chemistry

Trigonal bipyramidal molecular geometry

Trigonal bipyramidal molecular geometry 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 Trigonal bipyramidal molecular geometry rather than just read about it. In short: In chemistry, a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular bipyramid. This is one geometry for which the bond angles surrounding the central atom are not identical (see also pentagonal bipyramid), because there is no geometrical arrangement with five terminal atoms in equivalent positions.

Key takeaways

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

Reference excerpt

In chemistry, a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular bipyramid. This is one geometry for which the bond angles surrounding the central atom are not identical (see also pentagonal bipyramid), because there is no geometrical arrangement with five terminal atoms in equivalent positions. Examples of this molecular geometry are phosphorus pentafluoride (PF5), and phosphorus pentachloride (PCl5) in the gas phase.

Axial (or apical) and equatorial positions

The five atoms bonded to the central atom are not all equivalent, and two different types of position are defined. For phosphorus pentachloride as an example, the phosphorus atom shares a plane with three chlorine atoms at 120° angles to each other in equatorial positions, and two more chlorine atoms above and below the plane (axial or apical positions). According to the VSEPR theory of molecular geometry, an axial position is more crowded because an axial atom has three neighboring equatorial atoms (on the same central atom) at a 90° bond angle, whereas an equatorial atom has only two neighboring axial atoms at a 90° bond angle. For molecules with five identical ligands, the axial bond lengths tend to be longer because the ligand atom cannot approach the central atom as closely. As examples, in PF5 the axial P−F bond length is 158 pm and the equatorial is 152 pm, and in PCl5 the axial and equatorial are 214 and 202 pm respectively. In the mixed halide PF3Cl2 the chlorines occupy two of the equatorial positions, indicating that fluorine has a greater apicophilicity or tendency to occupy an axial position. In general ligand apicophilicity increases with electronegativity and also with pi-electron withdrawing ability, as in the sequence Cl < F < CN. Both factors decrease electron density in the bonding region near the central atom so that crowding in the axial position is less important.

Related geometries with lone pairs The VSEPR theory also predicts that substitution of a ligand at a central atom by a lone pair of valence electrons leaves the general form of the electron arrangement unchanged with the lone pair now occupying one position. For molecules with five pairs of valence electrons including both bonding pairs and lone pairs, the electron pairs are still arranged in a trigonal bipyramid but one or more equatorial positions is not attached to a ligand atom so that the molecular geometry (for the nuclei only) is different. The seesaw molecular geometry is found in sulfur tetrafluoride (SF4) with a central sulfur atom surrounded by four fluorine atoms occupying two axial and two equatorial positions, as well as one equatorial lone pair, corresponding to an AX4E molecule in the AXE notation. A T-shaped molecular geometry is found in chlorine trifluoride (ClF3), an AX3E2 molecule with fluorine atoms in two axial and one equatorial position, as well as two equatorial lone pairs. Finally, the triiodide ion (I−3) is also based upon a trigonal bipyramid, but the actual molecular geometry is linear with terminal iodine atoms in the two axial positions only and the three equatorial positions occupied by lone pairs of electrons (AX2E3); another example of this geometry is provided by xenon difluoride, XeF2.

Berry pseudorotation Isomers with a trigonal bipyramidal geometry are able to interconvert through a process known as Berry pseudorotation. Pseudorotation is similar in concept to the movement of a conformational diastereomer, though no full revolutions are completed. In the process of pseudorotation, two equatorial ligands (both of which have a shorter bond length than the third) "shift" toward the molecule's axis, while the axial ligands simultaneously "shift" toward the equator, creating a constant cyclical movement. Pseudorotation is particularly notable in simple molecules such as phosphorus pentafluoride (PF5).

Illustrations

Worked examples

Example 1 — a first encounter with Trigonal bipyramidal molecular geometry

Start with the simplest possible case. Write down what Trigonal bipyramidal molecular geometry 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 Trigonal bipyramidal molecular geometry 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 Trigonal bipyramidal molecular geometry 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 Trigonal bipyramidal molecular geometry

In research: Trigonal bipyramidal molecular geometry 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 Trigonal bipyramidal molecular geometry 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: Trigonal bipyramidal molecular geometry is common in secondary-school and first-year university syllabi. It links to neighbouring topics Molecular geometry, Stereochemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Trigonal bipyramidal molecular geometry 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 Trigonal bipyramidal molecular geometry in 20 minutes

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

Frequently asked questions

What is Trigonal bipyramidal molecular geometry in simple terms?

Why does Trigonal bipyramidal molecular geometry 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 Trigonal bipyramidal molecular geometry?

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 Trigonal bipyramidal molecular geometry.