chemistry

Pentamethylantimony

Pentamethylantimony 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 Pentamethylantimony rather than just read about it. In short: Pentamethylantimony or pentamethylstiborane is an organoantimony compound containing five methyl groups bound to an antimony atom with formula Sb(CH3)5. It is an example of a hypervalent compound.

Key takeaways

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

Reference excerpt

Pentamethylantimony or pentamethylstiborane is an organoantimony compound containing five methyl groups bound to an antimony atom with formula Sb(CH3)5. It is an example of a hypervalent compound. The molecular shape is trigonal bipyramidal. Some other antimony(V) organometallic compounds include pentapropynylantimony (Sb(C≡C−CH3)5) and pentaphenyl antimony (Sb(C6H5)5). Other known pentamethyl-pnictides include pentamethylbismuth and pentamethylarsenic.

Production Pentamethylantimony can be made by reacting Sb(CH3)3Br2 with two equivalents of methyl lithium. Another production route is to convert trimethylstibine to the trimethyl antimony dichloride, and then replace the chlorine with methyl groups with methyl lithium.

Sb(CH3)3 + Cl2 → Sb(CH3)3Cl2 Sb(CH3)3Cl2 + 2 LiCH3 → Sb(CH3)5 + 2 LiCl

Properties Pentamethylantimony is colourless. At -143 °C it crystallizes in the orthorhombic system with space group Ccmm. Unit cell dimensions are a=6.630 Å b=11.004 Å c=11.090 Å. There are four formula per unit cell. Unit cell volume is 809.1 Å3. The trigonal bipyramid shape has three equatorial positions for carbon, and two axial positions at the peaks of the pyramids. The length of the antimony-carbon bond is around 214 pm for equatorial methyl groups and 222 pm for the axial positions. The bond angles are 120° for ∠C-Sb-C across the equator, and 90° for ∠C-Sb-C between equator and axis. The molecules rapidly change carbon atom position, so that in NMR spectrum as low as −100 °C, there is only one kind of hydrogen position. Pentamethylantimony is more stable than pentamethylbismuth, because in lower energy trimethylbismuth, the non-bonding pair of electrons is more shielded due to the f-electrons and the lanthanoid contraction. Trimethylantimony is higher in energy, and thus less is released in a decomposition of pentamethylantimony. Pentamethylantimony can be stored as a liquid in clean glass at room temperature. Pentamethylantimony melts at -19 °C. Although it decomposes when boiling is attempted and can explode, it has a high vapour pressure at 8 mmHg at 25 °C. There are two absorption bands in the ultraviolet at 2380 and 2500 Å.

Reactions Pentamethylantimony reacts with methyl lithium to yield a colourless lithium hexamethylantimonate in tetrahydrofuran.

Sb(CH3)5 + LiCH3 → Li(thf)Sb(CH3)6 Pentamethylantimony reacts with silsesquioxanes to yield tetramethylstibonium silsesquioxanes. eg (cyclo-C6H11)7Si7O9(OH)3 yields (cyclo-C6H11)7Si7O9(OSb(CH3)4)3. The reaction happens quickly when there are more than two OH groups. Phosphonic acids and phosphinic acids combine with pentamethylantimony to yield compounds like (CH3)4SbOP(O)Ph2, (CH3)4SbOP(O)(OH)Ph and (CH3)4SbOP(O)(OH)3, eliminating methane. Stannocene Sn(C5H5)2 combines with pentamethylantimony to produce bis(tetramethylstibonium)tetracyclopentadienylstannate ([(CH3)4Sb]2Sn(C5H5)4). Pentamethylantimony reacts with many very weak acids to form a tetramethylstibonium salt or tetramethylstibonium derivative with the acid. Such acids include water (H2O), alcohols, thiols, phenol, carboxylic acids, hydrogen fluoride, thiocyanic acid, hydrazoic acid, difluorophosphoric acid, thiophosphinic acids, and alkylsilols. With halogens, pentamethylantimony has one or two methyl groups replaced by the halogen atoms. Lewis acids also react to form tetramethyl stibonium salts, including [(CH3)4Sb]TlBr4, [(CH3)4Sb][CH3SbCl5], Pentamethylantimony reacts with the surface of silica to coat it with Si-O-Sb(CH3)4 groups. Over 250 °C this decomposes to Sb(CH3) and leaves methyl groups attached to the silica surface.

References

Illustrations

Worked examples

Example 1 — a first encounter with Pentamethylantimony

Start with the simplest possible case. Write down what Pentamethylantimony 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 Pentamethylantimony 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 Pentamethylantimony 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 Pentamethylantimony

In research: Pentamethylantimony 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 Pentamethylantimony 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: Pentamethylantimony is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hypervalent molecules, Methyl complexes, Organoantimony compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Pentamethylantimony 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 Pentamethylantimony in 20 minutes

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

Frequently asked questions

What is Pentamethylantimony in simple terms?

Pentamethylantimony or pentamethylstiborane is an organoantimony compound containing five methyl groups bound to an antimony atom with formula Sb(CH3)5. It is an example of a hypervalent compound.

Why does Pentamethylantimony 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 Pentamethylantimony?

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 Pentamethylantimony.