Cobalt clusters

Cobalt shows a dramatic size dependence(JJ cJ that resembles the behavior of iron more so than that of vanadium or niobium. The smallest cluster to react is the trimer and the 5-9 atom clusters are significantly more inert than any of the larger clusters. Cobalt also has a significant dip in reactivity between 19 and 22. A theoretical calculation rationalized the onset in reactivity at the trimer to be associated with energetic stability of the products(29). 

The room-temperature chemistry of high-surface-area CoO-MgO solid solutions is dominated by the adsorption of CO on edges and steps. Co-ordinatively unsaturated Co2+ and O2 ions react primarily as 02 Co2+02 triplets with formation of [(CCh CoCO]2- species. In samples with high Co contents, the large amounts of clustered cobalt guest species are easily reduced by CO, even at room temperature, with formation of Co(CO)4 and carbonate-like species (377). The formation of polymeric radical anions of CO on high-surface-area CoO-MgO solid solution has also been reported (378). 

Bauvais LG, Shores MP, Long JR. Cyano-bridged Re8Qg (Q=S, Se) cluster-cobalt(II) framework materials versatile solid chemical sensors. J Am Chem Soc 2000 122 2763-72. 

Octahedral cluster iridiiam carbonyl derivatives have also been described (275), but these have been investigated in less detail than their cobalt or rhodium analogs. They are obtained by the alkali-metal reduction of Irit(C0)i2 in a manner somewhat similar to the preparation of octahedral cluster cobalt carbonyl derivatives by the alkali-metal reduction of Coit(C0)i2 Reaction of Irij(C0)i2 with sodium metal in tetrahydrofuTcin first gives the tetrahedral cluster anion Hirij (CX7) i" aind then the dark brown, octahedral cluster anion Ir0(CO)i5 , which can be isolated as its tetraethylammonixam salt (275). ... 

The cobalt carbonyls are prepared by the disproportionation reaction of [Co2(CO)g] in the presence of Lewis bases or by the reduction of cluster cobalt carbonyls with the alkali metals. The iridium compounds are obtained during reduction of [Ir4(CO)i2] with sodium in ether solution. The rhodium carbonyls are usually synthesized by reduction of [Rh2Cl2(CO)4] or [RhClg] " with carbon monoxide in basic medium or by nucleophilic attack of bases on the carbonyl group of carbonyl clusters (see preparation of [M4(CO)i2] and [M6(CO)i6]). 

They also used an octatopic porphyrin, tetrakis(3,5-dicarboxyphenyl)porphine (OCPP), to connect with various metal clusters, cobalt trigonal prisms... 

Parks E K, Winter B J, Klots T D and Riley S J 1992 Evidence for polyicosahedral structure in ammoniated iron, cobalt and nickel clusters J. Chem. Rhys. 96 8267... 

Bucher J P, Douglass D C and Bloomfield L A 1991 Magnetic properties of free cobalt clusters Phys. Rev. Lett. 66 3052... 

Billas I M L, Chatelain A and de Heer W A 1994 Magnetism from the atom to the bulk in iron, cobalt, and nickel clusters Science 265 1682... 

CNTs were also synthesized at lower temperatures starting from some metal phthalocyanines [18]. Nickel-, cobalt- and iron-phthalocyanines were deposited in vacuum and CNTs were grown perpendicularly on a quartz substrate at 700 and 800°C at relatively high yield. At the base of the nanotubes, a cluster of metal... 

Because of possible catalytic and biological relevance of metal-sulfur clusters, several such compounds of cobalt have been prepared. The action of H2S or M2S (M = alkali metal) on a non-aqueous solution of a convenient cobalt compound (often containing, or in the presence of, a phosphine) is a typical route. Diamagnetic [Co6Ss(PR3)6] (R = Et, Ph) comprise an octahedral array of metal atoms (Co-Co in the range 281.7 to 289.4pm), all faces capped by atoms,and show facile redox behaviour... 

I. M. L. BiUas, A. Chatelain and W. A. de Heer, Magnetism from the atom to the brdk in iron, cobalt and nickel clusters . Science 265.T682 (1994). 

M. PeUarin, B. Baguenard, J. L. Vialle, J. Lerme, M. Broyer, J. Miller and A. Perez, Evidence for icosahedral atomic shell structure in nickel and cobalt clusters. Comparison with iron clusters , Chem. Phys. Lett. 217 349 (1994). 

Cobalt, nickel and copper naked metal clusters and olefin chemisorption models. G. A. Ozin, Coord. Chem. Rev., 1979, 28, 117-146 (45). 

The metal concentration, matrix, and temperature effects that favor clustering of the cobalt group of metal atoms have been assessed by... 

Ozin, Hanlan, and Power, using optical spectroscopy (49,121). In view of the marked temperature-effect observed for the cobalt system, we shall focus on this cluster system here. Evidence for cobalt-atom aggregation at the few-atom extreme first came from a comparison of the optical data for Co Ar — 1 10 mixtures recorded at 4.2 and 12 K (see Fig. 4). A differential of roughly 8 K in this cryogenic-temperature regime was sufficient to cause the dramatic appearance of an entirely new set of optical absorptions in the regions 320-340 and 270-280 nm (see Fig. 4). Matrix variation, from Ar, to Kr, to Xe, helped clarify atom-cluster, band-overlap problems (see Fig. 5). 

The effect of deposition temperature on the clustering ability of Co atoms to form small, cobalt clusters was most revealing in terms of optical assignments, as well as activation-energy considerations. A series of runs with Co Ar — 1 10 deposited at 4.2, 12, 20, 25, 30, and 35 K (see Fig. 4) nicely demonstrated the gradual progression from isolated Co atoms to mixtures of Co/Coj to mixtures of C0/C02/C03. The most... 

Iridium, the heaviest element of the cobalt group, was found to display the least tendency towards cluster formation at 10-12 K (49), which, as already mentioned, was quite facile for Co and Rh (49). Considering the plethora of sharp, well-defined, atomic-resonance lines observed for Ir (see Fig. 6) compared to those of Co and Rh, the remarkably impressive correlation with the representation of the gas-... 

To conclude, we shall mention some metal-atom reactions with boranes (172) and carboranes (173). When cobalt atoms reacted with pentaboraneO) and cyclopentadiene, a number of new metalloborane clusters were formed (172), two of which were 65115003(17-05115)3 and cyclopentyl-B5H40o2(i7-05H5)3. Possible structures for the former are shown in Fig. 42. The reaction of cyclopentadiene, pentaboraneO), and 2-butyne with cobalt atoms yielded the metallocarborane species illustrated in Fig. 43 (173). 

The third reason for favoring a non-radical pathway is based on studies of a mutant version of the CFeSP. This mutant was generated by changing a cysteine residue to an alanine, which converts the 4Fe-4S cluster of the CFeSP into a 3Fe-4S cluster (14). This mutation causes the redox potential of the 3Fe-4S cluster to increase by about 500 mV. The mutant is incapable of coupling the reduction of the cobalt center to the oxidation of CO by CODH. Correspondingly, it is unable to participate in acetate synthesis from CH3-H4 folate, CO, and CoA unless chemical reductants are present. If mechanism 3 (discussed earlier) is correct, then the methyl transfer from the methylated corrinoid protein to CODH should be crippled. However, this reaction occurred at equal rates with the wild-type protein and the CFeSP variant. We feel that this result rules out the possibility of a radical methyl transfer mechanics and offers strong support for mechanism 1. 

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