Reactions of Clusters with Unsaturated Ligands

Several factors affect the nature of the products in a reaction between a transition metal cluster and an alkyne or alkene. In this section, the various synthetic routes to alkyne or alkene-substituted clusters will be presented, and these will be used to analyze the changes in reactivity of the cluster systems when one or more of the important reaction parameters is altered. In order to simplify the discussion, tri-, tetra-, and higher nuclearity clusters will be treated separately. Finally, in this section, there is a brief description of the chemistry of alkylidyne-substituted clusters since synthetic routes to alkyne-containing complexes may involve these species. 

In many cases the use of mono-substituted alkynes in reactions with M3(CO)l2 (M = Fe, Ru, Os) gives products very similar to those obtained with disubstituted alkynes (43-48). Nevertheless, the hydrogen atoms a to the triple bond may undergo a transfer from the ligand to the metal framework (49-52). Another interesting chemical transformation occurs in the reaction of Fe3(CO)12 with 1-pentyne (53). One of the products obtained, in very low yield, is the pentanuclear carbide Fe5C(CO)15 [Eq. (2)]. 

There are few reports of reactions between alkynes and trinuclear clusters of metals other than iron, ruthenium, or osmium. Some rhodium, platinum, and mixed-metal clusters undergo metal-metal bond rupture in reactions with alkynes (54-56), while in other cases the alkyne coordinates to the trinuclear unit without causing any major changes in framework geometry (56-59), as illustrated in Eq. (3). 

The reaction of M3(CO)12 with both open-chain and cyclic poly-alkenes has attracted some attention, especially in the case of Ru3(CO)i2. In most of the examples reported, the organic fragment bonds to the metal framework in such a way as to interact with more than one of the three metal atoms (68-77). There are some exceptions to this general statement, however. One is the reaction of Ru3(CO)j 2 with cyclopentadiene, in which a mononuclear complex is obtained (78). In other cases, tetranuclear and hexanuclear compounds are obtained (79 81). Cluster breakdown has also been observed in the case of a rhodium complex upon reaction with ethylene (55) as shown in Fig. 3. 

It is important at this stage to mention that most reactions involving the use of alkynes or alkenes have been carried out in hydrocarbon solvents, such as hexane or octane, or in aromatic ones, such as benzene. When polar solvents are employed there are sometimes variations in the number of products and in the yields obtained (28,31). For example, in hydrocarbon solvents, the reaction between Ru3(CO)12 and diphenylacetylene leads to the isolation of Ru3(CO)9(PhCCPh) as the major product. When the same reaction is carried out in basic aqueous methanol, the hydrido complex Ru3H2(CO)9(PhCCPh) is obtained in reasonable yield. 

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