Cobalt-metal bonding

Cobalt compounds have been in use for centuries, notably as pigments ( cobalt blue ) in glass and porcelain (a double silicate of cobalt and potassium) the metal itself has been produced on an industrial scale only during the twentieth century. Cobalt is relatively uncommon but widely distributed it occurs biologically in vitamin B12 (a complex of cobalt(III) in which the cobalt is bonded octahedrally to nitrogen atoms and the carbon atom of a CN group). In its ores, it is usually in combination with sulphur or arsenic, and other metals, notably copper and silver, are often present. Extraction is carried out by a process essentially similar to that used for iron, but is complicate because of the need to remove arsenic and other metals. 

Naturally occurring compounds with carbon-metal bonds are very rare The best example of such an organometallic compound is coenzyme Bi2 which has a carbon-cobalt ct bond (Figure 14 4) Pernicious anemia results from a coenzyme B12 deficiency and can be treated by adding sources of cobalt to the diet One source of cobalt IS vitamin B12 a compound structurally related to but not identical with coen zyme B12... 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

It is interesting to note that the products of these reactions obey the 18-electron rule. Cobalt has 27 electrons, and it acquires 6 from the three CO ligands and 3 from NO, which gives a total of 36. It is easy to see that Fe(CO)2(NO)2 and Mn(CO)4NO also obey the 18-electron rule. Because NO is considered as a donor of three electrons, two NO groups usually replace three CO ligands. This may not be readily apparent in some cases because metal-metal bonds are broken in addition to the substitution reactions. 

It would be fitting at this stage to define in detail the various carbon species for this review, as often different terms are used in the literature. A representation of the various carbon species is shown in Figure 4.2. Surface carbide or atomic carbon can be defined as isolated carbon atoms with only carbon-metal bonds, resulting from CO dissociation or disproportionation, the latter of which is not favored on cobalt at normal FTS conditions. Recent theoretical and experimental work has indicated that the CO dissociation is preferred at the step sites, so absorbed surface carbide is expected to be located near these sites.44-46... 

Two aspects of porphyrin electrosynthesis will be discussed in this paper. The first is the use of controlled potential electroreduction to produce metal-carbon a-bonded porphyrins of rhodium and cobalt. This electrosynthetic method is more selective than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with cyclic voltammetry, can be used to determine the various reaction pathways involved in the synthesis. The electrosynthetic method can also lead to a simultaneous or stepwise formation of different products and several examples of this will be presented. 

Further breaking of metal-metal bonds gives rise to the rectangular arrangement which is exemplified by the cobalt compounds, Co4(CO)10Oi4-E)2 already shown in Fig. 1. It is interesting that on breaking another metal-metal bond, the three residual... 

A proposed mechanism of this reaction was reported by Magnus and Principle [10], which is nowadays widely accepted (Scheme 1). Recently, negative-ion electrospray collision experiments have confirmed this mechanism in detail [11]. Starting with the formation of the alkyne-Co2(CO)6 complex 2, olefin 3 coordination and subsequent insertion takes place at the less hindered end of the alkyne. The in situ formed metallacycle 4 reacts rapidly under insertion of a CO ligand 5 and reductive elimination of 6 proceeds to liberate the desired cyclopentenone 7. It is important to note that all the bond-forming steps occur on only one cobalt atom. The other cobalt atom of the complex is presumed to act as an anchor which has additional electronic influences on the bond-forming metal atom via the existing metal-metal bond [12]. 

Recently proof has been reported for a heterometallic bimolecular formation of aldehyde from a manganese hydride and acylrhodium species [2], Phosphine free, rhodium carbonyl species show the same kinetics as the cobalt system, i.e. the hydrogenolysis of the acyl-metal bond is rate-determining. Addition of hydridomanganese pentacarbonyl led to an increase of the rate of the hydroformylation reaction. The second termination reaction that takes place according to the kinetics under the reaction conditions (10-60 bar, 25 °C) is reaction (3). The direct reaction with H2 takes place as well, but it is slower on a molar basis than the manganese hydride reaction. 

Silicon and germanium hydrides react with cobalt, manganese and rhenium carbonyls affording complexes having a silicon (or germanium)-metal bond. These reactions, described previously for inactive compounds have been used in the synthesis of optically active silyl and germyl-transition metals ... 

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

E.s.r. measurements and X-ray studies show that the extra electron in the tricobalt compound occupies an antibonding orbital made up primarily of a combination of two cobalt 3d orbitals localized in the plane of the three cobalt atoms. Replacement of one Co by Fe again leads to a decrease in the metal-metal bond lengths [0.039 A (av)]. The effect is therefore not so marked as with the sulphur analogues and the effect of replacing sulphur by selenium in [C03-(CO)gS] is to decrease the metal-metal distances by 0.021 The relevant data for the three compounds are given below ... 

Compounds with Metal-Metal Bonds. There is one clear example of the addition of a compound with a metal-metal bond to an olefin. Cobalt octacarbonyl reacts with tetrafluoroethylene to form a symmetrical adduct (102). 

Compounds with Metal—Metal Bonds. Additions of compounds with metal-metal bonds to acetylenes are rare. Perhaps the addition of acetylenes to cobalt octacarbonyl (29) should be considered an insertion reaction even though the metal-metal bond is not broken since the acetylene finally is bonded to both metal atoms. 

Metal Hydrides. It is likely that the reduction of aldehydes to alcohols by cobalt hydrocarbonyl (27) is an example of a carbonyl insertion reaction with a metal hydride. It is not clear which way the hydrocarbonyl adds to the carbonyl groups —whether it forms a cobalt-carbon bond (2), or a cobalt-oxygen bond (90). 

One complex with a metal—metal bond that has been added to an olefin is cobalt octacarbonyl. It reacts with tetrafluoroethylene and it seems reasonable that this is an insertion reaction but again it has not been proved. 

Percentage d-character Considering the electronic structure of metals thus derived, Pauling then calculates the percentage d-character of the metallic bonds, the percentage d-character being an indication of bond strength. As examples, we have chosen cobalt, nickel and copper (Table I). 

---