reduction of their dioxides by carbon or calcium carbide in an electric furnace, and then purified further by zone-refining.
- Tin and Lead are obtained by carbon reduction of their oxides or sulphides. I ffuther purification is necessary they can be dissolved in acid and redeposited electrolytically.
- Silicon and germanium are relatively inert but the following reactions occur:
Si + 2X2 SiX4 (X2 is a halogen)
Si + excess OH- silicates
Si + excess HX no reaction except HF will give SiF62-
Germanium is somewhat more reactive and will dissolve in sulphuric or nitric acids.
Tin and lead dissolve in several acids, hot alkalis and also react with halogens.
Compounds MH4 all exist as spontaneously flammable gases and are not very important.
Compounds MCl4 are all colourless liquids except PbCl4 which is yellow. They are hydrolysed easily to give hydrous oxides. In hydrochloric acid the lower members, tin and lead, give MCl62- ions in aqueous solution. The compounds are intermediates in the synthesis of organo compounds for example the infamous tetraethyl lead.
- Silica comes on three forms: quartz and crystobalite which are both crystalline, and silica glass. The glass has a very low coefficient of expansion and high melting point so it is relatively resistant to heat and sudden temperature changes. It is also transparent to a large part of the ultraviolet spectrum and therefore used in cells for spectrophotometry in that region.
- There are several important oxides of lead: PbO exists in a red from, litharge, and a yellow form, massicot. It is the most used Pb source of lead for synthesis. Pb2O3 which behaves like a mixture of PbO and PbO2 although it is a well defined structure is called "red lead". It is used as an anti-rust coating for steel. Lead dioxide, PbO2 is maroon in colour and has a structure similar to rutile (TiO2). It is one of the electrode materials in lead/acid batteries.
- The oxides vary from acidic for SiO2 to basic for tin and lead.
Silicon forms a very stable fluoroanion:
SiO2(s) + 6HF(aq) 2H+(aq) + SiF62-(aq) + 2H2O
The same anion is formed by the incomplete hydrolysis of SiF4:
SF4 + 2H2O SiO2 + SiF62- + 2H+ + 2HF
The other MF62- ions are hydrolysed by bases or even water in the case of the lead
complex ion. All the elements give analogous chloroanions, except silicon. The other fairly important anionic complex is obtained with oxalate, [M(ox)3]2-.
Cationic Complexes and Neutral Adducts
Can be formed with chelating uninegative oxygen donor ligands, e.g. [Pb(acac)3]+.
The MX4 compounds are Lewis acids and can form adducts which are sometimes neutral MX4L or MX4L2 molecular compounds but can also be ionic [MX2L2]X2.
Alkoxides, Carboxylates and Oxo Salts
Typical preparative routes are, for alkoxides:
MCl4 + 4ROH + 4(Et)3N M(OR)4 + 4(Et)3NHCl
Note the use of the triethylamine to "remove" the HCl which would be formed in its absence.
Carboxylates can be made by direct reaction:
Pb3O4 + 8CH3COOH Pb(CH3COO)4 + 2Pb(CH3COO)2 + 8H2O
Lead tetraacetate is used as an oxidizing agent in certain organic reactions.
Tin and Lead (IV) salts are hydrated e.g Pb(SO4)2.2H2O and subject to extensive hydrolysis in aqueous solution.
The Divalent State
- This oxidation state becomes more and more stab;le down the group.
- Silicon dihalides are only transient species. GeF2 and GeCl2 can be isolated.
- Tin II fluoride and chloride are well known and useful. (SnF2 is the active ingredient in many fluoridated toothpastes.) In solution, tin II is easily oxidized by air. The sulphates and nitrates are heavily hydrolysed: Sn3(OH)4(NO3)2 and Sn3(OH)2SO4.
- Lead II is the best defined divalent state. Most lead II salts are not very water soluble. The exception are the nitrate and the acetate. The solid halides are always anyhdrous.
Silenes and Other Organic Compounds
It is possible to make some compounds containing a Si=Si or Ge=Ge double bonds.
The earliest attempts tried the reaction:
2R2SiCl2 + Na/K in THF R2Si=SiR2 + 4K/NaCl
Without the bulky R groups cyclic polymers are generated typically with 6 silicons in a ring. By using sufficiently bulky organic groups, it was possible to prepare transient dimers, R2Si=SiR2, or monomers R2Si.
The first silene stable enough to isolate was prepared by the photochemical decomposition of (mes)2Si(SiMe3)2:
2(mes)2Si(SiMe3)2 (mes)2Si=Si(mes)2 + Me3Si-SiMe3
Unlike the carbon analogs, the molecule is not perfectly flat. The Si=Si double bond is 9% shorter than a normal Si-Si single bond, c.f. 13% in an olefin.