1 From Electron Transfer Reactions to the Effects of Backbonding Henry Taube
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Department of Chemistry, Stanford University, Stanford, CA 94305-5080
The initial motivation for extending the chemistry of ruthenium ammines was to provide new reagents for research on electron transfer reactions. Quite early in pursuing the preparative chemistry, an unexpected capac -ity ofRu(II)for backbonding was uncovered, and the systematic investi -gationof its influence on physical and chemical properties became a goal in its ownright.The new directions led to the discovery of a num -ber of novel reactions, which in some cases led to novel products. The research has provided documentation of the effect of backbonding on ligand-to-metal charge transfer, ligand-metal distances, the intensities of infrared absorption byπ-acceptorligands, the acid-base properties of such ligands, the properties of co-ligands exerted by the electronic-with -drawing power ofπ-acceptorligands, and on Ru(III)/Ru(II) redox potentials and enthalpies of complex formation. Some early results, which demonstrate the superior capacity of the osmium ammines to engage in backbonding, are also described.
ONE OF THE THEMES OF MY EARLY RESEARCH at Stanford, begun late in
1961, was the investigation of the mechanisms of substitution reactions of metal complexes by the application of isotope effects, mainly of oxygen. For example, we carried out kinetic and tracer studies in the alkaline hydrolysis of [ C o ( N H ) ( 0 C C F ) ] (I) and attempted to generate intermediates such as might be implicated in S 1 mechanisms (e.g., we hoped to generate [Co(NH3) ] by the nitrosation of [ C o i N H ^ N ^ P ) (2). 3
5
2
3
2 +
N
5
3+
Mechanisms of Electron Transfer The major emphasis of the research in my group, however, was the investiga tion of the mechanisms of electron transfer reactions. In key experiments done © 1997 American Chemical Society
In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.
1
2
E L E C T R O N TRANSFER REACTIONS
at the University of Chicago, the substitution characteristics of the I ^ N H ^ g Co(III)/(II) and Cr(III)/Cr(II) couples had been exploited to reveal the partici pation of atom bridging groups (3, 4) in the electron transfer act, especially for the reaction in eq 1 Cr
2 +
+ [(NH3) CoCl] -> [ ( N H 3 ) C o - C l - C r ( H 0 ) 14+ } 5
2+
5
-> C o ( H 0 )
Downloaded by ADAMS STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch001
2
6
2 +
+ 5NH
2
4
+
5
+ (H 0) CrCl 2
5
(1)
2 +
and the work had proceeded to the point that electron transfer through poly atomic bridging groups had been demonstrated (5, 6). Of particular interest is the possibility of electron transfer over extended primary bond systems, for which unambiguous examples, if any, were rare at that time. Our objective was to extend the scope of the investigation to metal centers with different elec tronic structures. The need to take this direction was recognized i n part because the principles of ligand field theory had by then become a part of the scientific culture i n the field. The redox active orbitals of both Co(III) and Cr(II) have σ symmetry, and a promising next step was to seek a replacement metal ion capable of accepting an electron in an orbital of π symmetry instead of the Co(III) oxidant. The oxidant is selected to be substitution-inert, and the bridging ligand can be varied systematically, enabling the effect of such changes on reaction rates to be studied. A n additional requirement on the replacement is that the metal ion driving force for reduction match that of the cobalt ammines.
Reactivity ofRu(II)/(III) Ammine Complexes A survey of the literature suggested that the Ru(II) ammines offered the best prospects of meeting all three conditions. The kinetic inertness and the nd vacancy were recognized characteristics of Ru(II) ammines, but information on the redox properties of Ru(II)/(III) ammines was lacking. Preparative work, mainly on Ru(III) tetraammines by Morgan and Burstall (7), and on the Ru(III) pentaammines and the tetraammines by Gleu and co-workers (S), had been done. A major theme of this early work with the Ru(III) tetraammines is the similarity of the Ru(III) isomers to those of the Co(III) tetraammines, which had been described much earlier. In fact, terms such as violeo-, roseo-, purpureo-, and praseo-, descriptive of the colors of the Co(III) ammines, were being used to imply geometrical arrangements, not to describe color. The only Ru(II) ammine complexes described in this early work contain S 0 , HSO3, or SO|" as ligands (#), but the significance of the stability of these species i n air was not at all obvious to me i n my early reading of this literature. Very little relevant additional work on ruthenium ammines had appeared when, ca. 1961, I began to look for nd redox reagents. [An exception is an abstract (9) published in 1959, in which the preparation of [ R u t N H ^ H S O ^ J and [Ru(NH3) (NO)]2
5
In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.
