8
The Nature of Peroxo-Bridged Dicobalt Complexes G. L. GOODMAN, H. G. HECHT, and J. A. WEIL
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Argonne National Laboratory, Argonne, Ill.
A literature survey of investigations of the peroxo -bridgeddicobalt complexes is given, with primary interest devoted to the rather unusual, para magnetic species. Chemical work relative to the composition, preparations, and reactions of these salts is briefly discussed. Recent investigations by optical, polarographic, and electron para magnetic resonance techniques have shown that the electron hole characterizing the paramagnetic series of complexes is delocalized over both cobalt atoms and probably over the peroxo bridge and also, to some extent, over the other ligands. Possible structures proposed for this class of com pounds are discussed in terms of the experimental results.
m considerable amount of attention has been directed in recent years to the compounds discussed here. The material reviewed does not constitute the last word relating to the peroxo-bridged dicobalt complexes; on the contrary, work on these compounds is actively in progress in a number of laboratories throughout the world. Interesting results have emerged, and it is hoped that this review will reliably summarize the present status of this field of research and serve as a guide to subsequent work. Chemical Aspects of Complexes There exists a series of diamagnetic complexes, typified by the dark brown decaammine-jm-peroxodicobalt ( III ) ion [(NH ) Co—0 —Co(NH )5] 3
6
2
3
+4
(diamagneticA )
obtainable by the oxidation of ammoniacal solutions of cobalt (II) salts. This salt has been known for a long time, having beenfirstreported in 1852 by Fremy ( 12) ; it contains two cobalt atoms of valence +3. A related, paramagnetic series of com pounds characterized by their green colors is now known to be derived from the compounds of the above type by further oxidation. The oxidized form of was described by Maquenne (23) and by Vortmann (38), and its structure was formulated erroneously as a protonated version of (d-1). Subsequently, however, 90 COLBURN; FREE RADICALS in Inorganic Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
GOODMAN ET AL.
91
Peroxo-Bridged Dicobalt Complexes
Werner (41, 43) published the results of his work on these compounds and pro posed the structure accepted today: [(NH ) Co—0 —Go(NH ) ] 3
6
2
3
5
+6
(paramagneticA )
which differs from that of Maquenne and Vortmann just by removal of one proton. The complexes (d-l) and (p-1) are representative of a large class of compounds containing various ligands, some of which act as bridges between the cobalt atoms. Some examples of the paramagnetic series are: π +4
o
o
2
2
(NH ) Go ^ \ ο ( Ν Η ) \ / NH 3
4
3
4
en Go
2
Go
2
en2
(P-2) (*-3)
o
2
(NH ) Go—OH—Go(NH ) 3
3
3
NH
Go C1(NH )
(NH ) C1 Go 3
3
3
3
NH
2
3
2
(/>-5)
Gmelin (17) gives a complete account of the early work, syntheses, and properties of these compounds. Numerous bridging groups are known, including not only 0 , OH, and N H , but also N 0 , S0 , and C H C 0 in the diamagnetic forms. Werner succeeded in resolving optical isomers of several dicobalt com plexes, including (ρ-3) (42). Additional studies (1,6,20) by chemical means have confirmed the composi tion and bond arrangement of proposed by Werner, who had interpreted this formulation by assigning tri- and quadrivalencies, respectively, to the two cobalt atoms: [ ( N H ) C o ( 0 ) C o ^ N H ^ ] * . The same electron structure was postulated for the other analogous complexes. Quadrivalent cobalt is not found in any other compound, and hence would be a feature peculiar to this class of complexes containing a peroxo bridge; it is this characteristic which accounts for the interest in these inorganic free radicals shown by a large number of workers. An alternative interpretation of these results was first suggested by Gleu and Rehm (16) and supported by Jakob and Ogorzalek (20). These workers pointed out that on the basis of the available chemical evidence, one can interpret the results equally well by assuming that the two cobalt atoms are trivalent and joined by an 0 ~ radical: [ ( N H 3 ) C o ( 0 - ) C o ( N H ) 5 ] + . Another alternative, initially proposed by Malatesta (22), is essentially a modification of Werners interpretation in terms of modern concepts of resonance. From this viewpoint, the two cobalt atoms are thought of as being equivalent and of valency intermediate between 3 and 4. Still other interpretations are possible. It is doubtful that a distinction can be made between the various charge distributions proposed by purely chemical means, although this might be possible in principle. We thus rely on physical methods such as optical studies, x-ray analysis, polarography, magnetic susceptibility, and electron paramagnetic resonance (EPR) studies for a determination of the structure of these complex cations. This is not to say that the chemistry of these compounds is no longer of 2
2
3
2
5
in
2
4
3
2
5
2
5
m
2
m
3
5
COLBURN; FREE RADICALS in Inorganic Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
92
ADVANCES IN CHEMISTRY SERIES
interest. Thompson and Wilmarth (35) have studied extensively the oxidationreduction properties of several of these compounds and have cleared up a number of uncertainties concerning the structures reported in previous work. In particular, the red diamagnetic peroxo complex assumed by Werner (41 ) to have the imido-bridged structure (d-2) was shown to be just the acid salt (d-3)
