Nickel carbonyl species

Nickel carbonyl charged, or formed in the carbonylation reaction mixture, can catalyze the carbonylation of methanol (11). To maintain the activity of the nickel carbonyl catalyst high temperature and pressure are required (12-14). However, certain promoters can maintain an active, soluble, nickel carbonyl species under much milder conditions. The most reactive promoters are phosphines, alkali metal salts, tin compounds, and 2-hydroxypyridine. Reaction rates of 2 to 7 X 10-3(mol/1.sec) can be achieved without the use of high concentration of iodine (Table II). in addition, high reaction rates... 

When dicobalt octacarbonyl, [Co(CO)4]2, is the catalyst, the species that actually adds to the double bond is tricarbonylhydrocobalt, HCo(CO)3. Carbonylation, RCo(CO)3- -CO—>RCo(CO)4, takes place, followed by a rearrangement and a reduction of the C—Co bond, similar to steps 4 and 5 of the nickel carbonyl mechanism shown in 15-30. The reducing agent in the reduction step is tetra-carbonylhydrocobalt HCo(CO)4, ° or, under some conditions, H2. When HCo(CO)4 was the agent used to hydroformylate styrene, the observation of CIDNP indicated that the mechanism is different, and involves free radicals. Alcohols can be obtained by allowing the reduction to continue after all the carbon monoxide is... 

Although Ni(CO)4 was discovered many years ago, no neutral Ni2(CO)x compound has ever been synthesized in macroscopic amounts. However, several communications report ionic species such as [Ni2(CO)8l+, [Ni2(CO)7], and [Ni2(CO)6]+, where structures with one or two bridging carbonyls are proposed.2418 Plausible structures for neutral Ni2(CO)x (x = 5, 6, 7) have been investigated by theoretical methods, and decomposition temperatures well below room temperature have been predicted.2419,2420 Tetra-, penta-, and hexanuclear nickel carbonyl clusters have been investigated by means of molecular orbital theory. It is found that the neutral forms are more stable than the corresponding anionic forms but the anionic forms gain in stability as the nuclearity rises.2421 Nickel carbonyl cluster anions are manifold, and structural systematics have been reviewed.2422,2423 An example includes the anion [Ni9(CO)i6]2- with a close-packed two-layer metal core.2424... 

Nickel carbonyl radicals show an even greater tendency than cobalt carbonyls to cluster in a krypton matrix. Three binuclear nickel carbonyls have been detected by EPR spectroscopy in the products of y-irradiated Ni(CO)4 in Kr, yet no mononuclear species has been positively identified (65). 13C hyperfine structure has... 

A nickel(II) species is also though to be an intermediate in the carbonyla-tion reaction of iodobenzene with (V-methylbenzaldimine and nickel carbonyl. Two addition modes of an ensuing aroylnickel(II) complex to the C=N double bond can be envisaged as routes to l-methyl-2-phenylindol-3-one and an ethylenediamine derivative (Scheme 47).7 5 The scope of this simple indole synthesis has not been assessed. 

The formation of l,3,4-triarylpyrroline-2,5-diones from the reaction of diphenylcyclopropenone and N-sulfinylarylamines with nickel carbonyl has been described earlier (see Scheme 36 in Section IV,A,2).64 In contrast with this result, 2,4,5-triphenyl-3-isothiazolone 1-oxide is produced in this type of process using iron pentacarbonyl, and the analogous cyclohexyl derivative is formed in a nickel carbonyl-mediated reaction (Scheme 115).64 It is probable that metallocyclic species (cf. 93) are intermediates in these transformations. 

In the case of phosphine, especially tri-n-butyl and triphenyl phosphines, an active phosphine complex is formed in the reaction medium via reaction with nickel carbonyl. This complex is a very active species provided that the optimum concentration of phosphine is used. Low phosphine concentration results in a loss of the effective nickel concentration through the formation of nickel tetra-carbonyl, nickel metal or nickel iodide. The absolute concentration of phosphine is less important than the P/Ni ratio. In addition to form the stable Ni-P catalyst, the phosphine has to compete with other ligands in the reaction mixture for nickel. With high carbon monoxide partial pressure, there is more CO in solution to compete with phosphine favoring the formation of the carbonyl, which is inactive under the reaction conditions. Hence with high carbon mon-... 

The activity of the nickel catalyst is affected by major variations in carbon monoxide partial pressure. With very low carbon monoxide partial pressure, nickel precipitates as a metal powder and occasionally as nickel iodide. Stability of the catalyst is improved with higher CO partial pressure up to a point above which the catalyst activity drops linearly. The optimum level of carbon monoxide is different from one catalyst mixture to another. This behavior is characteristic of all the nickel catalyzed carbonylation reactions we studied. In the Li-P system, optimum carbon monoxide partial pressure is in the range of 700 to 800 psi (Table V). On the other hand, the optimum carbon monoxide partial pressure for the Li-Sn system is in the range of 220 to 250 psi, at 160 C, and 450 psi at 180 C (Table VI). It is presumed that the retarding effect of higher carbon monoxide partial pressure is associated with stabilizing an inactive carbonyl species. 

