edlted by
JOHN H. WOTlZ Southern Illinois University Carbondale, Illinois 82901
Mechanisms of Ligand Substitution Reactions Of Cobalt(lll) Complexes Ralph G. Pearson University of California Santa Barbara, 93106 In 1946 1 had returned to the Chemistry Department of Northwestern University after wartime service in the Air Force. Fred Basolo had just joined the department after two venrs as a research chemist at Rohm and Haas. I began . some research in physical-organicchemistry, following up my thesis work with W. V. Evans. hut usiue kinetic methods t o study mechanisms. In late 1941Basolo suggested to me that the field of reaction mechanisms of metal complexes was ripe for investigation, and we agreed to collabora&. Our first choices for studv were the stable complexes of cohalt(III), especially the chlorammines. Basolo was an expert in the synthesis and characterization of these compounds, as a result of his training in the laboratory of J. C. Bailar a t Illinois. The first student to work on the project was E. L. Muetterties, who was an undergraduate wishing to do some senior honors research. His problem was to study the stereochemical changes in the mercuric ion induced aquation of cis and trans Co(en)zClz+. His success was slight, due to the great experimental of working with inoreanic compounds a t .--.~ ~difficulties ~~~~ this time. Practically the oniy methods"availabl~wereelemental analysis and visible uv spectroscopy. I remember my own frustration on going from organic compounds to inorganic compounds as subjects of study. Not even melting points, as criteria of purity, could he used. The colors of these cobalt complexes were not only s t r i k i i , but also useful, as they changed during the course of chemical reaction and allowed for easy studies of the rates. A rule of thumb was that cis dichloraknines were violet and that the trans isomers were preen. The products of hydrolysis were a mlxture of ris and t r a m rhloraquo complexes which were red. One could easily estimate the half-liie for hydrnlysis of the t r n m d~chloridessinw the equimolar mixture ot red and green gave n nearly colorless solution (red and green arL. complementary colors). Sometimes the simple rules were deceptive. At one stage we prepared an alleged tram-dichlorohis(dipyridyl)cohdt(III) chloride, and because of its beautiful green color and analysis, we were sure that it was indeed the trans isomer. Later work by others showed that it was in fact the cis isomer. I t was not until 1958that crvstal field theorv. heaan - to explain the colors of inorganic complexes in detail. In late 1949, or early 1950,Basolo and I received a grant of $7,000 from the Atomic Energy Commission to support our joint project. While miniscule by today's standards, this was a princely sum for those days. Graduate students J. G. Bergmann, C. R. Boston, and B. D. Stone were enticed into working on the problem. Slowly results began to he obtained, and our first papers appeared in 1952. Of course there had been some early work on the kinetics of reactions of complex ions, particularly by J. N. Brmnsted, F. J. Garrick, and A. B. Lamb. Much of this was concerned with salt effects on the behavior of ionic reactants. However, ~
720 1 Journal of Chemical Education
people interested more in mechanistic detail were heginning to appear. In 1950 a symposium on "Rate and Equilibrium Behavior of Complex Ions" was held a t the University of Chicago. H. Tauhe, A. W. Adamson, E. L. King, R. G. Wilkins, C. S. Garner, G. M. Harris, and R. E. Connick were in attendance (as I recall) and some of them had remarks to make about rate behavior. Particularly Tauhe gave a stimulating analysis of the lability or non-lability of almost all of the metal ions in their complexes. Meanwhile in England, C. K. Ingold of University College, London, had persuaded R. S. Nyholm to join forces with him in an attack on the mechanisms of reaction of octahedral complexes. Ingold was already the acknowledged high priest of organic reaction mechaniims, exrept perhaps for dissent~ng v o ~ e 4from K. Itdinson. S. Winstein. and a few others. Ha\,inc worked out the rules for nucleophilic substitution reactions a t tetrahedral carbon. Ineold was keen to ao on to new geometries and coordinati& numbers. He