Stereochemistry and Mechanisms of Reactions of Werner Complexes

28 Stereochemistry and Mechanisms of Reactions of Werner Complexes F R E D BASOLO

Downloaded by TUFTS UNIV on December 7, 2015 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/ba-1967-0062.ch028

Northwestern University, Evanston, Ill.

Any detailed description of the mechanism of an octahedral substitution must also account for the stereochemical changes that accompany reaction. Werner recognized this and made use of it in his discussions of the stereochem istry of reactions of cobalt(III) complexes. The available experimental results can be explained on the basis of pos sible molecular rearrangements, and some cautious predictions can even be made. The base hydrolysis of cobalt(III)ammines appears to be unique in that it often occurs with rearrangement; it also affords the few known examples of optical inversion. These results can be ex plained by formation of a 5-coordinated species with a trigonal bipyramidal structure. Optically active metal complexes racemize by either an intramolecular or an in termolecular process. Substitution reactions of platinum metal complexes often occur with retention of configuration.

C tereoehemistry has played a major role in the development of chemistry, and it continues to be most significant. Werner made extensive use of the information available to him on the stereochemistry of metal com plexes in developing his coordination theory. He made the first meaning ful attempt to understand the mechanisms of substitution reactions of these systems on the basis of the stereochemical changes accompanying such reactions. The paper (49) he wrote in 1912 is a real milestone and should be read by anyone interested in octahedral substitution reactions. It is valuable because of the large amount of experimental data it contains on reactions of cis and 2rans-cobalt(III) complexes, but it is also of interest because it shows how closely Werner's ideas on the mechanisms of these reactions anticipate our current views on the subject. 408 In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.


28.

BASOLO

Mechanisms of Reactions

409

Werner called attention to the fact that cobalt(III) complexes frequently react with rearrangement and suggested that a satisfactory explanation for this is necessary. In his own words, "two main questions must be answered, namely: (1) which causes require a rearrangement; and (2) how does the rearrangement take place?" It is only fair to say that after 54 years we are still not able to answer these questions unequivocally. The answers offered by Werner are very similar to those given today. He suggested that rearrangement was not caused by the formation of the more stable product because examples were known in which the less stable geometrical isomer was preferentially formed. It was further suggested that a dissociation process is not involved for, if the vacated coordination position were merely occupied by the entering ligand, then there would be no rearrangement. Alternatively, the active intermediate may have time to rearrange to its more stable configuration, whereupon the ligand enters to give the more stable isomer. Since rearrangements do occur, and since the more stable isomer is not always formed, Werner concluded that such a dissociation mechanism is not involved. Werner favored what we now call a ligand interchange mechanism. In his own words, "—then when such a molecule (in the second sphere) gets included in the first sphere an acid group becomes transferred from the first sphere into the second." (The terms first and second spheres used by Werner correspond to what is now called the inner and outer coordina tion spheres (or shells), respectively.) A modern representation of this statement is shown by Figure 1. It was suggested that a metal complex has a residual attraction for groups in a second sphere, and it was assumed this attractive effect is greater toward certain directions than toward others. Thus, the direction favored by this interaction determines the position occupied by the entering ligand and may be the cause of rearrange ment during substitution.

Figure 1. Ligand interchange mechanism as modified from Reference JjS. The circles represent the outer ligand spheres and Y may be either solvent or other reagent. Heavy dots, M • • X, are to represent the greater importance of bond-breaking relative to bond-making, M • • • Y

Having answered the question of th^ cause of rearrangement, Werner then addressed himself to the second question—how the rearrangement takes place. H i s conclusions on this can best be described with reference

In Werner Centennial; Kauffman, G.; Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

410

WERNER CENTENNIAL

to Figure 2 which he proposed for the reaction of L*-/3-[CO(trien)(OH) ]+ + CI" 2

(10)

arrangement can take place as is shown in Figure 7 by the attack of water at Z 1,3 of intermediate (B). Attack at Z 1,3 over Z 1,2 is preferred due to the greater stability of the 0 form compared with the a form. In sup port of this is the observation (33) that L*-/3-[Co(trien)Cl ]+ reacts with O H ~ with almost complete optical retention to give L*-/3-[CO (trien) (OH) ]+. T h e optical inversion reactions observed i n liquid ammonia (3 33) solu tions can also be explained by this same type of mechanism. 2


2

}

OH

H

(L*-/3) Figure 7. Conjugate base mechanism for the optical inversion of D * - « [Co(trien(OCHl} into L*-(3-[Co(trien)(OH) ]+ by water attack of (B) at Zl,3; trien = NH C HJSfHC2H NHC2H4NH2 +

2

2

Cis-Trans

2

4

Isomerization

In addition to the stereochemical changes that accompany chemical reaction, these systems can be rearranged without chemical change. Several such examples of geometrical and of optical rearrangements are known for metal complexes. A brief account is given of some examples of geometrical (or cis-trans) isomerization i n this section, and the next section contains a few examples of optical isomerization (or racemization).


