---
title: "Olefin Metathesis | Inorganic Chemistry II"
description: "Olefin metathesis is a catalyst-driven exchange of alkene fragments that makes new C=C bonds, a core reaction in Inorganic Chemistry II catalysis."
canonical: "https://fiveable.me/inorganic-chemistry-ii/key-terms/olefin-metathesis"
type: "key-term"
subject: "Inorganic Chemistry II"
unit: "Unit 10"
---

# Olefin Metathesis | Inorganic Chemistry II

## Definition

Olefin metathesis is a transition-metal-catalyzed reaction where alkene fragments swap partners to form new alkenes. In Inorganic Chemistry II, it is a classic example of homogeneous catalysis and organometallic reactivity.

## What It Is

Olefin metathesis is an organometallic reaction in Inorganic Chemistry II where two alkene-containing molecules exchange parts of their carbon-carbon double bonds and form new alkenes. Instead of a simple addition or substitution, the reaction reshuffles the alkene partners, which is why the product mixture can look like the starting materials have been "recombined."

The chemistry usually happens with a transition metal catalyst, often a molybdenum or tungsten complex in older systems, or a ruthenium catalyst in many modern lab settings. The metal does not just sit there speeding things up in a vague way. It forms a reactive metal-carbene species that can interact with the alkene and set up the bond exchange.

The key intermediate is a metallacyclobutane. That ring forms when the catalyst and alkene combine, then breaks apart in a new pattern. This step is what lets one C=C bond become another C=C bond in a different place or with a different partner. If your instructor draws the mechanism, the metallacyclobutane is usually the centerpiece because it explains how the carbon skeleton gets rearranged without fully breaking the molecule apart first.

A common way to think about the reaction is that the alkene pieces are "swapping ends." If two identical alkenes react, you can get homodimerization products. If two different alkenes react, you get cross metathesis products, which is useful when you want to build a specific unsaturated molecule instead of a random mix. Selectivity depends on the catalyst, the alkene structure, and often the reaction conditions.

In inorganic chemistry, the big takeaway is not just the organic product. Olefin metathesis shows how a well-designed metal complex can control bond making and bond breaking in a highly organized way. It is a clean example of homogeneous catalysis because the catalyst and substrates are in the same phase and the metal center is doing real mechanistic work, not just acting as a black box.

## Why It Matters

Olefin metathesis matters in Inorganic Chemistry II because it ties together several ideas from the course at once: transition metal catalysis, organometallic intermediates, and mechanism-based thinking. It is one of the clearest examples of how a metal complex can change the route a reaction takes, not just the speed.

This term also gives you a real case for studying catalyst design. Small changes to the metal center or ligands can change how stable the catalyst is, how selective it becomes, and whether it survives air, moisture, or other functional groups. That makes metathesis a good example of why chemists care so much about the structure of the catalyst, not just the substrate.

You will also see metathesis when the course moves into catalysis applications. It connects to how industry makes polymers, pharmaceuticals, and fine chemicals from simpler alkene feedstocks. If you can explain why a catalyst can rearrange alkenes cleanly, you are showing that you understand both the mechanism and the practical reason inorganic chemists study it.

It is also a useful concept for comparing reaction pathways. Once you know the metallacyclobutane mechanism, you can separate metathesis from ordinary addition reactions, insertion chemistry, or reactions that just form a new bond without exchanging alkene partners. That kind of comparison is exactly what comes up in reaction-mechanism questions and class discussions.

## Connections

### Catalyst

Olefin metathesis depends on a catalyst because the uncatalyzed exchange of alkene fragments is not practical. In this topic, the catalyst is not just a reaction booster, it is the species that creates the metal-carbene intermediate and controls how the alkene partners reorganize. When you study metathesis, you are really studying how a catalyst changes the pathway.

### Alkene

Alkenes are the substrates in olefin metathesis, so you need to recognize which double bonds can react and what products are possible. The structure of the alkene affects selectivity, especially in cross metathesis where two different alkenes are competing to form new products. Steric hindrance and substitution pattern often shape the outcome.

### Cross Metathesis

Cross metathesis is the version of olefin metathesis where two different alkenes exchange fragments to make mixed products. It is the version students often see in synthesis examples because it is useful for building a target molecule with a specific carbon chain. The main challenge is controlling which partners react and how much side product forms.

### [carbene ligands](/inorganic-chemistry-ii/key-terms/carbene-ligands)

Carbene ligands matter because many metathesis catalysts operate through metal-carbene chemistry. The metal-carbene unit is the reactive site that interacts with the alkene and leads to the metallacyclobutane intermediate. If you are tracing the mechanism, identifying the carbene-like character of the catalyst helps you follow why the double bonds can be rearranged.

## On the AP Exam

A quiz or problem-set question usually asks you to identify olefin metathesis from a reaction scheme, name the catalyst type, or draw the metallacyclobutane intermediate. You might also be asked to compare homodimerization and cross metathesis, or explain why a certain alkene pair gives a better product mix. In a mechanism problem, the move is to track how the metal-carbene interacts with the alkene, then show the bond reorganization step by step. If the instructor gives product structures, you should be able to work backward and spot which alkene fragments were exchanged. In discussion or a short answer, use the vocabulary of homogeneous catalysis, organometallic intermediates, and selectivity instead of describing it as a vague "swap" reaction.

## olefin metathesis vs Hydrogenation

Olefin metathesis is often confused with hydrogenation because both involve alkenes and catalysts, but they do very different things. Hydrogenation adds hydrogen across a double bond and removes unsaturation, while metathesis rearranges alkene partners and keeps double bonds in the products. If the product still contains C=C bonds, that is a big clue that it is metathesis, not hydrogenation.

## Key Takeaways

- Olefin metathesis is a catalyst-driven exchange of alkene fragments that forms new alkenes.
- The reaction is a major example of homogeneous catalysis in Inorganic Chemistry II because the metal complex and the alkene react in the same phase.
- A metallacyclobutane intermediate is the mechanistic step that explains how the double bonds are rearranged.
- Cross metathesis joins two different alkenes, while homodimerization involves similar or identical alkene partners.
- Catalyst choice and alkene structure strongly affect selectivity, side products, and how useful the reaction is in synthesis.

## FAQs

### What is olefin metathesis in Inorganic Chemistry II?

Olefin metathesis is a transition-metal-catalyzed reaction that swaps alkene fragments to make new alkenes. In Inorganic Chemistry II, it is a model reaction for understanding homogeneous catalysis, organometallic intermediates, and catalyst selectivity.

### What intermediate forms in olefin metathesis?

The classic intermediate is a metallacyclobutane. That four-membered ring forms and then breaks apart in a different bonding pattern, which is what allows the alkene partners to be exchanged.

### How is cross metathesis different from homodimerization?

Cross metathesis combines two different alkenes, so the product is a mixed alkene. Homodimerization happens when identical or very similar alkene molecules react with each other, which can give more symmetrical products. The distinction matters because it changes how selective the synthesis is.

### Why are transition metals used for olefin metathesis?

Transition metals can stabilize the reactive species needed for the reaction and support the bond rearrangement through metal-carbene chemistry. Molybdenum and tungsten were classic choices, and many modern systems use ruthenium because they can be effective and easier to handle in the lab.

## Related Study Guides

- [10.2 Homogeneous Catalysis](/inorganic-chemistry-ii/unit-10/homogeneous-catalysis/study-guide/V2P8VG3o6SceBfu1)

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