had already developed a theory of edge displacement for octahedral complexes, hut the war prevented him from testing this out until 1949. His new ally, Nvholm. was a skilled coordination chemist, trained inAustralia by F. P. Dwyer. The man who did the actual work was, of course, a maduate student named D. D. Brown. The first papers hy ~ r i w nIn, gold, and Nyholm appeared in 1953. They dealt with substitution reactions of cis-Co(en)zClz+ by various anions in methanol solvent. Their conclusions as to mechanism were almost a t complete variance with ours. They found a duality of mechanism, with S N ~or, dissociation processes, operating for weak nucleophiles, and SN2, or displacement mechanisms for stronger, more basic nucleophiles. The methoxide ion was found to he the most nowerful nucleo~hile. . . as mieht he expetted, being some lo5 times more reactive than methanol. The Northwestern group, working with different systems, had found only S N mechanisms ~ for all reactants. Our explanation for the unusual reactivity of the OH- ion in water, analogous to that of the CH30- ion in methanol, was that reaction occurred via aspecial S N ~ C mechanism. B This name of the conjugate base of stands for unimolecula~dissoci~tion the original complex. For example ~~~~
~
C O ( N H ~ ) ~+COH~ ~ +F== CO(NH~)~NH~CI+ + Hs0 faat
(1)
This mechanism had been . nrooosed . earlier hv Garrick. hut on little evidence. We had heard of the work a t Universitv Colleee as earlv as 1951, and knew that differences of opinio~existe~. I had sient the 1951-52 academic year a t Oxford and had gone to London on several occasions. However, Nyholm was in Australia and Ingold proved elusive. I did have a long talk with E. D. Hughes, Ingold's great collaborator on organic chemistry. Afterwards I realized that while I had told Hughes all I knew, he had told me nothing in return. He pleaded ignorance of all matters inorganic. In 1953 the Reaction Mechanisms Conference (Organic
Chrm~stry)was hrld at Northwrstcrn and lnpold was one of the speakers. He summar~zedh ~ rerent s work, with Hughrs and uther;. u n the nitration mechanism of henlene. His lecturr was brilliant, and ovrrpowering in its a b ~ l n yro convince the liqtener. A portion was devoted to demoltshing the work of one or two others who worked on the same problem. Face to face, Sir Christopher was surprisingly gentle and courteous. This was apparently always the case. He never displayed any rancor or arrogance except in his scientific papers and lectures. He was clearly a brilliant man who would think thines throueh to his own satisfaction. He was then impatimt with anyone who could not see things as rlearly as hedid. Hut h? would not display this impat~ence,except in the proper forum. Between 1954 and 1957. Basolo and I, together with R. E. hlwker and \\'. K. Matnl~sh,dld a numhrrofrxprrim+:ntsthat convinced us that the SxlCH merhanwn wascorrrct. P. M. Henry worked with r i and ~ trans Co(en)?CI?' In methanol and disnwrred that ingold and Nyholm had made an elemmtary error. They failed to buffer their solutions, so that a solution of sodium azide, for example, would generate a trace of methoxide ion. This, of course, caused a rapid solvolysis of the chloride hound to cobalt, and gave the impression that Nnwas a moderately active nucleophile. Another complication in methanol, discovered by Henry and ignored by Ingold, Nyholm and Brown, was that ion-pairing occurred and obscured the kinetics. Ingold also tended to discount ion-pairing in purely organic reactions, and sometimes got into difficulties as a result. It must be recalled that between 1925 and 1950 the DebyeHiickel theory of complete dissociation of salts was in command. Except in solvents of very low dielectric constant, such as benzene or dioxane, ion-pairing of univalent electrolytes was considered to be negligible. Coordination chemists, with their concepts of both inner-sphere and outer-sphere comolexes. led the way in changing this attitude. Ingold was unimpreis+tdhy any of' our work and continued to lecture and puhl~shpapers which explained the r~:sultsby dual m e ~ h a n i ~ mHewould s. quote our papers and explain them away in footnotes. In fact, he was not particularly astringent, hut instead acted more like an indulgent