o
o
2
en2Co
Go en
(Ν0 ) ·ΗΝ0
2
3
3
2
en Go
Go en ( N O 3 V H N O 3
2
3
(d-2)
2
(d-i)
of the reduced form of the amido-bridged dicobalt-peroxo complex (p-3). In later work (19), a new type of dicobalt-peroxo complexes, containing cyanide as ligands, has been reported. Both the diamagnetic form (d-4) and the paramag netic oxidized form (p-6) were prepared. [(CN) Co—0 —Go(CN) ]-e
[(CN)5Go—0 —Go(CN) ]-
(d-4)
(p-6)
5
2
6
2
6
5
Charles and Barnartt (7) have studied the reaction by which the rather unstable [(NH ) Co—0 —Co(NH ) ]+ ion is decomposed in dilute sulfuric acid solutions to form various products, including some [(NH ) Co—0 —Οο(ΝΗ ) ]+ . By contrast, the oxidized +5 ion is stable in acidic solution, although unstable in bases (32). Recent work by Sykes (33) deals with the kinetics of the reduction of the complex (p-2) by ferrous ion in aqueous solution, and gives some evidence for the protonation of the peroxo bridge in acid solution. Other kinetic work, dealing with the oxidation of Co (III) perchlorate by H 0 , has been interpreted in terms of a dimeric cobalt complex species bridged by oxygen (I). Several intermediates in the oxidation of certain cobalt chelates have been found, which are formed by the reversible absorption of oxygen (5,15, 24, 34, 44, 45). These compounds are of interest to a large number of investigators because of their similarity to naturally occurring substances which act as oxygen carriers in biological systems. The intermediate formed in these reactions appears to be very similar to the diamagnetic peroxo-bridged ammine complexes—e.g., both are very unstable at low pH's, diamagnetic, and brown in color, and contain two cobalt complexes per oxygen molecule. The diamagnetic intermediate formed by treating the Co(II)-histidine complex with oxygen gas has recently been isolated in crystal line form and its properties have been studied by Sano and Tanabe (29). We have found no investigations of possible paramagnetic peroxo-bridged products obtained by further oxidation of the peroxodicobalt(III) complexes. One factor which has probably been a deterrent to progress in the study of these compounds has been the long involved preparations used, which resulted in poor yields. Some improvements in these preparations have been made. A yield of about 30 to 50% of the monobridged peroxo +5 complex (p-1) can be ob tained in the form of [ ( Ν Η ) 0 ο ( Ο ) 0 ο ( Ν Η ) ] (S0 ) HS0 .3H 0 by the use of ammonium persulfate as an oxidizing agent (16,21). A relatively short prepara tion which yields approximately the same quantity of the nitrate of this compound has been developed in this laboratory (39). The desired product is obtained by passing a 3% ozone in oxygen stream for about 30 minutes through an ammoniacal cobalt (II) nitrate solution containing ammonium ion. Ozone in oxygen has also 3
5
2
3
5
4
3
2
3
6
2
5
2
3
2
8
5
4
2
4
2
COLBURN; FREE RADICALS in Inorganic Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
5
δ
GOODMAN ET AL.
Peroxo-Bridged Dicobalt Complexes
93
been used instead of air in the conventional preparation (41) of [(NH ) Co(0 ,NH )Co(NH ) ](N0 ) . This technique reduces the time required for the preparation of this dibridged complex, although the yields are still rather poor (about 10 to 15%). 3
2
2
3
4
3
4
4
Physical and Theoretical Aspects of Complexes We now turn to the electronic structures of these complex ions, relying chiefly on the physical methods mentioned above. A number of investigations of the magnetic susceptibility of the peroxo com pounds have been reported (2, 11, 16, 22, 26), which indicate the paramagnetic nature of the oxidized complexes in contrast to the diamagnetism of the reduced forms. Studies at various temperatures indicate that the paramagnetic compounds follow Curie's law closely and each possesses a permanent magnetic moment of about 1.7 Bohr magnetons, indicative of the presence of one unpaired electron. Following the classic investigations of Werner and others, the first work deal ing with optical properties of these compounds was that of Mathieu (25), who studied the absorption spectra, optical rotation, and circular dichroism (Cotton effect). The absorption spectra of aqueous solutions of some of these compounds have also been recorded by other workers (7,21,31, 32, 33, 46), several of whom included comparisons with spectra of more conventional cobaltic ammine com plexes. A polarizing microscope has been used by Yamada, Shimura, and Tsuchida (46) to study the dichroism of small single crystals of the monobridged peroxo complexes (d-1) and (p-1). These workers discussed their results for the oxidized complex in terms of two cobalt atoms in different valence states with electrons moving through the peroxo radical so as to make the two cobalt atoms indistinguish able in the ground state. Unfortunately, however, no attempt to interpret these optical phenomena in terms of the details of any electronic structure has yet been published. The infrared spectrum of (p-2) is briefly mentioned by Chatt et ah (8,14). As afirststep in any discussion of the electronic structure of the peroxo com plexes, it is necessary to know or estimate the position of the nuclei of cobalt, oxygen, and other bridging atoms. Unfortunately, only rather sketchy, pre liminary information is available from x-ray structural investigations. Okaya (27) has found a Co-Co distance of 4.51 A. for (p-1) pentanitrate, and very tentatively suggested a cis structure:
CO
Ο—Ο
1.45
db
Co—Ο
2.0
± 0.1 A.
CO
