Oxidative addition is a process where a metal complex increases its oxidation state and coordination number by addition of two ligands. It is the reverse of reductive elimination. It requires the metal to have available orbitals and be in a lower oxidation state. There are four mechanisms for oxidative addition: concerted, SN2, radical, and ionic. Oxidative addition and reductive elimination are important steps in many catalytic cycles in organometallic chemistry and homogeneous catalysis.

1. Presented By-
Swapnali Borah
M.Sc 4th Semester
Roll No. 17

2. 1. Concerted, or three‐center, oxidative addition mechanism
2. SN2 mechanism
3. Radical mechanisms
4. Ionic Mechanisms
Introduction
Conditions for oxidative addition
Characteristics of oxidative addition
Examples
Mechanisms-
Applications
Conclusion
References

3. IT IS A PROCESS BY WHICH A METAL CENTRE IN A COORDINATIVELY UNSATURATED
COMPLEX INCREASES ITS OXIDATION NUMBER AS WELL AS COORDINATION NUMBER.
Ln M + X Y Ln M
X
Y
16 e- 18 e-
O.S. = + 2
C.N. = +2
THE REVERSE OF THE OXIDATIVE ADDITION IS CALLED REDUCTIVE ELIMINATION.
OXIDATIVE
ADDITION
REDUCTIVE
ELIMINATION

4. • Metal must possess non-bonding electron pair.
• The complex must be co-ordinatively
unsaturated.
• Suitable orbitals should be available.
• The higher oxidation state of the metal should
be energetically accessible and stable.

5. • Two new anionic ligands are added.
• Coordination number increases by two units.
• Oxidation number increases by two units.
• The oxidation state of the metal is low.
• The more electron rich the metal, the more
the addition becomes easier.
• The substrate behaves both as a Lewis acid
and a Lewis base.

6.  Reaction of Vaskas complex with molecular
hydrogen

7. • Oxidative addition without cleavage of the
X-Y bond.

8. X-X
H2, Cl2, Br2, I2
C-C
Ph3C-CPh3, MeC(CN)3
H-X
HCl, HBr, HI, HCN
C-X
CH3I, CCl4, C6H5I
M-X
HgCl2, R3SiCl, Ph3PAuCl
ATOMS SEPARATE ATOMS REMAIN ATTACHED
O2,
SO2,
CF2=CF2
RNCS, RNCO
CS2
RN=C=NR’
Table : Substances that can be added oxidatively.

9. OctahedralOctahedral
OctahedralpyramidalSq
OctahedralTBP
OctahedralplanarSq
planarSqlTetrahedra
planarSqLinear
XL
X
XL
X
XL
X
22
2.2
.
.
.
2
2
2
2
2
2
,2
,
,2
 

 

 








10. MECHANISM OF OXIDATIVE ADDITION
 There are four mechanisms :
o Concerted or three-centered
o SN2
o Radical
o Ionic
1. CONCERTED MECHANISM :
 Follows when X-Y is non-polar.
 Proceeds through two steps-
a. The associative step which involves formation of a Ϭ- complex.
b. Oxidative part of the reaction in which metal electrons are formally
transferred to the σ* orbital of A−B .
16e- ,M(0) 18e- , M(0) 18e-, M(II)
a b

11. 2. SN2 Mechanism
• Often found in addition of polar ligands like methyl,
allyl, benzyl halides.
• Proceed via a polar transition state.
• Accelerated in polar solvents.
16e- , Ir(I) 16e- , Ir(III)
16e- , Ir(III)

12. 3. Radical mechanism
 Two types are there :
1.Non- chain radical mechanism.
• Addition of certain alkyl halides to
proceeds through this mechanism.
2.Radical chain mechanism
• A radical initiator is required to keep the
process going.
PtL3
fast
PtL2
PtL2 + RX
slow
PtXL2 + R
PtXL2 + R
fast
RPtXL2
 33ht 
.
..
Pt(II)
Pt(0)
.

13. Ir(I) Ir(II) Ir(III)

14. A. the one in which the starting metal complex adds to
H prior to the addition of the halide X .
B. the other type, in which the halide anion X− adds to
the starting metal complex first, and then the
addition of proton H+ occurs on the metal complex.
4. IONIC MECHANISM
This mechanism for the oxidative addition reaction is
common to the addition of hydrogen halides
(HX) in its dissociated H+ and X− forms.
There are two pathways-

15.    233343 )()()( 33
PRHPtClPRHPtClHPRPt PRPR
   
     
  222 )()()( ClLcodHIr
fast
LcodIrClClHLcodIr slow
A. When cationic part attach first to the metal centre.
• The metal should be electron efficient .
• Ligands should have Ϭ-donating capacity.
Pt(0) , 18e- Pt(II) , 16e- Pt(II) , 18e-
Tetrahedral Sq. Planar Sq. Planar
B. When attack of anion takes place first .
The metal centre should be electron deficient .
Complexes with ∏-acceptor ligand having +ve
charge on it would follow this pathway .
Ir(I) , 16e- Ir(I) , 18e- Ir(III) , 18e-
Sq. planar TBP Octahedral

16.  An important step in many homogeneous catalytic cycles.
• Hydrogenation of alkenes-Wilkinson catalyst
• Pd catalyzed Cross coupling of Ar-B(OH)2and Ar-X –Suzuki
Coupling
R3P Pd PR3
Br
Pd
Br
Ph3P PPh3
Pd0
Pd2+

17.  Thus, both O. A. and R. E. are important processes in
organometallics and catalytic chemistry of transition metal
complexes.
 O. A. is more prominent in case of metal complexes with
low oxidation state. Reverse case applies in R.E.
 O.A. is accompanied by increase in oxidation number and
coordination number
 Oxidation number, coordination number of the metal
increases in O.A. ,vice versa in R.E.
 In both the processes, stereochemical aspects also holds
importance.

18.  Advanced Inorganic Chemistry,13th Edition, Vol ||, Gurdeep Raj, 837-844, 2012,
KRISHNA Prakashan Media Pvt. Ltd.
 Inorganic Chemistry, 3rd Edition,Gary L. Miessler, Donald A. Tarr, 534-536, 2015,
Pearson India Education Services Pvt. Ltd.
 Inorganic Chemistry, 4th Edition, James E. Huheey, Ellen A. Keiter, Richard L.
Keiter, Okhil K. Medhi, 637-642, 2013, Dorling Kindersley ( India ) Pvt. Ltd.
 Advanced Inorganic Chemistry, 3rd Edition, F. A. Cotton, G. Wilkinson, 772-774,
1972, John Wiley & Sons, Inc.
 Michael S. Driver, John F. Hartwig, Organometallics, vol. 17, Issue 6, 1134-1143,
1998.
 Colin Eaborn, Kalipada Kundu, Alan Pidcock, Journal of Chemical Society, 5, 1223-
1232, 1981.
 https://doi.org/10.1021%2Facs.organomet.5b00565