The active catalyst is presumably formed through reduction of the Ni species, aided by hydrogen or group VIB metals and their carbonyls, to the nickel carbonyl. Nickel carbonyl is converted to the active catalyst by ligand dissociation. The exact nickel species is the result of complex equilibria, equations 11-17. The zero valent complex adds methyl iodide, after CO dissociation. This is believed to be the rate-determining step and has first order kinetics with respect to both iodide and Ni. It was found that temperature greater than 100 C is needed for the oxidative addition (29). 

Nickel carbonyl is by far the best-known of these compounds. All the data support a tetrahedral structure with four equivalent, linear Ni—C—0 arrangements. The Raman (3, 62, 61) and infrared spectra (8, 53, 135, 136) have been investigated and various assignments of the fundamental frequencies have been put forward on the basis of Td symmetry (53, 136, 208). Several approximate coordinate analyses have been made using these assignments, but the spectra of isotopically substituted species have not been measured, so exact evaluations of force constants have not yet proved possible. The Ni—C and C—0 stretching force constants are particularly important and the values obtained by workers using various approximations are included in Table I. 

In summary, it can be seen that the structural chemistry of the zero-valent complexes has to date been dominated by investigations on nickel carbonyl. Preliminary studies on other compounds indicate that the tetrahedral structure may not be the only possibility, and thorough investigation of the unsaturatcd species is likely to develop. Back-donation may be examined by study of force constants, in favorable cases merely of observed frequencies, and it would be interesting to see more cases of competition for back-donation, as in (MeNC)3Ni(CO). 

Initial attack of hydroxide ions onto Ni(CO)4 gives as yet uncharacterized species,1 which readily condense with unreacted Ni(CO)4 to give variable mixtures of [Ni5(CO)12]2- and [Ni6(CO)12]2-. Hydrolysis of [Ni5(CO)12]2- in water produces [Ni6(CO)12]2-, according to the above reaction. The dianion [Ni CO) 2]2- is isolated as the tetramethylam-monium salt in a crystalline state in 70% yield. The [Ni6(CO)12]2 dianion is a starting material for the synthesis of several other homo- and heteromet-allic nickel carbonyl cluster complexes.1-9... 

The usual activation of carbon monoxide by coordination appears to involve complexes in which the caibon atom bonded to the metal is rendered slightly positive, and thus more readily attacked by electron rich species such as ethylemc or acetylenic linkages. An example is seen in the reaction of nickel carbonyl and aqueous acetylene, which results in the production of acrylic add. 

The nickel hydride species would be present in either case because of the equilibria in (8). The hydride is likely responsible for the isomerization of the initially formed 3-butenoate ester into the 2-butenoate. If a base such as a tertiary amine is added to the carbonylation reaction mixture, only the 3-butenoate is formed, since the HC1 is then neutralized preventing formation of significant concentrations of the hydride. 

Examples of nickel complexes are rare and are confined to three reported nickel carbonyl clusters containing interstitial germanium or tin atoms (90). The reaction between [Ni6(CO)l2]2 and GeCl4 affords two species, [Ni10Ge(CO)2o]2 , 79, and [Ni12Ge(CO)22]2, 80. The former structure is based on a pentagonal antiprismatic framework of nickel atoms with an... 

A second example of metal transport by single-component gas involves the formation of metal carbonyls that are unstable in carbon monoxide. Shen et al. (72) discovered that supported nickel particles are not stable in carbon monoxide. Under certain conditions, CO combines with the metal in the particles to form volatile metal carbonyls, such as nickel carbonyl. These volatile species carry the metal out of the reactor, resulting in a rapid net loss of metal. In some cases, metal is not carried out of the reactor, but new metal particles form at pore mouths, blocking them and effectively deactivating the catalyst. 

Furthermore, the binding energy difference between the 4a and 5a carbon monoxide molecular orbitals, A(4o-5o), varies by only 0.3 eV ( 7 7, 82-88). The vibrational spectra show tremendous differences, however. Both nickel (89) and palladium (68) form multiply coordinated carbonyl species at low CO exposures and the atop species are only seen at high coverage. 

The carbonylation of alcohols to give the next higher carboxylic acid is catalyzed by nickel. Catalytic systems are based on Ni(CO)4, NiX2, or metallic nickel in the presence of a halogen (86,87). In all of these cases, though, the active species can be derived from nickel(O) species, for example... 

Figure 3 shows FTIR spectra of CO adsorption on nickel magnesia catalysts. On Nim Mg(,2>oO and Ni/MgO catalysts, linear (2100-2000 cm ), bridge (2000-1850 cm ) and physisorbed Ni(CO)4 (2057 cm ) were mainly observed. In contrast, on NidojMgo O nickel monomer and dimer carbonyl species which are interacted with MgO were mainly observed as previously reported[10]. These species were increased with the CO pressure, therefore they are found to be formed via CO induced structural change. On NioojMgj O solid solution, Ni metal particles seem to be highly dispersive. 

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