11: Organometallic Chemistry
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- 326251
Organometallic chemistry is a subfield of inorganic chemistry involving the study of compounds containing, and reactions involving, metal-carbon bonds. The metal-carbon bond may be transient or temporary, but if one exists during a reaction or in a compound of interest, we’re squarely in the domain of organometallic chemistry. In addition to M-C bond, bonds between metals and the other common elements of organic chemistry also appear in organometallic chemistry: metal-nitrogen, metal-oxygen, metal-halogen, and even metal-hydrogen bonds all play a role. Metals cover a vast swath of the periodic table and include the alkali metals, alkali earth metals, transition metals, the main group metals, and the lanthanides and actinides. We will focus most prominently on the transition metals in this chapter.
Why is the subject worth studying? Organometallic complexes are catalysts for a wide range of industrially important chemical reactions from polymers to pharmaceuticals. There’s a reason the “organo” comes first in “organometallic chemistry”—These catalysts usually the aid in the creation of new bonds in organic compounds, building up complex products from simple starting materials. The fact is that you can do things with organometallic chemistry that you cannot do using straight-up organic chemistry. Case in point:
The venerable Suzuki reaction...unthinkable without palladium!
The establishment of a bond between two phenyl rings seems unthinkable to the pure organic chemist, but with a palladium catalyst it si a commonplace reaction. Bromobenzene looks like a potential electrophile at the bromine-bearing carbon, and if you’re familiar with hydroboration you might see phenylboronic acid as a potential nucleophile at the boron-bearing carbon. Catalytic palladium makes it all happen. Organometallic chemistry is full of these mind-bending transformations, and can expand the synthetic toolbox of the organic chemist considerably.
Contributors and Attributions
Learning Objectives
- Illustrate the orbital overlap involved in metal-ligand bonding for different classes of organometallic ligands
- Determine the metal electron count and use that to predict reactivity of the metal complex
- Recognize the different classes of reactions important to organometallic catalysis
Thumbnail image shows Wilkinson's catalyst (CC0; Benjah-bmm27 via Wikimedia Commons)
- 11.1: Organometallic Ligands
- There are some classes of ligands and modes of bonding that are important and in some cases unique to organometallic complexes and reactions. Before discussing reactions of organometallic complexes we will start with an overview of ligands common to organometallic complexes. In organometallic reactions, ligands can be either spectators, not chemically involved or changed during the reaction, or actors, chemically changed during the reaction.
- 11.2: The 18 Electron Rule
- Electron counting is important because the number of electrons in a complex can tell us a lot about the stability and reactivity in a coordination compound. In addition, it allows us to predict and understand structures to a certain extent. Electron counting sounds trivial, but it is not as trivial as it seems, actually there are even two different methods for electron counting. Each method leads to the same result, whichever method you prefer is your choice.
- 11.3: Oxidative Addition
- How important are oxidative additions? Very. The addition of dihydrogen (H2) is an important step in catalytic hydrogenation reactions. Organometallic C–H activations depend on oxidative additions of C–H bonds. In a fundamental sense, oxidative additions of organic compounds are commonly used to establish critical metal-carbon bonds.
- 11.4: Reductive Elimination
- Reductive elimination is the microscopic reverse of oxidative addition. It is literally oxidative addition run in reverse. Chemically, reductive elimination and oxidative addition share the same reaction coordinate. The only difference between their reaction coordinate diagrams relates to what we call “reactants” and “products.”
- 11.5: Migratory Insertion- 1,2-Insertions
- Insertions of π systems into M-X bonds establish two new σ bonds in one step, in a stereocontrolled manner. As we saw in the last post, however, we should take care to distinguish these fully intramolecular migratory insertions from intermolecular attack of a nucleophile or electrophile on a coordinated π-system ligand. The reverse reaction of migratory insertion, β-elimination, is not the same as the reverse of nucleophilic or electrophilic attack on a coordinated π system.
- 11.6: β-Elimination Reactions
- In organic chemistry class, one learns that elimination reactions involve the cleavage of a σ bond and formation of a π bond. A nucleophilic pair of electrons (either from another bond or a lone pair) heads into a new π bond as a leaving group departs. This process is called β-elimination because the bond β to the nucleophilic pair of electrons breaks. Transition metal complexes can participate in their own version of β-elimination, and metal alkyl complexes famously do so.
- 11.7: Organometallic Catalysts
- The different types of organometallic reactions discussed in the previous sections can be combined together into catalytic cycles to produce valuable chemicals or perform otherwise difficult chemical reactions. While the organometallic catalyst will undergo a number of transformations over the course of the cycle, it will return to its original active state at the end of the reaction. By definition catalysts are reactive, because of this the metal complex added to a reaction is at times a more s