Ammonia trans effect

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

Kinetic inertness, evidently caused by the electronic configuration, leads to a remarkable unreactivity of the Rh-H bond to hydrolysis. In the absence of air, it is unaffected by ammonia solution in dilute solution, the ammonia tram to hydride is reversibly replaced by water, showing that the hydride has a trans-effect parallel to its trans-influence. 

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],... 

The trans-effect can be used synthetically. In the reaction of Br- with Au(NH3)4+, the introduction of the first bromine weakens the Au—N bond trans to it so that the introduction of a second bromine is both sterospecifically trans and rapid. (A similar effect occurs in the corresponding chloride.) The third and fourth ammonia molecules are replaced with difficulty, permitting the isolation of AuBr2(NH3)2 (second-order rate constants at 25°C are k] = 3.40, k2 = 6.5, k2 = 9.3 x 10-5 and k4 — 2.68 x 10 2lmor s l at 25°C) [141]. 

It appears that one of these rate constants (kc or kt) is 10 times as large as the other and anyone who has worked in platinum chemistry will say, of course, that chloride has a greater trans effect than ammonia. These are both weak trans directors, but one would expect the fast reaction site to be the cw-chlorides in other words, the chloride which is trans to a chloride, the cis-diaquo isomer, is the one which should be formed rapidly. 

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. 

In 1926, Chernyaev introduced the concept of the trans effect in platinum chemistry. In reactions of square-planar Pt(II) compounds, ligands trans to chloride are more easily replaced than those trans to ligands such as ammonia chloride is said to have a stronger trans effect than ammonia. When coupled with the fact that chloride itself is more easily replaced than ammonia, this trans effect allows the formation of isomeric... 

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,... 

The cis isomer has been prepared by the action of aqueous ammonia on ammonium or potassium tetrachloroplati-nate(II), while the trans isomer has been prepared by the action of heat or concentrated hydrochloric acid on tetraammineplatinum(II) chloride. These reactions are in accord with Peyrone s rule and J0rgensen s rule, respectively, both of which are included in Chernyaev s trans effect. 

In reaction (a), after the first ammonia is replaced, the second replacement is trans to the first Cl. In reaction (b), the second replacement is trans to d (replacement of ammonia by chloride is also possible, resulting in formation of the reactant [PtClJ ). The first steps in reactions (c) through (f) are the possible replacements, with nearly equal probabilities for replacement of ammonia or pyridine. The second steps of (c) through (f) depend on the trans effect of Cl . Both steps of (g) and (h) depend on the greater lability of chloride. By using reactions such as these, specific isomers can be prepared. Chemyaev prepared a wide variety of compounds and established the order of trans-effect ligands ... 

---