1. TAUBE
C 1 H 0 are reported but with no particular reference to their chemical prop erties.] Considerable work on 1,10-bipyridine and phenanthrohne (phen) com plexes of ruthenium had also been reported, notably by Dwyer and co-workers (10). This work included the study of the redox chemistry and showed that the ruthenium 3+/2+ couples were much more strongly oxidizing than those of the cobalt 3H-/2+ complexes, which had been used i n the electron transfer research. Along with the new work that was initiated on ruthenium ammines soon after my arrival at Stanford, we continued our earlier research on cobaltammine oxidants with extended bridging groups. This research suffered a severe setback when it became apparent that a number of interesting effects, reported by an unusually enterprising co-worker and me (11) i n the period 1959-1961, could not be reproduced, and their further exploration was my first priority at Stanford. A good part of the early effort was devoted to setting the record straight, and I take this opportunity to express my appreciation to those who contributed to this essential but, for them, unrewarding, task (12-14). 2
Downloaded by ADAMS STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch001
3
Electron Transfer Reactions and Effects of Backbonding
2
Mechanisms of Electron Transfer with Extended Bridging Groups The fact that organic bridging groups containing conjugated bond systems can lead to greatly enhanced rates of reduction of their pentaamminecobalt(III) complexes by Cr (aq) had been observed in numerous cases. A comparison of electron transfer results for different bridging groups led to the hypothesis that reducibility of the ligand was the key issue (IS) and that, for the most part, reduction of Co(III) when high rates are observed involves stepwise transfer [i.e., electron hopping rather than the presumed resonance transfer (5)]. The revised interpretation of electron hopping as the preferred mechanism of elec tron transfer through conjugated bond systems for the Co(III)/Cr(II) reactions was bolstered by a considerable number of studies, and several completed somewhat later were particularly persuasive (16-19). [This subject is dealt with in some detail in a review article by Meyer et al. (20).] The Co(III)-Cr(II) case implied that a stepwise electron transfer mechanism (electron hopping) is enforced by the symmetry mismatch between the carrier ligand π* orbitals and the metal ion σά orbitals. Resonance transfer would occur if the redox active orbital of at least one of the metal ions is of π symmetry, which in fact proves to be the case when [Co(NH3) ] is replaced by [Ru(NH3) ] (21). 2+
5
3+
5
3+
During this period studies on electron transfer reactions of ruthenium compounds had begun even before the additional incentive of understanding the mechanism of electron transfer (electron hopping vs. resonance transfer) had been defined. The first publication on the redox chemistry of ruthenium ammines deals with the reduction of [ R u ( N H ) ] , [ R u ( N H ) C l ] , and [ R u ( N H ) ( H 0 ) ] by Cr (aq) (22-24). Several new observations that proved to have important implications for later work are reported i n those studies. 3
3
5
2
3+
6
3+
3
5
2+
In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.
2 +
4
E L E C T R O N TRANSFER REACTIONS
First, the reaction of C r with [Ru(NH3) Cl] takes place with C I " transfer, similar to the analogous reaction with [ C o ( N H ) C l ] , whereas that with [ R u ( N H ) ] of necessity involves an outer-sphere-activated complex. The rate advantage of the inner-sphere path, however, is reduced to 80 for the Ru(III) system, compared to the factor of 10 observed for Co(III) complexes. Observations made on the properties of the Ru(II) products, such as the inert ness of the R u ( I I ) - N H bond and the lability of the R u ( I I ) - H 0 bond, are par ticularly relevant to later work. (The stability of the R u ( I I ) - N H bond had already been demonstrated in [ R u ^ H ^ S O ^ ] * , but because S 0 is a π acid, the inertness in [ R u ( N H ) ( H 0 ) ] could not be ensured.) Second, catalysis by [ R u ( N H ) ( H 0 ) ] of the Ru(III) substitution reaction 2 +
2+
5
3
3
2+
5
3+
6
9
3
2
3
2
3
Downloaded by ADAMS STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch001
3
5
5
2
2
2+
2+
2
^(ΝΗ3) (Η 0)] 5
2
+ C l " -> [ R u i N H ^ C l p + H 0
3 +
(2)
2
was observed, which implies that the R u ( I I ) - H 0 bonds are quite labile and implies also that electron transfer between Ru(II) and Ru(III) ammines is facile. Third, and to our astonishment, [ R u ( N H ) ( H 0 ) ] was found to be incompatible with C l O j (in 1.0 M C l O j , the half-life for Ru(II) at 25 °C is
