Clusters redox condensation

The reaction of a carbonylmetalate with a neutral metal carbonyl has been labeled a redox condensation by Chini et al. (40, 41) and has been as widely used as a pyrolysis reaction for synthesizing mixed-metal clusters. Carbonylmetalates usually react rapidly with most neutral carbonyls, even under very mild conditions. A large number of mixed-metal hydride clusters have been formed via this type of reaction, primarily because the initial products are anionic clusters that in many cases may be protonated to yield the neutral hydride derivative. 

A logical extension of the synthesis of [Co6(CO)i5]2- from [Co(EtOH) J [Co(CO)4]2 has been successfully applied to the preparation of mixed-metal clusters. Thermal decomposition of [Ni(EtOII) J[Co (CO) 4]2 prepared in situ gives the red hexanuclear dianion [Ni2Co4(CO) n]2", through the following redox condensation and redistribution processes (44) ... 

Finally, cobalt carbide-carbonyl clusters have recently been isolated through a two-step synthesis. First of all, the well-known Co3(CO)9CCl is prepared from Co2(CO)8 and CCh, and then the hexanuclear carbide dianion [Co6(CO)i5C]2- is obtained in good yields (9) by further reaction with Xa[(, o(CO)4] in diisopropylether [see Eqs. (18) and (19)]. Further redox condensation between [Oo6(CO)i5C]2-and Co4(CO)i2 [see Eq. (11)] gives the square antiprismatic octanuclear cluster [Cdo8(Cdt )i8 C]2- (13). Both these carbide derivatives, as well as all of the other cobalt high nuclearity clusters, are sensitive to air and react with carbon monoxide at atmospheric pressure. 

Oxidation of HNCC often results in cluster fragmentation or, as a consequence of redox condensation, cluster expansion as discussed in Section II,B,3. The systematic formation of oxidized species of unchanged nuclearity has been investigated only recently. Depending on the reagent, the products of oxidation reactions may or may not incorporate the oxidant and these reactions are classified below accordingly. 

As with oxidation, the reduction of HNCC can be complicated by redox condensation reactions and by cluster degradation induced by free carbon monoxide. The latter is most commonly observed when reduction is effected... 

Few successful reactions are known by which an organometallic nitrosyl product has been formed from the condensation of mononuclear metal nitrosyl complexes. The unique cluster (Tj -C5H5)3Mn3(NO)4 (39), which contains the only i3-NO known (7) (40), is prepared using photochemical or thermal (41) decarbonylation of (Tj -C5H5)2Mn2(CO)2(NO)2 [Eq. (25)]. The use of the redox condensation of a nitrosyl carbonylmetallate with a neutral metal carbonyl cluster to produce a nitrosyl cluster has... 

Little information exists on the redox properties of rhodium clusters despite the fact that electron transfer is implicated in the redox condensation reactions of these compounds 192). 

The reaction of metal carbonyl complexes with carbonyl anions is a widely used technique for synthesis of mixed-metal clusters. This reaction is also termed reductive condensation or redox condensation. The metal carbonyl anion may be generated in situ or may be separately isolated. The mixed-metal product is usually an anion protonation can be used to generate a neutral cluster hydride. 

Redox condensation can occur by the simple addition of a mononudear carbonyl cationic spedes to an anionic cluster as shown in equation (3.18). Such condensation reactions allow for a stepwise and controlled construction of higher nuclearity clusters whereby the regeneration of a suffidently high negative charge on the cluster can be accomplished by appropriate reduction or deprotonation steps in the condensation sequence. [168]... 

The most striking class of redox condensation reactions is that in which two otherwise stable clusters readily and quantitatively collapse into one cluster upon being brought in contact with each other. Some remarkable more recent examples of homo- and heterometallic clusters formed by this process are shown in equations (3.19) and (3.20). [209, 210]... 

All the above hydrides, regardless of their stereochemistry, have been obtained either by direct protonation of the preformed parent anion, e.g. Equation (3.21), [324] or by protonation of a precursor anion which subsequently undergoes a more or less straightforward redox condensation, as schematically indicated in Equation (3.22). [121] At least one example for the synthesis of interstitial hydride clusters through the activation of molecular hydrogen is known and is represented in Equation (3.23). [328, 329]... 

A few of the synthetic methods adopted so far are closely related to the previously examined methods used for the synthesis of clusters containing interstitial or exposed main group elements or elemental organic fragments. For example, as shown in Scheme 3-11, the very peculiar [Bi4Fe4(CO),3] cluster (see Section 3.2.6) has been obtained in two steps through classical redox condensation reactions or by the clockwise condensation induced by fragment elimination. [312] Related syntheses have also been used for the E-Co (E = Sb,Bi) [314, 315] and n-Fe [309] mixed clusters listed in Ihble 3-11. 

Redox condensation of an anionic mono or polynuclear carbonyl species with a neutral, cationic, or anionic fragment provides a valuable method for the synthesis of carbonyl metal clusters. 

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