Substitution trans effect

In the case of the trans-complex, only the two chloride ions are substituted, the trans-effect of ammonia being too low to give substitution with the result that white needle crystals of trans-[Pt(NH3)2(tu)2]Cl2 are formed [73],... 

Because Cl- has a stronger trans-effect than NH3, a group opposite to Cl-is replaced in the second substitution. Similarly, in the synthesis of the transisomer by heating Pt(NH3)4+ with Cl-(Figure 3.86), it is the ligand trans to chloride that is again replaced in the second step. 

The cis- and trans-isomers of [Pt(NH3)(N02)Cl2]- have been synthesized from PtCl - merely by choice of the order of ligand substitution (Figure 3.87). (In the second step, chloride trans to chloride is more labile.) The second substitution is dictated by N02 having a higher position in the trans-effect series than chloride [144],... 

The moderate specificity for forming m-2-butene initially (see Fig. 1) is again consistent with equatorial orientation of isopropyl the rather low cis specificity indicates only a moderate preference for equatorial orientation of the a-methyl, probably because of the offsetting weak repulsions caused by cis- 1,2-dimethyl-substitution. This effect is absent in the metathesis of tra i-4-methyl-2-pentene, and trans specificity for... 

In acidic aqueous solution the complexation of the chloroaqua derivatives of cis- and trans-DDP involves substitution of the aqua ligand with the nucleobase [15]. The trans derivative reacts 10 times faster than the cis isomer with inosine at pH 3, in line with the trans-effect Cl" > NH3. It has been shown that trans- [PtCl(NH3)2(H20)]+ behaves like a mono-... 

It has long been known that substitution at the anion of Zeise s salt, [Pt(CH=CH2)Cl3], is, thanks to the high trans effect of the coordinated ethene, very fast. Recent developments in low-temperature stopped-flow apparatus have now permitted the study of the kinetics of substitution at Zeise s and other [Pt(alkene)Cl3] anions in methanol solution. These substitutions obey the customary two-term rate law (i.e. with kohs = ki+ /s3[nucleophile]), with large negative AS values for the k2 term as expected for Sn2 processes (196). 

It is observed that insertion into a zirconacyclopentene 163, which is not a-substituted on either the alkyl and alkenyl side of the zirconium, shows only a 2.2 1 selectivity in favor of the alkyl side. Further steric hindrance of approach to the alkyl side by the use of a terminally substituted trans-alkene in the co-cyclization to form 164 leads to complete selectivity in favor of insertion into the alkenyl side. However, insertion into the zirconacycle 165 derived from a cyclic alkene surprisingly gives complete selectivity in favor of insertion into the alkyl side. In the proposed mechanism of insertion, attack of a carbenoid on the zirconium atom to form an ate complex must occur in the same plane as the C—Zr—C atoms (lateral attack 171 Fig. 3.3) [87,88]. It is not surprising that an a-alkenyl substituent, which lies precisely in that plane, has such a pronounced effect. The difference between 164 and 165 may also have a steric basis (Fig. 3.3). The alkyl substituent in 164 lies in the lateral attack plane (as illustrated by 172), whereas in 165 it lies well out of the plane (as illustrated by 173). However, the difference between 165 and 163 cannot be attributed to steric factors 165 is more hindered on the alkyl side. A similar pattern is observed for insertion into zirconacyclopentanes 167 and 168, where insertion into the more hindered side is observed for the former. In the zirconacycles 169 and 170, where the extra substituent is (3 to the zirconium, insertion is remarkably selective in favor of the somewhat more hindered side. 

The kinetics and mechanism of ligand substitution reactions of square-planar platinum(II) dimethyl sulfoxide complexes have been exhaustively studied (173), and these workers conclude that the cis and trans influences and the trans effects of Me2SO and ethylene are similar in magnitude whereas the cis effect of Me2SO is about 100 times as large as that of ethylene. The results for reaction (5), where the stability constants, Kt, are reported to be 1.5 x 108 (L = S-Me2SO) and 4.5 x 108 (L = ethylene) corroborate this analogy (213). 

Some kinetic data on the trans effect are now available. More detailed systematic studies are needed, but quantitative information does provide the magnitude of the trans labilizing ability of various ligands for substitution reactions in these systems. For complexes of the type [PtNH3LCl2]", where the leaving Cl" is trans to L, the trans effect order of L is approximately (28),... 

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