Organometallic Chemistry

1. ORGANOMETALLIC
CHEMISTRY
UNIT – 2 REACTION MECHANISM OF ORGANOMETALLIC
COMPOUNDS

2. Oxidative addition
 We now look at a general method for simultaneously introducing pairs of anionic
ligands,A and B, by the oxidative addition of an A−B molecule such as H2 or CH3‐I.
 In the oxidative addition direction, we break the A−B bond and form an M−A and an
M−B bond.

3.  The oxidation state (OS), electron count (EC), and coordination number (CN) all increase
by two units during the reaction.
 Oxidative additions proceed by a great variety of mechanisms , however , a vacant 2e site
is always requiredon the metal.
 can either start with a 16e complex or a 2e site must be opened up in an 18e complex by
the loss of a ligandproducing a 16e intermediatespecies.
 The change in oxidation state means that the starting metal complex of a given oxidation
state must also have a stable oxidation state two units higher to undergo oxidative
addition.

4. For example
There are many examples, however, one of the most‐studied cases is the addition of
H2 to the 16e square planar d8 species IrCl(CO)(PPh3)2 to give the 18e d6 octahedral
dihydrideIrCl(H2)(CO)(PPh3)2 .

5. Reductive elimination
 Reductive elimination is an elementary step in organometallic chemistry in which the
oxidation state of the metal center decreases while forming a new covalent bond
between two ligands.
 It is the microscopic reverse of oxidative reaction leads to the extrusion of A−B from an
M(A)(B)complex and is often the product‐forming step in a catalytic reaction.

6. Insertion & Elimination reaction
An insertion reaction is a chemical reaction where one chemical entity interposes itself
into an existing bond of typically a secondchemical entity e.g.: A + B–C → B–A–C.

7. Nucleophilic and electrophilic attack of ligands
Nucleophilic:
-Electron rich species(metal have a +ive chargeand ligand have a electron
withdrawing groups
Electrophilic:
- Electron deficient species(metal have -ive charge,ligand have electron
deficient species

8. Metallocenes
A metallocene is a compound typically consisting of two cyclopentadienyl
anions (C 5H −
5 , abbreviated Cp) bound to a metal center (M) in the oxidation state II, with
the resulting general formula (C5H5)2M.

9. Synthesis
 Using a metal salt and cyclopentadienyl reagents
Sodium cyclopentadienide (NaCp) is the preferred reagent for these types of
reactions. It is most easily obtained by the reaction of molten sodium and
dicyclopentadiene.
MCl2 + 2 NaC5H5 → (C5H5)2M + 2 NaCl (M = V, Cr, Mn, Fe, Co; solvent = THF, DME, NH3)
CrCl3 + 3 NaC5H5 → [(C5H5)2Cr] + 1⁄2 "C10H10" + 3 NaCl
NaCp acts as a reducing agent and a ligand in the reaction.

10.  Using a metal and cyclopentadiene
This technique provides using metal atoms in the gas phase rather than the solid
metal. The highly reactive atoms or molecules are generated at a high temperature
under vacuum and brought together with chosen reactants on a cold surface.
M + C5H6 → MC5H5 + 1⁄2 H2 (M= Li, Na, K)
M + 2 C5H6 → [(C5H5)2M] + H2 (M = Mg, Fe)

11. Properties
 Metallocenes generallyhave high thermal stability.
 Ferrocene can be sublimed in air at over 100 °C with no decomposition.
 Metallocenes are generally purified in the laboratory by vacuum
sublimation.

12. Uses
Polyethylene produced using a metallocene can be used as a very thin film which
has excellent optical properties and sealing performance, thus making it very
effective for wrapping foods.

13. The compound Fe(ɳ5-C5H5)2, bis (cyclopentadienyl)iron, popularly known as
ferrocene was discoveredin 1951. It has famous sandwich structure.
Ferrocene

14. Preparation
By treating iron (II) chloride with Grignard reagent
2(C5H5)-MgBr+ FeCl → (C5H5)-Fe-(C5H5) + MgBr2 + MgCl2
By treating iron halide with sodium cyclopentadienide(in THF)
C5H6 + Na→ C5H5Na + ½ H2
2C5H5Na + FeCl → (C5H5)-Fe-(C5H5) + 2NaCl
By reaction of iron halide with cyclopentadiene in presence of strong base
2C5H6 + FeCl2 + 2(C5H5)2NH → Fe (C5H5)2 + 2(C5H5)2NH2Cl

15. properties
 Ferrocene is an air -stable orange solid with a camphor-likeodor.
 As expected for a symmetric, uncharged species, ferrocene is soluble
in normal organic solvents, such as benzene,but is insoluble in water.
 It is stable to temperatures as high as 400 °C.

16. Structure of ferrocene
The staggered configuration is due to crystal packingforces so that carbon- carbon and
hydrogen- hydrogen repulsions between the two rings are minimum.