THE OXIDATIVE ADDITION REACTIONS
An oxidative addition reaction is one in which a neutral ligand adds to a metal center and in doing so
the
metal is oxidizes, typically by the loss of 2e-. The transferring of the two electrons from the metal to the
incoming ligand, breaks a bond in that ligand forming two new anionic ligands. At least one of these new
anionic ligands attached to the metal center.
two new anionic
hydride ligands
H
OC
Ph3P
Ir
+H2
PPh3
Cl
OC
H
Ir
Ph3P
H
PPh3
Cl
OC
Ph3P
Ir
PPh3
H
Cl
Ir(+3)
18e-
Ir(+1)
18e-
Ir(+1)
16e-
When a complex behaves simultaneously as Lewis acid (usual coordinate bond) and Lewis base (back bonding ),
the oxidative addition reaction can be occur in this system. The reaction can be written as
Ln M n
+
⇄
XY
Ln (X)(Y) (M n+2)
For an oxidative addition reaction to proceed there must be a non bonding electron density on the metal M, and two
vacant site on the complex Ln M to allow formation of two new bonds to X and Y with the metal M with its
oxidation state separated by two units. Many reactions of the compounds even of non metals, not usually thought of
as oxidative addition reactions, may be also designated as oxidative addition reactions. The most intensively studied
oxidative addition reactions of the transition metal are complexes of the metals are those of complexes of metals
with d8, and d10, electronic configuration, notably Fe0, Ru0, Os0 ,Rh 1, Ir 1, Ni 0, Pd 0, Pt 0, and Pt2. An
exceptionally well studied complex is the square planar trans Ir (I) Cl (CO) (PPh3)2 which undergoes reaction as
follows.
(CH 3)2 S
PF3
SnCl 2
+
+
+
Trans Ir (I)(Cl) (CO) (P Ph3)2
It may be noted that
I2
(CH 3)2 S I2
F2
PF5
CI 2
SnCl 4
+ HCl
Trans Ir (III) (Cl)2(CO) (H) (P Ph3)2
i) The coordinative unsaturated square complex of Rh1, Ir1 or Pt 2, with sixteen electrons on oxidative addition
reaction produces eighteen electron system. The reverse of such reaction (reductive eliminations) would reduce an
eighteen electron system to a sixteen electron system.
ii) In additions of the molecules such as H2, HCl, or Cl2, two new bonds are formed with the metal atom or ions with
the breakage of H-H or HCl or Cl-Cl bonds. However molecules that contain multiple bonds may be added to the
metal atom or ion with out cleavage of all the bonds present in the multiple bond-containing reagents.
The molecules which undergo the oxidative addition reactions are classified in to three categories
they are: Non-electrophillic, Non-electrophillic “intact”, and Electrophillic.
Non-electrophillic:- These molecules do not contain electronegative atoms and/or are not good oxidizing
agents. They are often considered to be “non-reactive” substrates. These molecules generally require the
presence of an empty orbital on the metal center in order for them to pre-coordinate prior to being activated for
the oxidative addition reaction. Examples for non electrophilic specious for oxidative addition reactions are H2,
C-H bonds, Si-H bonds, S-H bonds, B-H bonds, N-H bonds, S-S bonds, C-C bonds, etc. H2 has most important
catalytic applications, followed by Si-H bonds, B-H, N-H, and S-H bonds.
C-H bond activation and
functionalization is very important.
Non-electrophillic “Intact”: These molecules may or may not contain electronegative atoms, but they need to
have a double or triple bond present. One needs a metal center with an empty orbital (16e- or lower count).
Example [ Ir (CO)Cl(PPh3)3 ] in order to pre-coordinate the ligand before the oxidative addition occurs.
Typical “intact” ligands that can perform an oxidation addition without fragmenting apart are (O 2 can also act as
an electrophillic substrate): Examples for non electrophilic intact reagents are alkenes, alkynes, and O These
2
ligands often needs to have electron withdrawing functional groups on the alkenes or alkynes in order to
increase their electron-withdrawing ability in order to help promote the transfer of electrons from the metal to
the ligand. Unlike most of the other substrate molecules that break a single bond and form two separate anionic
ligands upon the oxidative addition, these ligands have double or triple bonds and only one of the  bonds is
broken leaving the σ - bond intact. The ligand pick up two electrons from the metal and becomes a dianionic
10
ligand. On oxidative addition of Pt complexes with alkyne , the Pt center from Pt 0 d (considering 6S orbital
8
remain empty) get converted to Pt+2 d and generated a new dianionic unsaturated alkenyl ligand.
metallocyclopropene
R
Me3P
Pt
R
PMe3
PMe3
.
