chemistry

Trans effect

Trans effect 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 Trans effect rather than just read about it. In short: In inorganic chemistry, the trans effect is the increased lability of ligands that are trans to certain other ligands, which can thus be regarded as trans-directing ligands. It is attributed to electronic effects and it is most notable in square planar complexes, although it can also be observed for octahedral complexes.

Key takeaways

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

Reference excerpt

In inorganic chemistry, the trans effect is the increased lability of ligands that are trans to certain other ligands, which can thus be regarded as trans-directing ligands. It is attributed to electronic effects and it is most notable in square planar complexes, although it can also be observed for octahedral complexes. The analogous cis effect is most often observed in octahedral transition metal complexes. In addition to this kinetic trans effect, trans ligands also have an influence on the ground state of the molecule, the most notable ones being bond lengths and stability. Some authors prefer the term trans influence to distinguish it from the kinetic effect, while others use more specific terms such as structural trans effect or thermodynamic trans effect. The discovery of the trans effect is attributed to Ilya Ilich Chernyaev, who recognized it and gave it a name in 1926.

Kinetic trans effect The intensity of the trans effect (as measured by the increase in rate of substitution of the trans ligand) follows this sequence:

F−, H2O, OH− < NH3 < py < Cl− < Br− < I−, SCN−, NO2−, SC(NH2)2, Ph− < SO32− < PR3, AsR3, SR2, CH3− < H−, NO, CO, CN−, C2H4 One classic example of the trans effect is the synthesis of cisplatin and its trans isomer. The complex PtCl42− reacts with ammonia to give [PtCl3NH3]−. A second substitution by ammonia gives cis-[PtCl2(NH3)2], showing that Cl- has a greater trans effect than NH3. The procedure is however complicated by the production of Magnus's green salt. As a result, cisplatin is produced commercially via [PtI4]2− as first reported by Dhara in 1970.

If, on the other hand, one starts from Pt(NH3)42+, the trans product is obtained instead:

The trans effect in square complexes can be explained in terms of an addition/elimination mechanism that goes through a trigonal bipyramidal intermediate. Ligands with a high trans effect are in general those with high π acidity (as in the case of phosphines) or low-ligand lone-pair–dπ repulsions (as in the case of hydride), which prefer the more π-basic equatorial sites in the intermediate. The second equatorial position is occupied by the incoming ligand; due to the principle of microscopic reversibility, the departing ligand must also leave from an equatorial position. The third and final equatorial site is occupied by the trans ligand, so the net result is that the kinetically favored product is the one in which the ligand trans to the one with the largest trans effect is eliminated.

Structural trans effect The structural trans effect can be measured experimentally using X-ray crystallography, and is observed as a stretching of the bonds between the metal and the ligand trans to a trans-influencing ligand. Stretching by as much as 0.2 Å occurs with strong trans-influencing ligands such as hydride. A cis influence can also be observed, but is smaller than the trans influence. The relative importance of the cis and trans influences depends on the formal electron configuration of the metal center, and explanations have been proposed based on the involvement of the atomic orbitals.

References

Further reading Quagliano, J. V.; Schubert, LEO. (1952). "The Trans Effect in Complex Inorganic Compounds". Chemical Reviews. 50 (2): 201–260. doi:10.1021/cr60156a001. Basolo, F.; Pearson, R. G. (1962). "The trans effect in metal complexes". Prog. Inorg. Chem. 4: 381–453. Hartley, F. R. (1973). "The cis- and trans-effects of ligands". Chemical Society Reviews. 2 (2): 163. doi:10.1039/CS9730200163.

Illustrations

Worked examples

Example 1 — a first encounter with Trans effect

Start with the simplest possible case. Write down what Trans effect 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 Trans effect 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 Trans effect 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 Trans effect

In research: Trans effect 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 Trans effect 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: Trans effect is common in secondary-school and first-year university syllabi. It links to neighbouring topics Coordination chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Trans effect 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 Trans effect in 20 minutes

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

Frequently asked questions

What is Trans effect in simple terms?

Why does Trans effect 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 Trans effect?

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 Trans effect.