chemistry

Organochromium chemistry

Organochromium chemistry is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Organochromium chemistry rather than just read about it. In short: Organochromium chemistry is a branch of organometallic chemistry that deals with organic compounds containing a chromium to carbon bond and their reactions. The field is of some relevance to organic synthesis.

Key takeaways

Organochromium chemistry belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Organochromium chemistry to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Organochromium chemistry from memory before moving on to harder problems.

Reference excerpt

Organochromium chemistry is a branch of organometallic chemistry that deals with organic compounds containing a chromium to carbon bond and their reactions. The field is of some relevance to organic synthesis. The relevant oxidation states for organochromium complexes encompass the entire range of possible oxidation states from –4 (d10) in Na4[Cr–IV(CO)4] to +6 (d0) in oxo-alkyl complexes like Cp*CrVI(=O)2Me.

History The first organochromium compound was described in 1919 by Franz Hein. He treated phenylmagnesium bromide with chromium(III) chloride to give a new product (after hydrolysis) which he incorrectly identified as pentaphenyl chromium bromide (Ph5CrBr). Years later, in 1957 H.H. Zeiss et al. repeated Hein's experiments and correctly arrived at a cationic bisarene chromium sandwich compound (ArH2Cr+). Bis(benzene)chromium itself was discovered around the same time in 1956 by Ernst Otto Fischer by reaction of chromium(III) chloride, benzene and aluminum chloride. The related compound chromocene had been discovered a few years earlier in 1953 also by Fischer.

Anet and Leblanc also in 1957 prepared a benzyl chromium solution from benzyl bromide and chromium(II) perchlorate. This reaction involves one-electron oxidative addition of the carbon-bromine bond, a process which was shown by Kochi to be a case of double single electron transfer, first to give the benzyl free radical and then to the benzyl anion.

G. Wilke et al. introduced tris-(η-allyl)chromium in 1963 as an early Ziegler–Natta catalyst, albeit of limited commercial success. Chromocene compounds were first employed in ethylene polymerization in 1972 by Union Carbide and continue to be used today in the industrial production of high-density polyethylene. The organochromium compound (phenylmethoxycarbene)pentacarbonylchromium, Ph(OCH3)C=Cr(CO)5 was the first carbene complex to be crystallographically characterized by Fischer in 1967 (now called a Fischer carbene). The first ever carbyne, this one also containing chromium, made its debut in 1973.

The first example of a proposed metal-metal quintuple bond is found in a compound of the type [CrAr]2, where Ar is a bulky aryl ligand.

Applications in organic synthesis Although organochromium chemistry is heavily employed in industrial catalysis, relatively few reagents have been developed for applications in organic synthesis. Two are the Nozaki-Hiyama-Kishi reaction (1977) (transmetallation with organonickel intermediate) and the Takai olefination (1986)(oxidation of Cr(II) to Cr(III) while replacing halogens). In a niche exploit, certain tricarbonyl(arene)chromium complexes display benzylic activation.

Organochromium compounds Organochromium compounds can be divided into these broad compound classes:

Sandwich compounds: chromocene Cp2Cr and Bis(benzene)chromium derivatives (ArH)2Cr. More commonly studied are half-sandwich complexes such as (η6-C6H5OMe)Cr(CO)3. Chromium carbenes (R1)(R2)C::CrLn and carbynes (RC:::CrLn) Chromium(III) complexes RCrL5. Complexes of chromium carbonyl anion and cation (e.g Na4Cr(CO)4).

Ethylene polymerization and oligomerization Chromium catalysts are important in ethylene polymerization. The Phillips catalyst is prepared by impregnating chromium(VI) oxide on silica followed activation in dry air at high temperatures. The bright yellow catalyst becomes reduced by the ethylene to afford a probable Cr(II) species that is catalytically active. A related catalytic systems developed by Union Carbide and DSM are also based on silica with chromocene and other chromium complexes. How these catalysts work is unclear. One model system describes it as coordination polymerization:

With two THF ligands the catalyst is stable but in dichloromethane one ligand is lost to form a 13 electron chromium intermediate. This enables side-on addition of an ethylene unit and a polymer chain can grow by migratory insertion. Chromium compounds also catalyse the trimerization of ethylene to produce the monomer 1-hexene.

References

Illustrations

Worked examples

Example 1 — a first encounter with Organochromium chemistry

Start with the simplest possible case. Write down what Organochromium chemistry claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Organochromium chemistry before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Organochromium chemistry ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Organochromium chemistry

In research: Organochromium chemistry appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Organochromium chemistry in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Organochromium chemistry is common in secondary-school and first-year university syllabi. It links to neighbouring topics Organochromium compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Organochromium chemistry outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Organochromium chemistry” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Organochromium chemistry in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Organochromium chemistry means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Organochromium chemistry out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Organochromium chemistry in simple terms?

Why does Organochromium chemistry matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Organochromium chemistry?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Organochromium chemistry.