Tetranuclear cobalt clusters

Table 3. Illustration of the noble gas rule for tetranuclear cobalt clusters... 

In the case of carbonyl metal clusters, the presence of free carbon monoxide notoriously affects the reduction processes inducing condensations as well as degradation of the clusters species. Thus the reduction of the tetranuclear cobalt cluster 04(00)4 with lithium, sodium, or potassium in which the final product is the hexanuclear cluster [Co6(CO)i5] is an illustrative example of the complexity of such reactions. 

The N,S-chelate 2-aminoethanethiolate (aet-) forms stable cobalt(III) complexes, including clusters where the S can take on a bridging role. Reaction of [Co(NH3)5C1]2+ with Ni(aet)2 in water for several hours affords the black tetranuclear compound Co4(aet)8, which features act in simple chelate and bridging roles (245).1083 The simple monomeric complex [Co(aet)2(en)]+ has been reported 1084 when heated in water at 50 °C, the trimer Co Co(aet)3 2 is one product, with the central Co surrounded by six bridging S atoms. 

Related substitution patterns are observed in tetranuclear cobalt and rhodium clusters. Thus, the small ligand, P(OMe) (L), occupies axial sites in [Co,(CO)1 L ] (x = 1,2) (17) whereas steric effects become important witfi (OPh) and the isomers shown in Fig. 2 are obtained with tetrarhodium derivatives. 

In mixed metal clusters, redistributions can occur in the mass spectrometer so that the highest observed peak arises from such a product. This is clearly exemplified by the heterometallic dodecacarbonyls containing metals of the cobalt triad. The thermal stability of this series of clusters decreases as the rhodium content increases and facile redistributions result in the formation of tetranuclear species containing less rhodium the very thermally stable Q I CO) does not rearrange184. ... 

In comparison with the remarkable stability of the anions [Co(CO)4], [Co6(CO)is]2- and [Co6(CO)14]4-, the low stability of the anionic tetranuclear clusters of cobalt seems extraordinary. Thus the low stability of the anion [Co4(CO)n(COOi-Pr)]- has prevented its isolation in a pure form61 ... 

The tetranuclear and trinuclear clusters will only be observed at low pressures [8], but all other species are very common under hydroformylation conditions. Complex 4 is an ionic complex that is formed in polar solvents [9] and even hexa-solvated, divalent cobalt species may form as the cation. Under practical conditions both the dimers and the hydrides are observed, thus depending on the hydrogen pressure there will be more or less of the hydride present. 

Involvement of two nucleophilic nitrogen atoms is thus typical for the amino heterocycles. The mutual disposition of the pyridine and amine nitrogen atoms allows the formation of chelate structures for the cobalt complexes of purine, 221 and 222. Structures with the N, iV -five-membered metal cycles were proven for the tri- and tetranuclear complexes of silver ) with 8-aminoquinoline (223) (92IC4370), and polymeric copper- and rhodium-acetate clusters (224). Another coordination mode can be found in the complexes of 4-amino-3,5-bis(pyridin-2-yl)-l,2,4-triazole, (225 or... 

Tetranuclear alkyne clusters involving cobalt can adopt a variety of geometries. Reductive dehalogenation of ( Bu)(PhC=C)PCI by [Co2(CO)8] affords 39 in which a chain of four cobalt centers is supported by the functionalized alkyne, as shown in Eq, (15).146-147... 

The majority of tetranuclear alkyne clusters containing cobalt adopt butterfly geometries.151152 This is illustrated by the molecular structure of [Co4(/L4-r72-HC2H)(/L-CO)2(CO)8] (Fig. 16), which can be prepared by the... 

Another series of tetranuclear compounds has been basically obtained by adding a ferrocenyl moiety to the tricobalt methylidyne cluster Co3(p3-CHXCO)9 [79, 80]. Fig. 31, which refers to Co3(C (C5H4)-Fe(C5H5) (CO)9, illustrates the main structural features of this class of compounds [80]. With respect to the symmetrical assembly of Co3(CH)(CO)9 [81], the presence of the sterically encumbering ferrocenyl substituent induces deformation from C3v symmetry displacing the capping-carbon atom towards one cobalt vertex. 

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