father explaining things to not-too-bright offspring. Nyholm disengaged himself from the whole argument as quickly as he could, though his name appeared on some of the papers up until 1962. M. L. Tobe became Ingold's chief inorganic ally a t University College. As a result, a great deal of beautiful experimental work was done, not only on rates of reaction and on stereochemical changes during reaction, but also in synthesis. Fortunately for Basolo and me, a number of other people became interested in the mechanism of substitution reactions of cobalt(II1) complexes, particularly base hydrolysis. Work bv Taube. bv A. Sareeson and D. Buckingham, and finally by ~ o b e ha; , ek.ablished the S N ~ C B mechanism as well as a mechanism can be estahlisbed. Other work by Tohe, W. K. Wilmarth, A. Haim, C. H. Langford, D. W. Watts and others, supported the general Sp~1mechanism for cohalt(II1) complexes.' C. K. Ingold retired in 1961 and had his name on his last paper on the suhject in 1965. Until his death in 1971, I do not
think that he admitted, even to himself, that he might have been mlstaken on anv of hii merhanisms. In retrncpert. l bcl~evethat he fell into thecommon trapof starting with a pet theory whirh hr wilnted toprovewas nght, ratherrhan totest critirall!.. He had a very convlncltlg model d v d r t displacement and had a vested lntrrest in finding SN2merhanicm.; to test the conclusion^.^ I ouote from C. W. S h o.~.o e e"Ineold .~ had extraordinarv imagination, insight, initiative and ingenuity, . . . he possessed one of the greatest intellects in chemistry of the 20th century, and he added a new dimension to organic chemistry." I wholeheartedly agree with these remarks and I will add that he also was a great stimulus to inorganic chemistry. The very fact that he interested himself in the suhiect of inorganic reaction mechanisms added great stature-to the to&. Both Basolo and I recall the sparse audiences to which we would lecture prior to 1953, and the crowds we drew after Ingold's entry into the field. As mentioned earlier, Nyholm was anxious to disassociate himself from Ingold's tenditious approach to science. His style was completely different. He played the role of bluff, hearty colonial to perfection, though actually he was a much more comnlex individual. He must have found collahoratine with ~nioiddifficulta t times. On one occasion, Ingold s e n i ~ o b e two manuscriuts. one of which had Ineold. Nvholm, and Tobe as authors, and the other had ~ ~ h oand l k~ o b e . ' ~ c c o m p a nvine , ,. the mss. was a note askine that thev be returned since Nyhdm had nut yet sren them! lngold personally wrote w e n ' oaoer that bore h ~ name. s excent tor thme based un his PhT) work with Thorpe, and the 1965 paper with B. Bosnich and Tobe. Nyholm quickly became as influential in inorganic chemistry as Ingold already was in organic. Unfortunately he often went out of his way to imply that mechanistic studies were not important for inorganic chemists. I n his lectures he would show a slide with this equation KINETICS - FACT (4) MECHANISM- FICTION This toneue-in-cheek nresentation would often be taken se" riously by a numher of European chemists, especially the senior ones. While Nvholm did no sewice to inorganic - mechanism studies, he redeemed himself by revitalizing inorganic chemistry in Britain in many other ways. Certainly no permanent damage was done, since Britain is now one of the leadine countries in the field of inorganic reaction mechanisms.'~should also point out that university College did not alwavs come out second-best in arguments with Northwestern uni;ersity about inorganic reaction mechanisms. But that would he another story behind the story! 'For literature references, see Rasolo, F.. and Pearson, R. G.. "Mechanisms of Inorganic Reactions," 2nd Ed.,John Wiley and Sons, New York, 1967, Chapters 3 and 4; Tobe. M. L., A c r s Chem. RP%, 3, 377 (1970).
2The term edge-displacement certainly sounds better than the American equivalent, "hack-side attack." Shoppee, C. W., "Biographical Memoirs of Fellows of the Royal Society,"Vol. 18, 348 (1972). 'I am indebted to Professor M. I.. Tohe for this story and other viewpoints from the other side of the Atlantic.
Volume 55, Number 11, November 1978 / 721