R
PMe3
Pt
-PMe3
R
PMe3
Electrophillic: These molecules contain electro negative atoms and are good oxidizing agents. They are often
considered to be “reactive” substrates. These molecules do not require the presence of an empty orbital (18eis OK) on the metal center in order to perform the oxidative addition reaction. X2 (X = Cl, Br, I), R-X, Ar-X,
H-X, O2, etc. The most common substrates used here are R-X (alkyl halides), Ar-X (aryl halides), and H-X.
An example of the oxidative addition of CH3Br to
[IrCl(CO)(PPh ) ]is shown below. Note that the starting
3 2
metal complex in this case is 16e-: Note that the H3C-Br bond is broken on the oxidative addition reaction
generating two new anionic ligands: CH3 and Br . If the starting metal complex is 16e- (as shown above)
both ligands will usually end up coordinated to the metal to make an 18e- complex. In the case of a starting 18ecomplex (shown below) only one of the two anionic ligands (usually the strongest binding) generated from the
oxidative addition will end up coordinated to the metal unless a separate substitution reaction occurs.
OC
PPh3
Ir
Ph3P
SN2 nucleophillic attack
 + 
+CH3Br
Ph3P
H
CO
C
Ir
Cl
Cl
Ir(+1)
16e-
H
PPh3
H
Ir(+1)
16etwo new anionic
ligands
two new anionic
ligands
CH3
OC
Ph3P
CH3
+Br
PPh3
Ir
Br
Cl
OC
Ir
Ph3P
PPh3
+ Br
Cl
Br
Ir(+3)
18e-
OC
OC
H
CO
oxidative
addition
H
+
Re
OC
Ir(+3)
16e-
Cl
OC
OC
Re(-1)
18e-
OC
CO
Re
CO
Re(+1)
18e-
-CO
OC
+ Cl
Re
OC
CO
CO
two new anionic
ligands
1) CO ligand dissociation
2) 1- to 3-allyl hapticity change
OC
Re(+1)
18e-
In this case the alkyl anion is the best donor ligand and easily “beats out” the more electronegative and poorly
donating Cl anion. Note that the alkyl ligand (-CH CH=CH ) initially coordinated to the Re after the oxidative
2
2
1
3
addition is a  -allyl ligand and that it can convert to the generally more stable π -allyl on CO ligand
dissociation.
General Features of Oxidative Additions
Oxidative addition involves oxidation (removal of electrons) of the metal center, If the metal is more electronrich, oxidative addition reaction becomes more easier. So in comparing two or more metal complexes to see
which will be the most reactive towards a particular substrate for oxidative addition we would pick the metal
center with the higher electron density. Also remember that the non-electrophillic ligands (Class A) and
“intact” ligands (Class C) usually require that there is an empty orbital (16e- or lower) on the metal center in
order to react.
iii) The compounds with eighteen electrons cannot undergo oxidative addition reactions with out expulsion of
ligands. Example
0
Ru (CO) 2 (PPh 3)3
+
I
2
I
Ru (CO) I 2 (PPh 3)3
+
CO
For oxidative addition reaction, to proceed, the main requirements for a metal complex are
Higher non bonding electron density on the metal, two vacant coordination site of the reacting metal complex Ln
M ,two stable oxidation state separated by two units. However, the molecules that contain multiple bonds may be
added oxidatively to a metal complex without cleavage. In such cases three member rings are formed with metal.
Oxidative addition reaction can be regarded a equilibrium of the following type
Lm Mn+ +
XY
⇌
Lm M n+2 (X)(Y)
In this reaction equilibrium lies on the reduced metal or the oxidized metal side depends on following facts.
1) Nature of metal and its ligands.
2) Nature of newly adding group XY and of the M-X ,M-Y bonds so formed.
3) The medium in which reactions conducted.
When the molecule XY adds without separation of X from Y, the two new bonds to the metal are in cis pattern.
When X and Y are separated, by the addition to the metal the product may be one of the several isomers, with cis or
trans MX and MY groups. The final product will be isomer or a mixture of isomers, which is most stable
thermodynamically under the reaction conditions. The ligands, solvent, temperature, pressure and the like will have a
decisive influence on this; consequently, the nature of final product does not give guidelines to initial product of this
addition reactions, since isomerisation of the initial product may occur under the conditions of the reaction.
Mechanisms of Oxidative addition reaction
Mechanism for oxidative addition reaction may vary according to the nature of X-Y bonds. If X-Y is non polar, as in
the case of H2, a concerted reaction leading to a three centered transition state is most likely occurring. If X-Y is an
electrophilic polar molecule such as CH3I the oxidative addition reaction tends to proceed SN 2 mechanism.
Ionic Mechanism
A purely ionic mechanism is favored by polar medium. In polar medium HCl or HBr would be dissociated and
protonation of square planar complex would first produce a five coordinate intermediate.
