DOUBLE SALTS OR LATTICE COMPOUNDS:

They are those additions or molecular compounds which looses their identity in aqueous medium i.e. when they dissolved into water, they completely dissociate into their constituents which are simple ions.

Double salts are those molecular compounds which exist only in crystal lattices and lose their identity when dissolved in water. These are formed by mixing two apparently saturated compounds. For example,

(1) When saturated solution of potassium chloride and magnesium chloride is evaporated, we get a new substance called Carnalite.

(2) When saturated solutions of potassium sulphate and aluminum sulphate are mixed and the solution is evaporated, we get the well known compound, Potash alum.

(3) When saturated solutions of ferrous sulphate and al Ammonium sulphate are mixed and the solution is evaporated, we get the well known compound, Mohr’s salt.

Shape and size of the crystals of a double salt are different from that of its component salts. As mentioned above, these compounds exist (are stable) in the solid state and as soon as the lattice is disrupted on dissolution in water or any other solvent or on melting, they decompose into their individual constituents. Thus their solutions will show the same physical and chemical properties as a mixture of solutions of their components. Thus, for example, aqueous solution of potash alum will give the tests of K⁺¹, Al⁺³ and SO₄ ^2- ions

ORGANOMETALLIC COMPOUNDS:

Compounds that contain at least one carbon-metal bond are called organometallic compounds. Zeisse, in 1830, prepared the first organometallic compound by the action of ethylene on a solution of potassium chloroplatinate (II). In the last four decades, enormous work has been done in this field any many fascinating compounds have been synthesised and investigated. Grignard reagent, R-Mg-X is a familiar example of organometallic compounds where R is an alkyl group. Diethyl zinc [Zn(C₂H₅)₂] , lead tetraethyl [Pb(C₂H₅)₄] , ferrocene [Fe(C₅H₅)₂] dibenzene chromium [CrC₆H₆)₂] metal carbonyls are other examples of organometallic compounds. The compounds of metalloids such as germanium and antimony and non-metallic elements such as boron and silicon are also included under this classification.

Organometallic compounds may be classified in three classes :

(1) Sigma -bonded complexes,

(2) Pi- bonded complexes,

(3) Sigma and Pi-bonded (Both) Complexes

Sigma and Pi-bonded Organometallic compounds:

Metal carbonyls, compounds formed between metal and carbon monoxide belong to this class. These compounds possess both Sigma and Pi-bonding. The oxidation state of metal atoms in these compounds is zero. Carbonyls may be monomeric, bridged or polynuclear. Carbonyls are mainly formed by the transition metals of VIth, VIIth and VIIIth groups.

Some well known examples are:

In a metal carbonyl, the metal-carbon bond possesses both the Sigma and Pi- character. A sigma bond between metal and carbon atom is formed when a vacant hybrid bond of the metal atom overlaps with an orbital on C atom of carbon monoxide containing a lone pair of electrons.

Formation of Pi-bond is caused when a filled orbital of the metal atom overlaps with a vacant antibonding (Pi-star) orbital of C atom of carbon monoxide. This overlap is also called back donation of electrons by metal atom to carbon. It has been shown below:

The Pi-overlap is perpendicular to the nodal plane of sigma-bond.

Pi-bonded Organometallic compounds :

These are the compounds of metals with alkenes, alkynes, benzene and other ring compounds. In these complexes, the metal and ligand form a bond that involves the Pi electrons of the ligand. Three common examples are Zeise’s salt, ferocene and dibenzene chromium. These are shown here:

Sigma Bonded Complexes:

In these complexes, the metal atom and carbon atom of the ligand are joined together with a sigma bond, i.e., the ligand contributes one electron and is, therefore, called one electron donor. Examples are :

(1)  Grignard reagent, R-Mg-X where R is an alkyl or aryl group and  X is a halogen.

(2)  Zinc compounds of the formula R₂Zn such as (C₂H₅)₂Zn. This was first isolated by Frankland in 1849. Other similar compounds are Sn(CH₃)_4, Pb(C₂H₅)₄, Al₂(C₂H₅)₆  Al₂(CH₃)₆  , Pb(CH₃)₄ etc.

      Al₂(CH₃)₆ is a dimeric compound and has a structure similar to diborane, B₂H₆ It is an electron deficient compound and two methyl groups act as bridges between two aluminium atoms.