inorganic chemistry Inorganic chemistry deals with chemical synthesis, synthesis and behavior of inorganic compound, inorganic and organometallic chemistry, organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subj ...

, the trans effect is the increased

lability Lability refers to the degree that something is likely to undergo change. It is the opposite ( antonym) of stability. Biochemistry In reference to biochemistry, this is an important concept as far as kinetics is concerned in metalloprotein ...

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s that are

trans Trans- is a Latin prefix meaning "across", "beyond", or "on the other side of". Used alone, trans may refer to: Sociology * Trans, a sociological term which may refer to: ** Transgender, people who identify themselves with a gender that di ...

to certain other ligands, which can thus be regarded as trans-directing ligands. It is attributed to

electronic effect An electric effect influences the structure, reactivity, or properties of a molecule but is neither a traditional bond nor a steric effect. In organic chemistry, the term stereoelectronic effect is also used to emphasize the relation between t ...

s and it is most notable in square planar

complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...

es, although it can also be observed for octahedral complexes. The analogous

cis effect In inorganic chemistry, the cis effect is defined as the labilization (or destabilization) of CO ligands that are ''cis'' to other ligands. CO is a well-known strong pi-accepting ligand in organometallic chemistry that will labilize in the ''cis ...

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 Ilya Ilich Chernyaev (21 January 1893 – 30 September 1966) was a Russian and Soviet chemist who worked on inorganic chemistry and identified what is termed in coordination chemistry as the trans effect. He was a specialist on the refining of rhodi ...

, 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 Cisplatin is a chemical compound with chemical formula, formula ''cis''-. It is a coordination complex of platinum that is used as a chemotherapy medication used to treat a number of cancers. These include testicular cancer, ovarian cancer, c ...

and its

trans isomer Trans- is a Latin prefix meaning "across", "beyond", or "on the other side of". Used alone, trans may refer to: Sociology * Trans, a sociological term which may refer to: ** Transgender, people who identify themselves with a gender that dif ...

. The complex PtCl₄²⁻ reacts with ammonia to give tCl₃NH₃sup>−. A second substitution by ammonia gives cis- tCl₂(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 Magnus's green salt is the inorganic compound with the chemical formula, formula t(NH3)4PtCl4]. This salt is named after Heinrich Gustav Magnus, who, in the early 1830s, first reported the compound. The compound is a linear chain compound, cons ...

. As a result,

is produced commercially via tI₄sup>2− as first reported by Dhara in 1970. : If, on the other hand, one starts from Pt(NH₃)₄²⁺, the ''

'' 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–d_π 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 X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

, 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. Transeinfluss

Kinetic trans effect

Structural trans effect

References

Further reading