Homo- and Heterometallic Transition Metal Clusters

The type and range of metal carbonyl clusters which have been isolated and characterized for each metal, or combination of metals, are illustrated in Ihble 3-3 and 3-4 respectively, and for each entry the most prominent examples have been included. 

Ikble 3-3. Some representative high nuclearity homometallic clusters.  

It should be clear, however, that a certain amount of overlap between these three classes is unavoidable, not only because the categorization of chemistry often leads to oversimplification, but also because the above categories have been set up without any support from mechanistic studies. Often partition has been made only on the basis of the small amount of miscellaneous information which is presently known about the experimental conditions and the kind of reagents employed in each case. 

1 Synthesis of High Nuclearity Clusters by Ligand Elimination 

The use of either an anionic carbonyl precursor or a suitable mixture of two anions, so as to set the metal/charge ratio from the outset, is not compulsory because the final metal/charge ratio can also be controlled or achieved by addition of a carefully calibrated amount of oxidizing or reducing agent to the reaction mixture. A recent example in which reductive conditions were introduced during thermal activation of a carbonyl anion precursor is shown in equation (3.5), [164] 

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