28.

BASOLO

421

Mechanisms of Reactions

Most of the investigations on the kinetics and mechanisms of cis-trans isomerizations of metal complexes deal with systems of the type M ( A A ) X , where A A and X are bidentate and unidentate ligands, respectively. The immediate question that must be answered i n such a study is whether isomerization takes place by an intramolecular or an intermolecular process. Fortunately, this is readily amenable to experimental testing by the use of labeled isotopes of the most labile ligand in the complex. If ligand ex change takes place at a rate which is slower than the rate of isomerization, then this requires an intramolecular mechanism (Figure 9), whereas if ex change is as fast or faster than isomerization then this permits an inter molecular process for rearrangement. The results obtained often support an intermolecular mechanism, and a possible rearrangement by this process is that shown i n Figure 8.


2

2

That geometrical isomerization of metal complexes can occur was reported as early as 1889 by J0rgensen (30) for the system represented by Equation 11. m-[Co(en) Cl ] ^± trans-[Co(en) C1 ]+ 2

2

+

2

violet

(11)

2

green

The violet and green solid chlorides are readily inter con verted. Thus, if as-[Co(en) Cl ]Cl is dissolved i n a small amount of water containing hydrochloric acid and the solution is concentrated on a steam bath, green crystals of £rans-[Co(en) Cl ]Cl-H 0 Cl (24) separate. Removing H 0 and HC1 from the green solid at elevated temperatures and dissolving it in water followed by evaporation on a steam bath restores the violet crystals of czs-[Co(en) Cl ]Cl. This interconversion occurs because the cis isomer is less soluble than the trans and separates from water solution, whereas the frYms-H 0 Cl adduct is less soluble than the cis and separates from a hydrochloric acid solution. 2

2

2

2

5

2

5

2

2

2

2

cis

trans

Figure 8.

Possible intermolecular cis-trans isomerization of [M(AA) X ] 2

2

One of the earliest experiments on the application of radioactive isotopes to the study of mechanisms of reactions of metal complexes was that of Ettle and Johnson (22). They added radiochloride ion to solu tions of [Co(en) Cl ] and found that cis-trans isomerization i n this system 2

2

+


422

WERNER CENTENNIAL

is accompanied by a complete random distribution of radiochloride ion i n the complex. This suggests that isomerization takes place by an inter molecular mechanism. Considering what is now known about the water solution chemistry of these complexes, it appears that rearrangements may take place by the equilibria shown i n Equation 12. czs-[Co(en) Cl ] + H 0 ^± cts-[Co(en) H OCl] 2

+

2

2

2

2

2+

+ CI"

(12)

W *rans-[Co(en) Cl ] + H 0 ^ cis- and *rans-[Co(en) H OCl] + C l 2

2

+

2

2

2+

2

i n addition to the data on the aquation reactions (Table I), it is also


known (5) that isomerization of the chloro-aquo products can take place. Equation 12 is sufficient to permit isomerization by an intermolecular process, but it is an over-simplification of what happens i n the experiments described above.

Equilibria for the replacement of the second chloro

group, as well as acid-base equilibria between aquo and hydroxo species, must also be involved.

Less complicated is the isomerization of

cis-

[Co(en) Cl ]+ i n methanol solution where the rate of rearrangement equals 2

2

the rate of chloride ion exchange (48). Because an intermolecular isomerization is nothing more than a sub stitution reaction, the problem of assigning a detailed mechanism is the same as that described above for stereochemical changes accompanying octahedral substitution. For a dissociation process, isomerization can readily take place by rearrangement to a common intermediate as is shown by Figure 8. This simple representation does not include any solva tion steps, nor does it show the presence of the leaving group near the intermediate. Oxygen-18 experiments show that geometrical isomerizations of the complexes [Co(en) (H 0) ] + (81) and [ C o ( e n ) N H H 0 ] + (87) may take place by a water exchange path. However, the trans —> cis rearrangement of [ C o ( e n ) N H O H ] occurs without any exchange of the hydroxo group with the solvent water. This intramolecular isomerization is believed to involve a chelate ring opening-closing process (Figure 9). A similar process may be responsible for rearrangements i n the system [Cr(C 0 )2(H 0) ]~~ at p H less than 2, whereas above this p H water exchange may account for the isomerization (25). F a y and Piper (28) have used N M R techniques to investigate the change cis —>

Stereochemistry and Mechanisms of Reactions of Werner Complexes

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