Oxidative Addition

Oxidative addition reactions involve increasing both the coordination number and the oxidation state of the metal by two. This reaction can occur cis, with the ligands adding on the same side, or trans, with the ligands adding to opposite positions. 

Oxidation is the loss of electrons. In organic chemistry, oxidation reactions involve a decrease in electron density. This can occur with an increase in C-O, C-X, or C-N  bonds and a decrease in C-H bonds.

Significance in Catalysis

• Reversible - The reverse reaction of oxidative addition is reductive elimination. This allows catalysts to regenerate and prevents deactivation. Reversible reactions also minimize waste.
• Formation and Breaking of C-H/C-C bonds - C-C bonds can be formed with Suzuki, Heck, and Sonogashira coupling. This is helpful in pharmaceuticals, green chemistry, and the fuel industry. 
• Hydrogenation - The addition of H₂ to metals is used in pharmaceuticals, industry, and organic synthesis. The transition metal catalyst allows for the process to be efficient and selective.
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  • Ex: Wilkinson's Catalyst

Mechanisms

[L_nM]⁺ⁿ + X-Y → [LnM-XY]⁽ⁿ⁺²⁾

Concerted

• Ligand binds initially as a σ complex, then the bond breaks due to back donation from the metal electrons
• Creates a transition state

SN2

• Addition of methyl, acyl, alyll, and benzyl halides
• Metal electrons break A-B bond by attacking the σ* orbital at the least electronegative atom
• More nucleophilic metals have greater reactivity
  • Alkyl>Aryl>Alkene
• Oxidation of the metal by 2, follwed by an increase in the total electron state by 2

Radical

• Metal-centered radical abstracts from the halide to leave the chain
• Can use photoinitiation
• Changes in substrate, complex, or impurities in reagents can change rate or mechanism

Mechanism for additions of alkyl halides R-X to Pt(PPh₃)₃, R=Me, Et, PhCh₂ X=I,Br

Examples

Concerted Addition of H₂ creates a transition state/intermediate. The hydride ligands add cis to one another.

Binuclear oxidative addition uses radical mechanisms (1 electron change, not 2)