Pentacyanocobaltates

It was suggested that this change in product distribution was due to the existence of an equilibrium between two types of complex, viz., a cr-butenyl-pentacyanocobaltate(III) and a 7r-butenyltetracyanocobaltate(III) 107, 109). However, further study of the kinetics and product distribution suggested the presence of two o-bonded complexes, viz., cr-but-l-en-3-yl and a-but-2-en-l-yl 24a). Direct evidence for the existence of a cyanide-dependent equilibrium between the a- and rr-bonded organocyanide complexes has been obtained from NMR studies of the complex prepared by the reaction of allyl halides with Co—H 109) (see also Section V,C). Both butadiene and crotyl chloride react with Co—H to give the same... 

Sulphur dioxide is reduced by pentacyanocobaltate is several stages (c/. the reduction of p-benzoquinone), viz. 

Pentacyanocobaltate ion reduces p-benzoquinone in several stages . An initial, fast reaction produces the bridged species... 

The absence of any effect of added iodide upon the products is in contrast with the reduction of H2O2 by pentacyanocobaltate (p. 462) and confirms that NH2 is the free radical intermediate rather than OH-... 

A similar type of oxygen complex has been observed during the oxidation of [Con(CN) s]-3 but it was not possible to show that this species was formed in the initial reaction step since with this system, as with the cobaloxime(II) system, the 1 1 adduct apparently reacts very rapidly with another molecule of pentacyanocobaltate(II) to form a diamagnetic binuclear complex with a bridging peroxide ligand 116). It appears that in the Bi2-system the bulk of the corrin ring does not allow formation of the diamagnetic binuclear complex. 

Thermal decomposition under hydrogen of a series of pentacyanocobaltate complexes (CN-, N02-, NO- or N3-ligands) revealed that the latter complex is the most exothermic by far. Presence of iron powder suppresses hydrogen cyanide formation. 

The [Co(CN)5]3 complex is an effective catalyst for some reactions, particularly the isomerization of alkenes. Newer and more efficient catalysts have been developed for some of the processes, but the catalytic behavior of the pentacyanocobalt(II) ion is also significant from a historical perspective. In reactions such as that shown in Eq. (22.10), two Co2+ ions increase one unit in oxidation state, instead of the more common situation in which one metal ion increases by two units in oxidation state. The cobalt complex also reacts with CIT3I, Cl2, and H202, which are indicated as X-Y in the equation... 

Establishment of a free radical mechanism via H-atom transfer for hydrogenation using HMn(CO)5 (see Section II,D), and possibly also HCo(CO)4 (see Section II,C), suggests that more serious consideration for such mechanisms should be given for other hydridocarbonyl catalyst systems, and indeed for other homogeneous catalysts systems in general. The pentacyanocobaltate(II) catalyst can certainly operate by such a mechanism (see Section II,D). 

The reactions of several Co(ii) complexes have been examined (Halpern, 1974), namely, pentacyanocobaltate(n) (Chock and Halpern, 1969 Halpern and Maher, 1964, 1965 Kwiatek and Seyler, 1965,1968 Kwiatek, 1967), bis-(glyoximato)cobalt(il) (Schneider et al., 1969), cobalt(li) Schiff s base (Marzilli et al., 1970, 1971) and bis(dioximato)cobalt(ii) (Halpern and Phelan, 1972) complexes. A halogen-atom-transfer mechanism has been proposed for most halides (158, 159), with the exception of the reaction of cobalt(ii) Schiflf s... 

The interconversion of a- and 7r-allyl complexes has been observed by Kwiatek, Mador, and Seyler in the interesting homogeneous catalytic system of potassium pentacyanocobaltate(II), K3Co(CN)5 (46). This solution absorbs molecular hydrogen to form the active hydride species, Kj[Co (CN)jH]. Addition of butadiene to the hydride results in the formation of a [Pg.36]

Addition of cyanide to Co(n)-salts under hydrogen produces an active hydrogenation catalyst which was subject of very intensive studies during the nineteen-sixties [13,14]. The catalytically active species is hydrido-pentacyanocobaltate formed according to eq. (3.3). 

Interesting other additives used in the pentacyanocobaltate(III)-catalyzed hydrogenations are the various cyclodextrins [17] - these reactions will be discussed in Chapter 10. 

Many transition metal complexes catalyze homogeneous activation of molecular hydrogen in solution, forming hydrido complexes. Such complexes include pentacyanocobaltate(II) anion, [Co(CN)5], many metal carbonyls, and several complexes of rhodium, iridium, and palladium. 

CN-stretching frequencies, 12 387 kinetic data for, 12 413 ligand substitution reactions, 12 406 Pentacyanocobalt(II) exchange reactions of, 10 201, 202 kinetic data for, 10 202... 

Carl H. Brubaker, Jr. I agree with Dr. Yalman that this represents a very complete piece of work, and I think, the majority of the conclusions are fairly clear cut. There is not much that can be added aside from speculation. I would hope that a little later Prof. Wilmarth or others will speculate about the structures of this transition state species, or several species of the pentacyanocobaltate(II) that are supposed to be the transition state complex, or an intermediate. 

Jack Halpem I would like to comment further on the question that Dr. Anbar raised about the possibility of generating the pentacyanocobalt(III) by electron transfer from pentacyanocobalt(II). 

Dr. Brubaker I have been interested personally in the work that has come from Australia in the last year by Betts and Winfield and others on the oxidation also of the pentacyano with oxygen. Here they find the oxygen binds a proposed intermediate species which may be a monoperoxo monomer, which this group scavenges very well for the pentacyanocobalt(II) and forms the familiar decacyano-/x-peroxyldicobalt(III) complex. So in this case too, the oxidant sticks, not water. 

III) or some X-pentacyanocobalt(III). But the sensitivity of the stoichiometries to the added anion is very small, and we can indicate this by a minus sign. I think that the only conclusion that one can reach from this type of data is that there is an intermediate in the system. 

Potassium pentacyanocobalt hydride reduces conjugated dienes to mono-olefins (61). The reaction mechanism probably involves an addition of the hydride to the diene followed by reduction with a second molecule of hydride (61). 

Similar acetylene addition reactions take place with bis-cydopentadienylnickel carbonyl dimer (93). Changing from carbonyl to cyanide ligands seems to allow the formation of a true vinyl derivative. Thus, potassium pentacyanocobaltate, which may react as a dimer with a cobalt-cobalt bond (20), reacts with acetylene to give the adduct XV (31). The product was thought to be the trans isomer, but the data were not conclusive. 

The pentacyanocobalt(II) ion reacts with acetylene to form a yellow crystalline salt K6[Co2(CN)io(C2H2)HH20 108). The nuclear magnetic resonance spectrum of the ion shows a single proton resonance line in the... 

Organic nitroxides (136a, 5), nitroparafins, and nitrolic acids (107) react with aqueous solutions of pentacyanocobaltate(II) salts to give alkyl pentacyanocobalt(III) nitroxide anion radicals, spin traps involving the formation of an N—Co bond (5). 

A variety of substrates have been catalytically hydrogenated at room temperature and 1 -atm. hydrogen pressure by pentacyanocobaltate(ll) anion. Conjugation is required for the reduction of C=C bonds The effects of detailed molecular structure on reducibility and of cyanide-cobalt ratio on mode of reduction have been noted Poisoning and reactivation of the catalyst as well as the effect of alkali are described, and mechanisms are tentatively proposed for these phenomena It is concluded that the aging reaction of pentacyanocobaltate(ll) is reversible A dimerization of acrylic acids at elevated temperatures was found ... 

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