MXL3 + H+ (solvated)
⇌
[MHXL3 ]+
Inter molecular isomerism followed by coordination of X - would then give final product
⇌
MHXL3+ + Cl - (solvated)
SN2 Mechanism
MHXL3Cl
In this case first step I is the attack of alkyl halide on metal complex, and formation of the
intermediate compound followed by dissociation of halide from the intermediate, and then attack of halide ion on
the metal.
Ln M
+
C R 1 R2 R3 X
Ln M
C
R2
R1
R2
R1
Ln M
C
X
+
X-
R3
R3
Ln M X (CR 1 R2 R3)
3. Several oxidative addition reactions are free radical in nature and can be initiated by free radicals sources such as
peroxides.
ML 4
ML 3
MXL 3
ML 3
+
RX
+ R'
+
L
M(X)L 3
+
M(X) (R)L
2
R'
+
L
The reaction of Ir Cl (CO) (PMe3)2 with Ph CHFCH2Br is initiated by the radical precursors such as O 2 The above
reaction is inhibited by radical scavengers like hydroquininone
4. Under non polar conditions, particularly with molecules that have no polarity (example O 2 or H2) one step
concerted processes give products with the new bonds formed to one another as in reaction
+
-
X------Y
Y
X
Ir
+
L------ -------L
L
L
It is observed that filled non bonding orbital of metal have proper symmetry to populate the antibonding orbital of
X----Y
Consider the following observations
i) The addition of HCl or HBr to trans Ir Cl(CO) (PPh 3)2 in non polar medium also gives cis addition products . If
polar medium are used the addition products are cis and trans mixtures.
The concerted mechanism is appropriate for the reactions in non –polar medium and ionic mechanism accounts the
reaction in polar medium. Many of the d 8 complexes react with molecular hydrogen. The attack of the molecular
hydrogen probably shifts electron density from the metal into the anti bonding MO of hydrogen helping to decrease
the H—H bond strength. This results in homolysis of the H-H bond. This mechanism implies initial coordination of
molecular hydrogen to the metal and indeed some complexes of this type are now characterized.
An alternative mechanism, which operates in other cases, is heterolytic cleavage of the H-H bond by base promoted
removal of H+
+
H
H
H2
Ln M
Ln M
Ln M
H
H
-H2

RuCl2 (PPh3 )3 +H2 +Et3 N
Ru H Cl (PPh3)3
+ [Et3 NH]+Cl-
Stereochemistry of Oxidative addition reaction
When XY molecule add without the separation of X from Y ,the two new bonds to the metal are necessarily in cis
position ,but when X and Y are separated the product may be one or more of several isomers with either cis or
trans MX and MY groups example
L1
L2
Rh
L4
L1
L1
+
X
L4
X
L1
Rh
L4
L2
L2
Rh
Y
L3
Y
X
L3
Y
X
L2
Y
Rh
L3
L4
L3
X
L2
Rh
Y
L4
L1
L3
The final product will be the isomer or isomer mixture that is the most stable thermodynamically under the reaction
conditions. The ligands, solvents, temperature, pressure etc. will have a deceive influence on the nature of products.
Factors favoring the oxidative addition reactions
i)
Low oxidation state of the metal.:- Oxidative addition reactions can be treated as a process undergoing
loss of electrons from the electron rich metal center. Hence a metal ion in a lower oxidation state in the
complex promotes oxidative addition generally. Any change in the metal complex, which enhance the
electron density at the central metal ion lead to an increase in reactivity towards oxidative addition
reactions.
ii)
Coordinate unsaturation. The oxidative addition reactions are generally accompanied by an increase in
the coordination number of the central metal atom or ion. It is therefore, expected that organo metallic
reagent should be coordinately unsaturated, Example Vaskas compound carbonyl chloride bis
(triphenylphosphane)iridium (I) [ Ir Cl (CO) (PPh3)2], Wilkinson catalyst chloride tris (triphenylphosphane)
rhodium [Rh Cl (PPh3)3]. The coordination unsaturation can be achieved either (a) by the generation of
vacant coordination site or (b) by the ability of a complex to exist in different coordination numbers.
iii)
Nature of the Metal There are many indications that the lighter metals in a given sub group are less basic
or nucleophiles than the heavier elements. For example, The complex [Ir Cl (CO) (PPh 3)2 ] (Vaska’s
compound) react completely and irreversibly with CH3I whereas the reaction is reversible for rhodium
analogue. This shows the lighter metal undergoes oxidative addition reaction in a greater extent.
