- Trans effect
In
inorganic chemistry , the trans effect is the labilization ofligand strans to certain other ligands, which can thus be regarded as trans directing ligands. It is attributed toelectronic effect s and it is most notable in square planar complexes, although it can also be observed for octahedral complexes.Coe, B. J.; Glenwright, S. J. Trans-effects in octahedral transition metal complexes. "Coordination Chemistry Reviews" 2000, "203", 5-80.]In addition to this "kinetic trans effect", trans ligands also have an influence on the ground state of the molecule, notably on bond lengths and stability. Some authors prefer the term trans influence to distinguish it from the kinetic effect,Cite book | author =
Robert H. Crabtree | title = The Organometallic Chemistry of the Transition Metals | year = 2005 | edition = 4th edition | isbn = 0-471-66256-9 | publisher = Wiley-Interscience | location = New Jersey] 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 , [Kauffmann, G. B. I'lya I'lich Chernyaev (1893-1966) and the Trans Effect. "J. Chem. Educ." 1977, "54", 86-89.] who recognized it and gave it a name in 1926. [Chernyaev, I. I. The mononitrites of bivalent platinum. I. "Ann. inst. platine" (USSR) 1926, "4", 243-275.]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
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 generally 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 where the ligand trans to the one with the largest trans effect is eliminated.tructural 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 atomic orbitals involved. [Anderson, K. M.; Orpen, A. G. On the relative magnitudes of the "cis" and "trans" influences in metal complexes. "Chem. Commun." 2001, 2682-2683. doi|10.1039/b108517b]References
Further reading
* Quagliano, J. V.; Schubert, Leo. The Trans Effect in Complex Inorganic Compounds. "Chem. Rev." 1952, "50", 201-260.
* Basolo, F. 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. doi|10.1039/CS9730200163
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