Chemistry 242 - Inorganic Chemistry II
Chapter 15 - Silicon, Germanium, Tin and Lead

Introduction

Multiple Bonding

Because of the great versatility of carbon, it is interesting to compare the other members of group 14.

Stereochemistry

See Table 15-1 for summary information about the tetravalent state: See table 15-2 for summary information about divalent states.

Isolation and Properties of the Elements

Hydrides

Compounds MH4 all exist as spontaneously flammable gases and are not very important.

Chlorides

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.

Oxygen Compounds

Complex Compounds

Anionic Complexes

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

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.