chemistry

Phosphirenium ion

Phosphirenium ion 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 Phosphirenium ion rather than just read about it. In short: Phosphirenium ions (R1R2C2PY1Y+2) are a series of organophosphorus compounds containing unsaturated three-membered ring phosphorus (V) heterocycles and σ*-aromaticity is believed to be present in such molecules. Many of the salts containing phosphirenium ions have been isolated and characterized by NMR spectroscopy and X-ray crystallography.

Key takeaways

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

Reference excerpt

Phosphirenium ions (R1R2C2PY1Y+2) are a series of organophosphorus compounds containing unsaturated three-membered ring phosphorus (V) heterocycles and σ*-aromaticity is believed to be present in such molecules. Many of the salts containing phosphirenium ions have been isolated and characterized by NMR spectroscopy and X-ray crystallography.

Synthesis The first series of phosphirenium ions were synthesized by reacting alkynes with methyl- or phenylphosphonous dichloride and aluminum trichloride. These reactions may be regarded as formal addition of "RClP+" to alkynes.

[2+1]-cycloaddition reactions between phosphaalkynes and chlorocarbene give phosphirenes, which serve as starting materials for the generation of phosphirenium species.

Treatment of diphenylphosphine oxide with diphenylacetylene affords phosphirenium species.

Phosphirenium ions can also be obtained from reaction between phosphiranes and alkynes, where "RClP+" is formally transferred from alkenes to alkynes.

Characterizations In the literature, 31P NMR spectra of phosphirenium ions show upfield shifts (−57.3 ppm when R1 = R2 = Y1 = CH3, Y2 = Cl). Large coupling constants J are also found in 1H NMR, and are comparable to those found in cyclopropenium ions. The first phosphirenium ion characterized by X-ray Crystallography has the following structural formula:

In the refined crystal structure, average phosphorus–cyclic carbon distance has been found to be 1.731(12) Å, roughly corresponding to a bond order of 1.5. For comparison, typical single- and double-bond P–C distances are 1.86 Å and 1.68 Å, respectively.

Reactivity Reminiscent of π-ligand exchange in coordination compounds, a phosphirenium ion may undergo alkyne exchange with other alkynes to give a mixture of phospinirenium species in equilibrium. Kinetically, elimination of alkyne from the cation is suggested to be rate-determining step.

In addition, the three-membered ring of phosphirenium ion may be broken. Successive reactions with suitable nucleophile are able to proceed on the electrophilic phosphorus atom. With the presence of an alkyne:

With the presence of water or alcohol:

Electrophilic B(C6F5)3 readily reacts with phosphinylalkynes at room temperature to give phosphirenium-borate zwitterions as intermediates, which then generate carbon-phosphorus σ bond activation products at higher temperature. The products are of interest to material science. Dotted line in product indicates weak interaction between boron and phosphor atoms (see frustrated Lewis pair).

σ*-Aromaticity Qualitative molecular orbital (MO) diagram of a phophirenium ion can be obtained by linear combination of orbitals from a PY+2 fragment and a bent RC=CR fragment. A low-lying σ* orbital from the former with ungerade symmetry interacts with both π and π* orbitals of the latter, creating a 2π-Hückel system, analogous to the one in cyclopropenium ion. This effect has been named as σ*-aromaticity. It is noteworthy that unlike the case of cyclopropenium ion, interaction between the filled σ orbital of the PY+2 fragment and π orbitals also leads to some degree of antiaromatic character. Therefore, net 3-center conjugative effect is a combination of both σ* stabilizing contribution and σ destabilizing contribution. Electronegativity of each substituent on phosphorus plays a role as more electron-donating ones give greater degrees of antiaromatic sigma destabilization. This has been confirmed by Natural Population Analysis (NPA), where the energy changes of the reactions below were calculated with interactions between the C–C double bond and phosphorus both turned on and off by manipulating Fock matrix elements:

Destabilization energies were the differences between corresponding reactions:

Destabilization energy = energy (1) − energy (2) Destabilization Energy with different Y groups: Y = F > OH > Cl > NH2 > Br > I > CH3 > H This series is in accordance with the trend of electronegativity of the ligand atoms. Natural bond orbital (NBO) analysis provides possible Lewis structures of a molecule and has been carried out to assess the structure of H2C2PF+2. Similar to the aromatic cyclopropenium ion, the phosphorus analog shows a resonance between the structure with carbon-carbon double bond (1, 72.02%) and the ones with carbon-phosphorus double bond (3a and 3b, 7.88% combined). In addition, the ring-opening forms 2a and 2b combined also occupy 9.08% weight.

References

Illustrations

Worked examples

Example 1 — a first encounter with Phosphirenium ion

Start with the simplest possible case. Write down what Phosphirenium ion 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 Phosphirenium ion 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 Phosphirenium ion 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 Phosphirenium ion

In research: Phosphirenium ion 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 Phosphirenium ion 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: Phosphirenium ion is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cations, Organophosphorus compounds, Quaternary phosphonium compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Phosphirenium ion 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 Phosphirenium ion in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Phosphirenium ion 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 Phosphirenium ion out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Phosphirenium ion in simple terms?

Why does Phosphirenium ion 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 Phosphirenium ion?

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 Phosphirenium ion.