Coordination compounds trans effect

DNA polymerases, 5, 1007 Trans effect, 1,16, 26, 315 metal complexes, 2, 705, palladium(II) amine complexes, 5, 1115 platinum complexes, 5, 353, 493 six-coordinate compounds. 1, 49 T ransestcrification metal alkoxide synthesis, 2, 340 Transferases zinc, S, 1002... 

Gs, trans, or metal effects occur when, e.g., an octahedral coordination compound ML6 [7] is transformed into a new complex, either MLSX [2], where a ligand has been altered, or M L6 [5], where the metal M has been replaced by an isovalent metal M. The comparison of either [2] with [7], or [2] with [7], discloses either cis and trans effects or metal effects inasmuch as the properties of the ligands L are altered by this variation in the first case, the substituent effect of X is subdivided into the cis and trans effects. 

The presence of large deposits of platinum ores in Russia led to an intensive study of the coordination compounds of platinum early in the development of coordination chemistry. As a result of these studies by tbe Russian school, the first stereospecific displacement reaction (and first example of the trans effect) was discovered. Consider two means of forming diamminedichlorop]atinum(ll) (1) displacement of Cl ions from [PtCIJ2- by NHj (2) displacement of NH3 from [Pt(NH3)4]2+ by CP ions. It is found that two different isomers are formed ... 

Only a few compounds containing Ru, Os, Rh or Ir do not possess type C. Notable are the metalloporphyrin dimers with Ru = Ru (entry 3) or Rh-In bonds (entry 34), the alkyl or acyl rhodium(III) (entries 31, 36, 38), alkyl-iridium(III) (entry 39) or arylruthenium(III) compounds (entry 12). All these species contain pentacoordinate noble metal ions, Type B. A reason for this unusual behavior of the metal ions is seen in the strong trans effect of the axial ligand L in these systems which is a strong o-donor (metalloporphyrinyl, alkyl, aryl, acyl) which precludes further coordination in the trans position of L. The reluctance of RhMe(OEP) to accept an axial ligand is shown by its crystallization as a re-dimer from n-hexane in presence of the base 1-Meim. 

Other theoretical considerations for synthetic coordination chemistry (isolobal and isoelectronic analogies, chelate, cis and trans effects, factors affecting the acid/ base properties of coordination compounds, bond theories, etc.) are covered in detail in excellent recent monographs [3,34b,104,106] and, for this reason, are not presented in this book. 

An important application of the trans effect is the synthesis of specific isomers of coordination compounds. Equations (3) and (4) show how the cis and trans isomers of Pt(NH3)2Cl2 can be prepared selectively by taking advantage of the trans effect order Cl > NH3. This example is also of practical interest because the cis isomer is an important antitumor drug, but the trans isomer is ineffective. In each case the first step of the substitution can give only one isomer. In equation (3) the cis isomer is formed in the second step because the Cl trans to Cl is more labile than the Cl trans to the lower trans effect ligand, ammonia. On the other hand, in equation (4) the first Cl to substitute labilizes the ammonia trans to itself to give the trans dichloride as final product. 

Platinum is found in compounds having a smaller range of oxidation states than is the case for many of the earlier elements in the periodic table. The differences between it and Pd are somewhat more marked than for analogous pairs of earlier elements. The coordination numbers see Coordination Numbers Geometries) tend to be lower than for earlier elements a CN of six is rarely exceeded and a CN of four is common. Many important concepts in coordination chemistry, such as square-planar coordination and the trans effect, were first discovered in Pt complexes. The high electronegativity of the element is reflected in a poor 7t-basic character, which helps account for the lack of a binary carbonyl. 

Ligand substitution is one of the most characteristic reactions of coordination compounds. If in the early stages of the development of coordination chemistry a ligand substitution reaction served as a synthetic method, then later on, especially after Werner, such reactions were widely employed both to solve structural problems (viz., geometric isomerism), and to elucidate the nature of a trans effect. 

Tetrahedral and Planar Complexes. Two isomers of [Pt(NH3)2Cl2] were known, and Werner (67) proposed that they were cis and trans isomers with a planar configuration about Pt(II). Study of the reactions of this and other Pt(II) compounds led Werner to introduce the concept we now call the trans effect. Because isomerism of the type expected for a planar configuration was known only for complexes of Pt(II) and a few examples for Pd(II), the existence of anything other than a tetrahedral configuration for 4-coordinate species was questioned. 

In order to confirm our SIMPLE NMR findings we recorded the Sn SIMPLE spectra of the 4-cis, 4 trans S ds, 5-trau and gem compounds. A secondary isotopic effect on the Sn chemical shift of +0.058, +0.036 and + 0.051 ppm is observed for compounds 4-cu, 5-m and 6-gan respectively (Figure 27). These isotopic shifts can reasonably result only from a two-bond effect through the pathway D-O- Sn and confirm the HO- Sn coordination. Compounds 4-trans and S-trans display no isotopic Sn shifts, in agreement with our SIMPLE results. 

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