Organometallic Reactions Part 2: Oxidative Addition

Summary

This video script provides an in-depth introduction to oxidative addition in organometallic chemistry. It explains the reaction mechanism, where a metal complex reacts with a molecule, breaking a bond and adding the resulting fragments as new ligands, thereby increasing the metal's oxidation state. The script covers both concerted and stepwise mechanisms, highlights common examples such as platinum and iridium complexes, and discusses the types of bonds typically involved (e.g., H-H, Si-H, C-H). It also touches on the limitations of the process, such as the importance of electron count and the inability of d0 metals to undergo oxidative addition.

Takeaways

• 😀 Ligand exchange and oxidative addition are important organometallic reactions that allow new ligands to attach to metals.
• 😀 Oxidative addition changes the oxidation state of the metal, typically seen in metals with stable oxidation states that are two apart (e.g., Pd(0) ↔ Pd(2+), Ir(1+) ↔ Ir(3+)).
• 😀 Oxidative addition occurs when a metal in its lower oxidation state reacts with a molecule (A-B), breaking the bond and resulting in two new ligands on the metal.
• 😀 The reaction often leads to a metal center with an oxidation state 2 higher than its initial state (e.g., from n to n+2).
• 😀 The most common form of oxidative addition is cis-concerted addition, where the two new ligands are positioned cis to each other.
• 😀 Oxidative addition is frequently seen with bonds like H-H or C-X (carbon-halogen), especially for nonpolar bonds, which undergo concerted addition.
• 😀 In the case of molecular hydrogen (H2), the sigma bond weakens due to electron donation and backbonding from the metal, leading to two hydrogens attaching to the metal in a cis configuration.
• 😀 Stepwise oxidative addition involves a two-step process, typically for polar bonds. The metal first reacts with the less electronegative fragment, forming an intermediate with a formal charge.
• 😀 In the stepwise mechanism, the nucleophilic portion of the molecule later attacks and coordinates to restore charge balance.
• 😀 The reaction can also proceed via a radical intermediate, instead of a cation, and careful attention to electron count is important in determining if the metal can accommodate additional ligands.
• 😀 C-H activation via oxidative addition is synthetically important, though more difficult. C-C bond activation is rare but has been observed in some cases.
• 😀 D0 metals (without d-electrons) cannot undergo oxidative addition because they cannot be oxidized in the same way as metals with available d-electrons.

Q & A

What is oxidative addition in organometallic chemistry?

Oxidative addition is a reaction in which a metal complex reacts with a substance, causing the metal to increase its oxidation state by two. This results in the attachment of new ligands to the metal and typically occurs when a bond between two atoms (A-B) breaks, with each fragment becoming a separate ligand on the metal.

Which metals are commonly involved in oxidative addition reactions?

Metals with stable oxidation states that are two units apart are commonly involved in oxidative addition reactions. Examples include palladium (Pd), rhenium (Re), and iridium (Ir), where Pd(0) and Pd(II), Re(I) and Re(III), and Ir(I) and Ir(III) are common oxidation states.

What are the two primary mechanisms of oxidative addition?

The two primary mechanisms of oxidative addition are concerted and stepwise. The concerted mechanism, which is more common for nonpolar bonds like H-H or C-H, involves both fragments of the bond adding to the metal simultaneously. The stepwise mechanism, more common for polar bonds, involves a two-step process where the less electronegative fragment initially binds to the metal, followed by the nucleophilic attack of the more electronegative fragment.

How does molecular hydrogen participate in oxidative addition?

In oxidative addition, molecular hydrogen can approach the metal's dz2 orbital and interact with it in a perpendicular fashion. This interaction weakens the H-H sigma bond, allowing the molecule to split into two hydrogen atoms that bind to the metal center, typically in a cis arrangement.

What structural change occurs in the metal complex during oxidative addition with hydrogen?

When molecular hydrogen undergoes oxidative addition to the metal, the metal's oxidation state increases by two, and one of the existing ligands (such as a chloride or carbonyl) is pushed to an axial position, resulting in the formation of an octahedral complex.

How does the stepwise mechanism differ from the concerted mechanism in oxidative addition?

In the stepwise mechanism, the metal first interacts with the less electronegative fragment of a polar molecule, leading to the dissociation of the more electronegative fragment. The metal's oxidation state increases by two, and the charge is temporarily altered. Then, the nucleophilic portion attacks and coordinates to restore the original charge. This contrasts with the concerted mechanism, where both fragments add simultaneously to the metal.

Can oxidative addition occur with C-H bonds?

Oxidative addition with C-H bonds is more difficult but is synthetically important. The process typically requires activation of the C-H bond, which may involve additional steps or catalysts to facilitate the reaction.

What types of bonds commonly undergo oxidative addition?

Oxidative addition commonly involves bonds like H-H, Si-H, B-B, B-H, Sn-H, Sn-Sn, N-H, O-H, S-H, and C-X. These bonds are prone to breaking, allowing the fragments to coordinate with the metal center.

What happens in an oxidative addition reaction with methyl iodide and platinum?

In this reaction, a platinum(II) complex reacts with methyl iodide, leading to an SN2-like transition state where the platinum attacks the methyl group and displaces iodide. The iodide then coordinates to the platinum, resulting in a complex with a 1+ charge and a methyl ligand attached to the platinum.

Why can't d0 metals participate in oxidative addition?

d0 metals, such as those with no d-electrons (e.g., Sc, Ti), cannot undergo oxidative addition because they lack the ability to be oxidized to a higher oxidation state. Oxidative addition requires the metal to increase its oxidation state by two, a process that d0 metals cannot achieve due to the absence of available d-electrons.