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.