# Trans effect

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{{Short description|In chemistry, tendency of trans ligands to change into other ligands}}
In [inorganic chemistry](/source/inorganic_chemistry), the '''trans effect''' is the increased [lability](/source/Lability) of [ligand](/source/ligand)s that are [trans](/source/Cis-trans_isomerism) to certain other ligands, which can thus be regarded as trans-directing ligands. It is attributed to [electronic effect](/source/electronic_effect)s and it is most notable in square planar [complex](/source/Complex_(chemistry))es, although it can also be observed for octahedral complexes.<ref name=coe>{{cite journal | doi=10.1016/S0010-8545(99)00184-8 | title=Trans-effects in octahedral transition metal complexes | date=2000 | last1=Coe | first1=Benjamin J. | last2=Glenwright | first2=Susan J. | journal=Coordination Chemistry Reviews | volume=203 | pages=5–80 }}</ref> The analogous [cis effect](/source/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,<ref name=crabtree>{{Cite book | author = [Robert H. Crabtree](/source/Robert_H._Crabtree) | title = The Organometallic Chemistry of the Transition Metals | year = 2005 | edition = 4th | isbn = 0-471-66256-9 | publisher = Wiley-Interscience | location = New Jersey}}</ref> while others use more specific terms such as '''structural trans effect''' or '''thermodynamic trans effect'''.<ref name=coe/>

The discovery of the trans effect is attributed to [Ilya Ilich Chernyaev](/source/Ilya_Ilich_Chernyaev),<ref>{{cite journal | doi=10.1021/ed054p86 | title=Il'ya Il'ich Chernyaev (1893-1966) and the trans effect | date=1977 | last1=Kauffman | first1=George B. | journal=Journal of Chemical Education | volume=54 | issue=2 | page=86 | bibcode=1977JChEd..54...86K }}</ref> who recognized it and gave it a name in 1926.<ref>{{cite journal |last=Chernyaev |first=I. I. |title=The mononitrites of bivalent platinum. I. |journal=Ann. Inst. Platine (USSR) |year=1926 |volume=4 |pages=243–275}}</ref>

==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<sup>−</sup>](/source/fluoride), [H<sub>2</sub>O](/source/water_(molecule)), [OH<sup>−</sup>](/source/hydroxide) < [NH<sub>3</sub>](/source/ammonia) < [py](/source/pyridine) < [Cl<sup>−</sup>](/source/chloride) < [Br<sup>−</sup>](/source/bromide) < [I<sup>−</sup>](/source/iodide), [SCN<sup>−</sup>](/source/thiocyanate), [NO<sub>2</sub><sup>−</sup>](/source/nitrite), [SC(NH<sub>2</sub>)<sub>2</sub>](/source/thiourea), [Ph<sup>−</sup>](/source/phenyl) < [SO<sub>3</sub><sup>2−</sup>](/source/sulfite) < [PR<sub>3</sub>](/source/phosphine), [AsR<sub>3</sub>](/source/arsine), [SR<sub>2</sub>](/source/thioether), [CH<sub>3</sub><sup>−</sup>](/source/methyl) < [H<sup>−</sup>](/source/hydride), [NO](/source/nitric_oxide), [CO](/source/carbon_monoxide), [CN<sup>−</sup>](/source/cyanide), [C<sub>2</sub>H<sub>4</sub>](/source/ethylene)

One classic example of the trans effect is the synthesis of [cisplatin](/source/cisplatin) and its [trans isomer](/source/Transplatin).<ref>{{cite book |volume=7|year=1963|author=George B. Kauffman |author2=Dwaine O. Cowan |title=Inorganic Syntheses |chapter=''cis'' - and ''trans'' -Dichlorodiammineplatinum(II) |pages=239–245|doi=10.1002/9780470132388.ch63|isbn=978-0-470-13238-8}}</ref> The complex PtCl<sub>4</sub><sup>2−</sup> reacts with ammonia to give [PtCl<sub>3</sub>NH<sub>3</sub>]<sup>−</sup>. A second substitution by ammonia gives cis-[PtCl<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub>], showing that Cl- has a greater trans effect than NH<sub>3</sub>. The procedure is however complicated by the production of [Magnus's green salt](/source/Magnus's_green_salt).<ref name=Alderden>{{cite journal|title=The Discovery and Development of Cisplatin| vauthors = Alderden RA, Hall MD, Hambley TW |s2cid= 29546931 |journal= Journal of Chemical Education|date= 2006|volume= 83 |issue=5 |page=728 |doi=10.1021/ed083p728|bibcode=2006JChEd..83..728A}}</ref> As a result, [cisplatin](/source/cisplatin) is produced commercially via [PtI<sub>4</sub>]<sup>2−</sup> as first reported by Dhara in 1970.<ref>{{cite journal|title=A rapid method for the synthesis of cis-[Pt(NH<sub>3</sub>)<sub>2</sub>Cl<sub>2</sub>] | vauthors = Dhara SC |journal=Indian J. Chem. |date=1970|volume= 8|pages= 123–134}}</ref>

:frameless|upright=3.0|Synthesis of cisplatin using the trans effect

If, on the other hand, one starts from Pt(NH<sub>3</sub>)<sub>4</sub><sup>2+</sup>, the ''[trans](/source/Cis-trans_isomerism)'' product is obtained instead:
:frameless|upright=3.0|Synthesis of transplatin using the trans effect

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–d<sub>π</sub> 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](/source/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.<ref name=crabtree />
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==Structural trans effect==

The structural trans effect can be measured experimentally using [X-ray crystallography](/source/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.<ref>{{cite journal | doi=10.1039/b108517b | title=On the relative magnitudes of cis and trans influences in metal complexes | date=2001 | last1=Anderson | first1=Kirsty M. | last2=Orpen | first2=A. Guy | journal=Chemical Communications | issue=24 | pages=2682–2683 }}</ref>

[[File:Transeinfluss.png|thumb|400px|center|Example of the structural ''trans'' effect: the effect induced by [triethylphosphine](/source/triethylphosphine) ligands is stronger than induced by [chloride](/source/chloride) ion ligands.]]

==References==
{{Reflist}}

==Further reading==
* {{cite journal |doi=10.1021/cr60156a001 |title=The Trans Effect in Complex Inorganic Compounds |date=1952 |last1=Quagliano |first1=J. V. |last2=Schubert |first2=LEO. |journal=Chemical Reviews |volume=50 |issue=2 |pages=201–260 }}
* {{cite journal |last1=Basolo |first1=F. |last2=Pearson |first2=R. G. |year=1962 |title=The trans effect in metal complexes |journal=Prog. Inorg. Chem. |volume=4 |pages=381–453}}
* {{cite journal |doi=10.1039/CS9730200163 |title=The cis- and trans-effects of ligands |date=1973 |last1=Hartley |first1=F. R. |journal=Chemical Society Reviews |volume=2 |issue=2 |page=163 }}

Category:Coordination chemistry

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