Pentacyanocobalt complexes

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. 

A number of rearrangements involving loss and gain of cyanide groups has also been observed (14), for example, the interconversion of the pentacyanocobalt complex (17) and tetracyanocobalt complex (18) (26). 

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... 

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. 

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... 

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]

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. 

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. 

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. 

It is possible that a small portion of the hydroxo complex is also formed by the reaction of pentacyanocobaltate(II) with hydrogen peroxide, which is known to be almost quantitative (4). No cyanocobaltate(III) species is known to activate hydrogen, and we have observed that the addition of hexacyanocobaltate(III) to CoH (H2 atmosphere) does not result in absorption of hydrogen. 

Initial complexes formed from the interaction of CoH and hydrogen peroxide, nitrobenzene, or anthiaquinone either react further with the excess CoH present to form pentacyanocobaltate(II) or are spontaneously hydrolyzed to yield hydroxypentacyanocobaltate(III). The latter species may then undergo the reverse aging process with CoH, forming pentacyanocobaltate(II), thereby effecting catalytic hydrogenation. 

A yellow form of the pentacyanocobaltate(II) ion has been observed in DMF solution. Various monomeric (NR4)3[Co(CN)5] complexes have been crystallized26 and a structural study on one shows the anion to possess a truly five-coordinate square-pyramidal geometry.27 Electron irradiation of solid K3[Co(CN)6] gives a presumed pentacyanocobaltate(II) ion,28 the visible spectrum of which is virtually identical with that of [Co(CN)5]3a ). The structural parallels between the green and yellow forms of [Co(CN)5]3 and isoelectronic [Co(CNR)5]2+ ions indicate that all the green forms observed both in the solid state and in solution are weakly coordinated in the sixth axial position. 

Although does not form inner-orbital 6-co-ordinate complexes, it does produce a spin-paired 5-co-ordinate pentacyanocobaltate(II) ion of bipyra-midal form (pp. 105 and 495) ... 

The hydrido complex may also arise from sources other than molecular hydrogen. The homolytic cleavage of water by pentacyanocobaltate (II) (9, 16) (Reaction 2) permits various substrates to be reduced stoichiometrically in the absence of molecular hydrogen (20). 

Alkyl Halides. Reaction with Co (CN) 5. Manya-bondedorgano-pentacyanocobaltate(III) complexes have been prepared recently by the reaction of organic halides with pentacyanocobaltate(II) (i3, 14, 21, 22) (Reaction 5). The second-order kinetics observed for this reaction (14)... 

Neopentyl and Neophyl Iodides. The halides mentioned above formed the corresponding organocobalt complexes on reaction with pentacyanocobaltate(II). While neopentyl radical is stable to rearrangement, the neophyl radical undergoes aryl migration (56) as follows ... 

Reaction with Co(CN)5H . General Considerations. Except for methyl iodide, primary, secondary, and tertiary alkyl iodides formed the corresponding alkanes, accompanied by minor quantities of alkenes, in good yield when added to well-stirred solutions of hydrido complex in a hydrogen atmosphere. Only about 35% of methyl iodide was converted to methane, the remainder forming the stable methylcobalt complex. This apparent preferential competition by pentacyanocobaltate(II) for the methyl radical has been used to prepare the methylcobalt complex (22). 

The reaction of organic halides with hydrido complex is less readily analyzed than the reaction with pentacyanocobaltate(II) since the two complex species are in equilibrium in a hydrogen atmosphere (Reaction 1). The question of whether alkanes are formed via hydride reduction of a radical (Reaction 11, Mechanism I) or via hydride-halide exchange (Reaction 13, Mechanism III) must be determined indirectly. 

Allylic Systems. Allylic Halides. Allylic halides also undergo homolytic carbon-halogen cleavage by pentacyanocobaltate(II) to form equimolar quantities of halo- and allylcobalt complexes (21, 22, 23). It is assumed that this reaction involves generation of the allylic radical (Reaction 19), which then reacts with pentacyanocobaltate(II) (Reaction 20). 

Butadiene, Using butadiene as the model substrate, it has been found that the catalytic hydrogenation of dienes occurs in three steps (1) reversible addition of hydrido complex to butadiene to form a butenylcobalt complex (Reaction 3) (2) cleavage of the butenylcobalt complex by hydrido complex to form butenes and pentacyanocobaltate(II) (Reaction 4) and (3) hydrogen absorption by pentacyanocobaltate(II) to reform hydrido complex (Reaction 1) 20, 21), The butenylcobalt complex has the same properties as the complex obtained via reaction of y-methyl-allyl bromide with pentacyanocobaltate(II) (21, 22). 

Table II indicates the isomer distribution of the butene product obtained at a cyanide to cobalt ratio of 5.1 under two reaction conditions. The upper set of figures for each substrate refers to the product distribution obtained in the presence of excess hydrido complex. The lower set of figures refers to results obtained with pentacyanocobaltate(II) in an inert atmosphere. Under such conditions, hydrido complex is formed via cleavage of water (Reaction 2).

Chlorodiphenylacetonitrile formed tetraphenylsuccinonitrile as well as a diflFerent type of complex. The infrared spectrum of the latter contained no nitrile absorption band (2210 cm." ) but exhibited a cyanide stretch at 2130 cm. S characteristic of inorganic pentacyanocobaltate(III) complexes 14,22), and a carbonyl band at 1575 cm. Its PMR spectrum indicated the presence of the (C6H5).)CH group. Diphenylacetamide precipitated on heating an aqueous solution of the complex. Apparently the resonance-stabilized radical initially formed in the reaction of this a-halonitrile with pentacyanocobaltate(II) may either dimerize or combine with pentacyanocobaltate(II) to form an N-keteniminocobalt complex. The latter is unstable in water, being converted to an N-amido-cobalt complex, which may be further hydrolyzed to the free amide (Reaction 23, paths d and e). Presumably the radical cannot add penta-... 

PMR spectra cannot indicate the presence of amide hydrogen because of rapid exchange of the proton with deuterium oxide solvent. We have found that nitrogen-cobalt bonded complexes with infrared absorptions at 1575 cm. may be formed when N-bromo primary amides react with pentacyanocobaltate(II). Comparison with the spectrum of a complex formed from an N-bromo secondary amide, in which no acidic hydrogen would be present, should help resolve this problem. 

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