Trans effect
Encyclopedia
In inorganic chemistry
, the trans effect is the labilization
(making unstable) of ligand
s that are trans to certain other ligands, which can thus be regarded as trans-directing ligands. It is attributed to electronic effect
s and it is most notable in square planar complex
es, although it can also be observed for octahedral 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.
The classic example of the trans effect is the synthesis of cisplatin
. Starting from PtCl42−, the first NH3 ligand is added to any of the four equivalent positions at random, but the second NH3 is added cis to the first one, because Cl− has a larger trans effect than NH3:
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.
, 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.
Inorganic chemistry
Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry...
, the trans effect is the labilization
Lability
Lability refers to something that is constantly undergoing change or something that is likely to undergo change.-Chemistry:The term is used to describe a relatively unstable and transient chemical species...
(making unstable) of ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s that are trans to certain other ligands, which can thus be regarded as trans-directing ligands. It is attributed to electronic effect
Electronic effect
An electronic effect influences the structure, reactivity, or properties of 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 the electronic structure and the geometry of a...
s and it is most notable in square planar complex
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
es, although it can also be observed for octahedral 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−FluorideFluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
, H2O, OH−HydroxideHydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
< NH3AmmoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
< pyPyridinePyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
< Cl−ChlorideThe chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
< Br−BromideA bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.-Natural occurrence:...
< I−IodideAn iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
, SCN−ThiocyanateThiocyanate is the anion [SCN]−. It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates...
, NO2−NitriteThe nitrite ion has the chemical formula NO2−. The anion is symmetric with equal N-O bond lengths and a O-N-O bond angle of ca. 120°. On protonation the unstable weak acid nitrous acid is produced. Nitrite can be oxidised or reduced, with product somewhat dependent on the oxidizing/reducing agent...
, SC(NH2)2ThioureaThiourea is an organosulfur compound of with the formula SC2 . It is structurally similar to urea, except that the oxygen atom is replaced by a sulfur atom, but the properties of urea and thiourea differ significantly. Thiourea is a reagent in organic synthesis. "Thioureas" refers to a broad...
, Ph− < SO32−SulfiteSulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...
< PR3PhosphinePhosphine is the compound with the chemical formula PH3. It is a colorless, flammable, toxic gas. Pure phosphine is odourless, but technical grade samples have a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphine...
, AsR3ArsineArsine is the chemical compound with the formula AsH3. This flammable, pyrophoric, and highly toxic gas is one of the simplest compounds of arsenic...
, SR2ThioetherA thioether is a functional group in organosulfur chemistry with the connectivity C-S-C as shown on right. Like many other sulfur-containing compounds, volatile thioethers have foul odors. A thioether is similar to an ether except that it contains a sulfur atom in place of the oxygen...
, CH3− < H−HydrideIn chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
, NONitric oxideNitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
, COCarbon monoxideCarbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, CN−CyanideA cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
, C2H4EthyleneEthylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
The classic example of the trans effect is the synthesis of cisplatin
Cisplatin
Cisplatin, cisplatinum, or cis-diamminedichloroplatinum is a chemotherapy drug. It is used to treat various types of cancers, including sarcomas, some carcinomas , lymphomas, and germ cell tumors...
. Starting from PtCl42−, the first NH3 ligand is added to any of the four equivalent positions at random, but the second NH3 is added cis to the first one, because Cl− has a larger trans effect than NH3:
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 crystallographyX-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
, 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.
Further reading
- Quagliano, J. V.; Schubert, Leo. The Trans Effect in Complex Inorganic Compounds. Chem. Rev. 1952, 50, 201-260.
- Basolo, F.Fred BasoloFred Basolo was an American inorganic chemist. He received his Ph.D. at the University of Illinois at Urbana-Champaign in 1943 with John C. Bailar. Basolo spent his entire professional career at Northwestern University...
; Pearson, R. G.Ralph PearsonRalph G. Pearson is a physical inorganic chemist best known for the development of the concept of hard and soft acids and bases ....
The trans effect in metal complexes. Prog. Inorg. Chem. 1962, 4, 381-453. - Hartley, F. R. The cis- and trans-effects of ligands. Chem. Soc. Rev. 1973, 2, 163-179.