Ph 3 P
CO
+
Ir
Cl
H3C
Ph 3 P
H3C
CO
I
Ir
P Ph 3
Cl
P Ph 3
I
Ph 3 P
CO
+
Rh
Cl
H3C
Ph 3 P
H3C
CO
I
Rh
P Ph 3
Cl
P Ph 3
I
Electronic nature of ligands:-- The presence of ligand which increase electron density around the metal atom or
ion in a complex also influence the rate of reaction. For example, Rh Cl (PPh 3)3 undergoes facile oxidative addition
reactions, but Rh Cl (CO) (PPh3)2 is much less active for oxidative addition reactions. The replacement of one
strongly donor and weakly acceptor PPh3 ligand in Rh Cl(PPh3)3 by a weakly donor and strongly acceptor CO ligand
should result in a lower electron density on the central rhodium atom in the latter complexes Rh Cl (CO) (PPh 3)2
and thus explain the difference in reactivity of the two complexes.
Nature of Addendum Oxidative addition reactions involving cleavage of carbon – carbon bond would obviously be
much less feasible both on the thermodynamic as well as kinetic considerations. However, such cleavage reactions
can be brought about under special conditions. For example, sterically crowded addendum such as hexa phenyl
ethane is cleaved as it results in the release of a large strain in C2Ph6 molecule.
+
Ni Ln
Ph
Ph
Ph
C
C
Ph
Ph
Ph
Ph
Ph
Ph
C
Ph
Ni
C
Ph
+ nL
Ph
Alternatively, electronic activation of carbon- carbon bonds in molecules like dimethyl oxalate and cyanogens
facilitate the cleavage reactions
COOMe
Pt (PPh3)4
+
COOMe
Pt (PPh3)2
COOMe
COOMe
+
2 PPh33
CN
CN
+
Pd (Ph 3 )4
Pd (Ph 3 )2
+
2PPh 3 3
CN
CN
Oxidative additions are not possible for a system of d0 configuration
Reductive Elimination Reactions
A reductive elimination reaction is the reverse of an oxidative addition. It is a reaction in which two cisoidal
anionic ligands on a metal center couple together. Each anionic ligand pushes one electron back onto the metal
center (in the case of a monometallic complex) to reduce it by 2e-. The coupled anionic ligands then usually fall
off the metal center as a neutral molecule.
two cisoidal anionic ligands that
can form a bond between them
H
H
OC
Ir
Ph3P
PPh3
OC
H
Ph3P
Cl
Ir(+3)
18e-
and eliminate
a neutral
ligand
H
Ir
PPh3
Cl
OC
Ir
Ph3P
PPh3
Cl
Ir(+1)
18e-
Ir(+1)
16e-
rarely observed intermediate
metal reduced
by 2 e-
+ H2
Since electron-rich metal complexes favor oxidative addition, the reverse is true for reductive elimination.
Since reductive elimination involves pushing electrons back onto the metal center from two anionic ligands
that are usually more electronegative than the metal center, it is best if the metal center is electron deficient.
This can be accomplished by having electron-withdrawing ligands (e.g., CO), cationic charge(s), and/or
coordinative unsaturation (sub-18e- counts). While reductive elimination can occur from saturated 18ecomplexes (so long as the two ligands that you want to reductively eliminate are cisoidal to one another), it has
been shown that reductive elimination can be promoted by a ligand dissociation generating an unsaturated and
more electron-deficient metal center.
In studying the above system, it was also found that one could have reductive elimination of CH3I from the
starting 18e- complex. This reaction, however, is very reversible due to the high reactivity of CH 3I for doing an
oxidative addition back reaction with the electron-rich neutral Pt(+2) complex to make the Pt(+4) octahedral
compound.
Ph2
P
CH3
Pt
P
Ph2 I
CH3
CH3
-I
Ph2
P

CH3
Pt
P
Ph2
Pt(+4)
18e-
Ph2
P
CH3
CH3
Pt
P
Ph2
Pt(+4)
16e-
CH3
+
CH3CH3
Pt(+2)
14e-
+I
Ph2
P
The dissociation of the I- generates
a cationic unsaturated complex. This
is electron deficient enough to help
promote the reductive elimination of
ethane (CH3CH3).
Pt
P
Ph2
CH3
I
Pt(+2)
16e-
reductive elimination
Ph2
P
The reductive elimination of the CH3I is
kinetically favored. This is because the
CH3
Pt
P
Ph2 I
Goldberg, J. Am. Chem. Soc.
1995, 117, 6889-6896
Ph2
P
CH3
CH3

orbitals around the iodide anion are
Pt
P
Ph2
CH3
CH3
+
CH3I
spherically symmetric and this makes
it much easier to overlap with the alkyl
group orbital to perform the reductive
elimination. The sp3 directed orbitals
Pt(+4)
18e-
Pt(+2)
16e-
on the two CH3 groups are more difficult
to overlap in order to get the reductive
elimination to occur. But the reductive
oxidative addition
elimination of the CH3CH3 is thermodynamically considerably more favorable
and the back oxidative addition much
more difficult.