chemistry

Non-coordinating anion

Non-coordinating anion 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 Non-coordinating anion rather than just read about it. In short: Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations.

Key takeaways

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

Reference excerpt

Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found as counterions for cationic metal complexes with an unsaturated coordination sphere. These special anions are essential components of homogeneous alkene polymerisation catalysts, where the active catalyst is a coordinatively unsaturated, cationic transition metal complex. For example, they are employed as counterions for the 14 valence electron cations [(C5H5)2ZrR]+ (R = methyl or a growing polyethylene chain). Complexes derived from non-coordinating anions have been used to catalyze hydrogenation, hydrosilylation, oligomerization, and the living polymerization of alkenes. The popularization of non-coordinating anions has contributed to increased understanding of agostic complexes wherein hydrocarbons and hydrogen serve as ligands. Non-coordinating anions are important components of many superacids, which result from the combination of Brønsted acids and Lewis acids.

Pre-"BARF" era Before the 1990s, tetrafluoroborate, hexafluorophosphate, and perchlorate were considered weakly coordinating anions. Usually, only by exclusion of conventional solvents were transition metal perchlorate complexes found to exist, for example. It is now appreciated that BF−4, PF−6, and ClO−4 bind to electrophilic metal centers of the type use in some catalytic such as cationic Zr(IV) centers. Other anions, such as triflates coordinate to metal cations. On that same theme, tetraphenylborate forms pi-complexes with utilizing its electron-rich phenyl groups as ligands.

Era of BARF

A revolution in this area occurred in the 1990s with the introduction of the tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ion, B[3,5-(CF3)2C6H3]−4, commonly abbreviated as B(ArF)4− and colloquially called "BARF". This anion is far less coordinating than tetrafluoroborate, hexafluorophosphate, and perchlorate, and consequently has enabled the study of still more electrophilic cations. Related tetrahedral anions include tetrakis(pentafluorophenyl)borate B(C6F5)−4, and Al[OC(CF3)3]−4.

In the bulky borates and aluminates, the negative charge is symmetrically distributed over many electronegative atoms. Related anions are derived from tris(pentafluorophenyl)boron B(C6F5)3. Another advantage of these anions is that their salts are more soluble in non-polar organic solvents such as dichloromethane, toluene, and, in some cases, even alkanes. Polar solvents, such as acetonitrile, THF, and water, tend to bind to electrophilic centers, in which cases, the use of a non-coordinating anion is pointless. Salts of the anion B[3,5-(CF3)2C6H3]−4 were first reported by Kobayashi and co-workers. For that reason, it is sometimes referred to as Kobayashi's anion. Kobayashi's method of preparation has been superseded by a safer route.

The neutral molecules that represent the parents to the non-coordinating anions are strong Lewis acids, e.g. boron trifluoride, BF3 and phosphorus pentafluoride, PF5. A notable Lewis acid of this genre is tris(pentafluorophenyl)borane, B(C6F5)3, which abstracts alkyl ligands:

(C5H5)2Zr(CH3)2 + B(C6F5)3 → [(C5H5)2Zr(CH3)]+[(CH3)B(C6F5)3]−

Other types of non-coordinating anions Another large class of non-coordinating anions are derived from carborane anion CB11H−12. Using this anion, the first example of a three-coordinate silicon compound, the salt [(mesityl)3Si][HCB11Me5Br6] contains a non-coordinating anion derived from a carborane.

References

Illustrations

Worked examples

Example 1 — a first encounter with Non-coordinating anion

Start with the simplest possible case. Write down what Non-coordinating anion 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 Non-coordinating anion 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 Non-coordinating anion 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 Non-coordinating anion

In research: Non-coordinating anion 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 Non-coordinating anion 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: Non-coordinating anion is common in secondary-school and first-year university syllabi. It links to neighbouring topics Coordination chemistry, Non-coordinating anions, so understanding it makes those chapters shorter.
In everyday life: Look for Non-coordinating anion 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.

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How to study Non-coordinating anion in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Non-coordinating anion 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.
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Frequently asked questions

What is Non-coordinating anion in simple terms?

Why does Non-coordinating anion 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 Non-coordinating anion?

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 Non-coordinating anion.