Cobalt complexes mononuclear

Rathke and Feder have employed Co2(CO)8 as the catalyst precursor in their studies. Samples withdrawn from reactions under pressure were analyzed for both total cobalt and for HCo(CO)4 (35) conversion to HCo(CO)4 was observed to the extent of 50-90%, varying according to (14) with temperature and hydrogen pressure. Experiments with different levels of catalyst showed that the overall rate of CO reduction was first-order in the HCo(CO)4 concentration, as determined by titration of reaction samples. Thus, there is substantial evidence that the catalyst in this system (or more precisely, the species present in the transition state of the rate-determining catalytic step) is a mononuclear cobalt complex. The observed kinetic dependences [Eq. 

The mononuclear cobalt complexes are stable and are able to be isolated in both 2+ and 3+ oxidation states. Cyclic voltammetric studies reveal reversible waves for both Co " 2+ and Co + i reduction couples. These redox couples are shifted anodically as the ligand substituents are changed from methyl to phenyl. Electrolytic and cyclic voltammetric studies before and after electrolysis support the idea that the integrity of the complexes is maintained during electrolytic cycles of the 2+/3+ oxidation states. The IpJIpa values of the Co + 2+ couple for the binuclear cobalt complexes are identical to those observed for the oxidation of the analogous iron complex. Attempts to produce the binuclear cobalt(III) species by exhaustive electrolysis have been limited by adsorption of the cobalt(III) complexes on the electrode surface [186, 187],... 

The mononuclear cobalt complexes also undergo a capping reaction with an additional HBF4 molecule forming a cage-like compound [437],... 

Cobalt(II) alkoxides are known and monomeric forms are part of a wider review.413 The interest in these compounds pertains to a potential role in catalysis. For example, a discrete cobalt(II) alkoxide is believed to form in situ from a chloro precursor during reaction and performs the catalytic role in the decomposition of dialkyl pyrocarbonates to dialkyl carbonates and carbon dioxide.414 A number of mononuclear alkoxide complexes of cobalt(II) have been characterized by crystal structures, as exemplified by [CoCl(OC(t-Bu)3)2 Li(THF)].415 The Co ion in this structure and close relatives has a rare distorted trigonal-planar coordination geometry due to the extreme steric crowding around the metal. 

Ammino-derivatives op Cobalt Salts—Cobaltous Salt Ammines—Cobaltic Salt Ammines—Mononuclear Cobalt-ammines containing One Atom of Cobalt in the Molecule—Cobaltic Salts with Trivalent Cation—Cobalt-ammines Containing Divalent Cation—Cobalt-ammines containing Monovalent Cation—Cobalt-ammines consisting of Non-dissociable Complex— Cobalt-ammines containing Monovalent Anion—Cobalt Salts containing Trivalent Anion—Polynuclear Cobalt-ammines containing Two or more Cobalt Atoms in the Molecule—Cobalt-ammines of Unknown Constitution— Ionisation Metamerism—Polymerisation Isomerism—Valency Isomerism —Co-ordination Position Isomerism—Isomerism due to Asymmetric Cobalt Atoms. 

It has long been known (93) that cobalt(II) complexes of phthalocyanines interact with molecular oxygen. The water-soluble tetrasulfonato derivative of the parent phthalocyanine selectively and catalytically oxidizes 2,6-di-tert-butylphenol to the benzoquinone and the dipheno-quinone in both homogeneous solution (94) and when polymer-supported (95). The active intermediate in the catalytic cycle is proposed to be the (as expected) mononuclear dioxygen complex of the cobalt-tetrasulfonatophthalocyanine system (92). It has been proposed that the formation of a peroxo-bridged dinuclear complex is responsible for the deactivation of the cobalt(II)-tetrasulfonatophthalocyanine system, since such a dinuclear system would be unable to further bind and activate dioxygen (96). Such deactivation results, ultimately, in loss of the catalyst and low turnover ratios. 

Complexes of other amino acids or their derivatives with cobalt(II) that have been investigated include dipeptides (120) these complexes have long been known to absorb dioxygen. For example, the mononuclear cobalt(II) complex of N, N,N", N "-diglycylethylenediaminete-traacetic acid (121) absorbs one mole of dioxygen per two moles of complex. This system has been proposed as a simple, convenient model system for the study of dioxygen complexes of cobalt(II) peptides in solution because of its relatively slow conversion to the irreversibly formed cobalt(III) dioxygen complex. 

It has been shown that mononuclear cobalt superoxo complexes react with 2,4,6-tri-t-butyl-phenol. The anion [Co(CN)5(02)]3 acts as a base in the oxidation of the phenol. The neutral complex Co(salptr)(Oz) forms a peroxy adduct with 2,4,6-tri-t-butylphenol. An X-ray study on this adduct has revealed the structure shown in (10).m... 

A reaction specific to mononuclear cobalt superoxo complexes is free radical abstraction. Much... 

A similar pattern has always been discussed for rhodium, with hydri-dotetracarbonylrhodium H-Rh(CO)4 as a real catalyst species. The equilibria between Rh4(CO)i2 and the extremely unstable Rh2(CO)s were measured by high pressure IR and compared to the respective equilibria of cobalt [15,16]. But it was only recently that the missing link in rhodium-catalyzed hydroformylation, the formation of the mononuclear hydrido complex under high pressure conditions, has been proven. Even the equilibria with the precursor cluster Rh2(CO)s could be determined quantitatively by special techniques [17]. Recent reviews on active cobalt and rhodium complexes, also ligand-modified, and on methods for the necessary spectroscopic in situ methods are given in [18,19]. 

Table 54 Structures Mononuclear Cobalt(IJI)-Superoxo Complexes...

The heterobinuclear complexes [MCo(/u,-FiCC2CF3)2(CO)3Cp] have been prepared by addition of octacarbonyldicobalt to the mononuclear alkyne complexes [M(F3CC2CF3)2(Cl)Cp] (M = Mo or W)24 demonstrating that cobalt-carbonyl fragments can coordinate to an alkyne already bound to another metal-ligand fragment see Eq. (5). 

Many of the structurally characterized -superoxo complexes are cobalt containing, or are iron complexes with sterically hindering porphyrins. Co compounds often react with dioxygen to form mononuclear superoxo complexes. 

The complex ions containing only hydroxo bridges (III and VI) are of interest because these are the only polynuclear cobalt (III) species which can be prepared directly from mononuclear cobalt (III) complexes. These ions are prepared by removing two moles of water from two moles of solid starting material—e.g., the sulfate of III is obtained by heating hydroxo-aquotetramminecobalt(III) sulfate (30) ... 

Binuclear complexes of naphthalene (VIII) and of benzene (IX) with [Os(NH3)5]2+ and [Ru(NH3)5]2+ moieties were reported by Taube et al. (19), in which anti- //-1,2-rj2 3,4- 2) coordination of the arene is present. This bonding mode was first observed by Pasman et al. (20) in the binuclear Rhenium complex X. Wolczanski et al. (21) obtained complex XI from the mononuclear complex [Ta(0—Si Bu3)3 2-(N,C)-pyridine ] and benzene. Each tantalum atom is bound unsymmetrically to one C=C bond of benzene with a weak interaction to a third carbon atom the bonding may be described as a distorted tj3-enyl. In the dinuclear cobalt complex XII, the xylene functions as a bis(enyl)-ligand (12,22). 

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