Reactions of Clusters

Reactions of clusters may lead to changes in their structures or they may only change the clusters coordination sphere. They may also cause changes within the ligands themselves. 

The dependence between the cluster structure and the number of its valence electrons shows that oxidation, reduction, or other reactions leading to changes in the valence electron number must cause rearrangement of the clusters skeleton, or that such reactions must either increase or decrease the multiplicity of the metal-metal bond involved. The structural changes of clusters may also be caused by association or dissociation of Lewis acid type ligands within the coordination sphere, that is, protonation or deprotonation. 

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Reactions of clusters with mononuclear or dinuclear metal complexes frequently provide a method of expanding the metal core nuclearity under controlled conditions. The majority of medium- and high-nuclearity homometallic clusters has been prepared from lower-nuclearity cluster precursors by thermolyses ("heat-it-and-hope ) reactions. This is less true of the heterometallic clusters in this... 

B. Survey of Complexes Formed in Reactions of Cluster Carbonyls with... 

Figure 10.4 Typical time-dependent spectral changes observed for the reaction of cluster [W3S4H3(dmpe)3]PFg with HCl in a CH2CI2 solution at 25.0° C. The data were recorded for 1000 s with a logarithmic time base. (Reproduced with permission from ref. 9.)...

Full spectroscopic and preparative details for clusters 141 and 142 are given, and the syntheses of positional isomers of MoCoPdPtCp(CO)6(dppm)2 are described. These noninterconverting isomers, which differ in the nature of the exoligated metal atom, are made either by reaction of cluster 138 with [Co-(CO)4] or by reaction of cluster 140 with [CpMo(CO)3] . [P. Braunstein, C. de Meric de Bellefon and M. Ries lnorg. Chem. 29,1181 (1990)]. 

Evidence that the CODH activity is a function of cluster C was obtained by rapid-freeze EPR studies [124], The reaction of cluster C with CO occurred within 10 ms, consistent with a role in CO oxidation reaction of the other clusters was much slower. 

The study of clusters has taken the path that is quite typical in physical chemistry research for a newly discovered system or state of matter (1) elucidation of energy eigenstates, both experimentally and theoretically, (2) elucidation of structure through experiments and calculations of various degrees of sophistication, (3) exploration of system dynamics, and (4) explorations of chemical reactivity within the new system. Indeed, previous review volumes covering cluster research have dealt mostly with eigenstates and structure, with some attention given to the dynamics and reactions of clusters (Bernstein 1990 Halberstadt and Janda 1990 Jena et al. 1987 Weber 1987). 

Another simple rhodium carbonyl complex also known to be Involved In the fragmentation and aggregation reactions of clusters Is Rh2(C0)s This species has been shown to participate In the reactions of neutral rhodium carbonyl species In either matrixes or solutions (equation 4), but It has not yet been Implicated In the chemistry of large anionic clusters. 

A second neutral species that has not previously been shown to be Involved In the reactions of clusters Is HRh(C0)4. This complex has recently been produced under high pressures of carbon monoxide and hydrogen by means of the reactions of Rh4(C0)i2 with CO and H2 (equations 5) or by bringing protonic acid In contact with CRh(C0)4] under 1000 atm. of C0 H2 (equation 6). 

The reasons above directed our attention to the role of [Rh(C0>4l] In the fragmentation-aggregation reactions of clusters. This anion Is generated during the fragmentation of CRh7(CO)- 6D3— under 600 atm of C0 H2 (Figure 1, equation 10). 

The parallel between the aggregation reactions of clusters with [Rh(C0)4] under ambient and high pressure conditions is more clearly shown by the comparison of the ability of these reactions to bring about the growth of clusters one atom and one negative charge at a time under both conditions. This behavior (equations 2 and 3) has been also noted under 537-840 atm (equation II, Ftgure 2). 12)... 

Photolytic and chemical methods of activation have also been employed in the reactions of clusters with unsaturated organic ligands. The photochemical activation of compounds containing metal-metal bonds has not received much attention until relatively recently (127). However, it is now being investigated in some detail. In some cases the photochemical products differ from those obtained by thermal activation alone (127). The reaction of Ru3(CO)12 with ethylene is an example of such behavior (128). 

It has been assumed and occasionally demonstrated that surface reactions of metal cluster complexes proceed in a manner similar to reactions in solution. The following examples are typical of those postulated for the reaction of clusters with surfaces. 

C. Reactions of Cluster-Bound Allenyl Groups I. Adduct Formation... 

The reactions of vdW molecules and clusters can be divided into intra- and intercluster processes, and further into neutral and ionic cluster reactions. The latter were recently reviewed by Mark and Castleman. Therefore the scope of this contribution will be limited to neutral species only. We distinguish between intra- and intercluster reactions. In intracluster processes reactions are induced inside a cluster, usually by light. Examples of such reactions are the reaction of excited mercury atoms with various molecules attached to them, reactions that follow photodissociation in the cluster, and charge transfers inside a large cluster. In intercluster reactions the cross molecular beam technique is usually applied in order to monitor scattered products and their internal energy. The intercluster reactions may be divided into three major categories recombination processes, vdW exchange reactions, and reactions of clusters with metal atoms. 

I am grateful to acknowledge support for the research on the reactions of clusters by the US-Israel Binational Science Foundation, the Fund for Basic Research administrated by the Israel Academy of Science, and the MINERVA foundation, Munich, Germany. Helpful suggestions and discussions with Professor Eli Poliak, Dr. Ori Cheshnovski, and Sidney Cohen are gratefully acknowledged. 

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