Trans-effect series

This would give rise to an order of the kind R2C=CR2, CO > CN- > N02 > SCN- > 1 > Br > NH3 > OH-. Therefore, a combination of a-and 7r-effects can be considered to give rise to the observed trans-effect series. 

A 7r-bonding explanation notes that several ligands high in the trans-effect series are good -acceptors and thus siphon off 7r-density, making the region trans to it electron deficient and thus attractive to ligands that are electron rich. 

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],... 

Like the isoelectronic Pd2+ and Pt2+, Au3+ exhibits both trans-effects and trans-influence. Table 4.13 (above) lists structural data for a number of complexes AuL3L, showing how the disparity in Au-X distances between cis-and trans-X depends on the position of L in the trans-effect series for the compounds listed, the effect is least noticeable in AuC13NH3 as these two ligands are proximate in the series. 

Now, consider leaving group orders in general. The order of inertness at Pt(II) (which corresponds to the theoretically comprehensible trans effect series) is largely opposite to the order of ligands in the well-known spectrochemical series. ... 

It is quite likely that both factors contribute to the order of ligands in the trans effect series, but as with other a- irarguments (Chapters 11.12. and 18), there is a wide spectrum of opinion regarding the relative importance of each factor. 

Basolo555 noted that reactions of Rh111 amine complexes were not dramatically accelerated by hydroxide ion, but did show that substitutions in base do follow the standard kobs = k, + k2[OH" ] format, with k, representing the first-order aquation observed in acidic solution, and k2 representing the second-order base-catalyzed path. Poe has studied the kinetics of the base hydrolysis of a variety of frans-[Rh(en)2XY]"+ complexes (Table 41). Studies on the base hydrolyses of trans-[Rh(en)2(OH)X]+ (X = Cl, Br, I) showed that the coordinated hydroxide has an intrinsic kinetic tram effect comparable to that of Cl" but that its position in a thermodynamic trans effect series is much higher.635 For oms-[Rh(en)2X2]+ (X = Cl, Br), virtually complete tram -+ cis isomerization occurs upon hydrolysis in base, and ca. 50% isomerization is observed when X = I.653 No such... 

Starting with [PtCU] , you need to perform two separate ligand substitution reactions. In one, NH3 will replace Cl" ion. In the other, NOi" ion will replace Cl" ion. The question is which substitution to perform first. According to the trans effect series shown in Section 21.4, the strength of the irons effect for die three ligands in question is... 

E21.14 (a) [PRPRjIj] + 2C1". The first Cl ion substitution will produce the reaction intermediate [Pt(PR3)3CI]". This will be attacked by the second equivalent of Cl" ion. Since phosphines are higher in the trans effect series than chloride ion, a phosphine trans to another phosphine will be substituted, giving m-[PtCl2(PRj)2]. 

C) c -IPt(NH3)j(py)2] " + 2Cr You should assume that NH3 and pyridine are about equal in the trans effect series. The first Cl" ion substitution will produce one of two reaction intermediates, cw-[PtCl(NH3)(py)2] or cis-[PtCl(NH3)2(py)]. Either of these will produce the same product when they are attacked by the second equivalent of cr ion. This is because Cl" is higher in the trans effect series than NH3 or pyridine. Therefore, the ligand trans to the first Cl" ligand will be substituted faster, and the product will be rra j-[PtCl2(NH3)(py)]. 

The trans-effect series has proved useful in rationalising known synthetic procedures and devising new ones. Let us consider the synthesis of cis and trans isomers of [Pt(NH3)2Cl2]. The cis isomer is... 

The trans effect series has proved very useful in rationalizing known synthetic procedures and in devising new ones. As an example we may consider the synthesis of the cis- and rranj-isomers of [Pt(NH3)2Cl2]. The synthesis of the ra-isomer is accomplished by treatment of the [PtCl4]2 " ion with ammonia (reaction 21-30). Since Cl- has a greater trany-directing Cl Cl Cl NH, Cl NH,... 

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