Mixed-metal clusters ligand substitution

Anionic clusters are good nucleophiles (see Section III,A) and are often easy to make. On the other hand, the electrophilic nature of most monometallic complexes is obvious from ligand substitutions. The combination of these properties makes a strategy for cluster expansion. This strategy was used for the first time by Hieber (130) in making Fe4(CO)fc from Fe3(CO),7 and Fe(CO)s. It is probably active in many syntheses of large metal carbonyl clusters because the Re, Os, Rh, Ir, Ni, and Pt clusters involved are almost always anionic. However, simple stoichiometries can rarely be written for such reactions (122). This route makes mixed metal clusters accessible, e.g.,... 

More coimnonly, it is to be expected that the reactivity of a mixed-metal cluster will be comparable to that of the most labile metal. For example, the rates of CO dissociation from (/u,-H)Ru3 Os (/u.-COMe)(CO)io fall in the series as Rus (4500) > RU2OS (1100) > RuOs2(220) OS3 (1). If this is the case, the relative rates for a series can provide information concerning the number of metal atoms which participate in such a transformation (see also Mechanisms of Reaction of Organometallic Complexes and Ligand Substitution). 

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