Download Oxidative Addition

Oxidative Addition
 Simultaneous introduction of a pair of anionic ligands, A and B, of an A−B molecule
such as H2 or CH3‐I. A−B bond is broken, and M−A and M−B bonds are formed.
▪ The oxidation state (OS), electron count (EC), and coordination number (CN) all
increase by two units during the reaction.
Requirements
1) A vacant 2e site is always required on the metal. We can either start with a 16e
complex or a 2e site must be opened up in an 18e complex by the loss of a ligand
producing a 16e intermediate species.
2) The starting metal complex of a given oxidation state must also have a stable
oxidation state two units higher to undergo oxidative addition.
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Oxidative Addition to Vaska’s Complex
HX
Cl
X
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Overview of Oxidative Addition
Mechanism Type of A-B
Stereochemistry
Concerted
Fairly non-polar substrates:
H-H, R3C-H, R3Si-H
cis-addition
SN2
Polarized substrates:
R3C-X
Also Cl2, Br2, I2
trans-addition
Radical
R3C-X, R3Sn-X
-
Ionic
H-X (largely dissociated in
solution)
-
 Non-polar substrates (e.g. H-H, C-H, Si-H) → Concerted
 Alkyl halides → Nucleophilic (SN2) or Radical
 Halogens (Cl2, Br2, I2) → Nucleophilic (SN2)
 Acids (HCl, HBr, HI) → Ionic
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Concerted Mechanism
 Two-Step Mechanism:
1) Incoming ligand first binds as a σ complex,
2) Bond breaking as a result of strong back donation from metal into the σ* orbital.
SN2 Mechanism (Non‐Concerted)
 The metal electron pair of LnM directly attacks the A–B σ* orbital at the least
electronegative atom.
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Reductive Elimination
 Reductive elimination, the reverse of oxidative addition, is most often seen in
higher oxidation states because the formal oxidation state of the metal is reduced
by two units in the reaction.
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Migratory Insertion
 A migratory insertion reaction occurs when a cisoidal anionic and neutral ligand on
a metal complex couple together to generate a new coordinated anionic ligand.
1) 1,1 insertion in which the metal and the X ligand end up bound to the same (1,1)
atom.
2) 1,2 insertion in which the metal and the X ligand end up bound to adjacent (1,2)
atoms of an L‐type ligand.
 A 2e vacant site is generated by insertion reactions. Conversely, elimination requires
a 2e vacant site.
 The insertion requires a cis arrangement of the ligands, while the elimination
generates a cis arrangement of these ligands.
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CO Insertion Reactions
13
13
13
 When the incoming ligand is 13CO, the product contains only one labeled CO, which
is cis to the newly formed acetyl group. This shows that the methyl group migrates
to a coordinated CO, rather than free CO attacking the Mn−Me bond.
 We can tell where the labeled CO is located in the product because there is a
characteristic shift of the ν(CO) stretching frequency to lower energy in the IR
spectrum of the complex as a result of the greater mass of 13C over normal carbon.
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Alkene Insertions Reactions
 η2‐ligands like alkenes give 1,2-insertion. This is the reverse of the familiar
β‐elimination reaction.
 Site Selectivity: The site selectivity of 1,2-insertion can be predicted using resonance
forms and partial charges.
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Eliminations
 Elimination reactions are just the reverse of migratory insertion reactions.
 β-hydride elimination: β elimination is the chief decomposition pathway for alkyls
that have β‐H substituents.
 α-hydride elimination: If an alkyl has no β hydrogens, it may break a C−H bond in
the α, γ, or δ position.
 carbonyl elimination or decarbonylation:
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