Carbonyl and Isonitrile Clusters

The NMR spectrum of Fc3(CO)i2 is a sharp singlet (62, 63) down to 123 K (33), demonstrating a very low barrier to equilibration of the five carbonyl environments of the molecular structure in the crystal (99), if that structure (Fig. 4) were maintained in solution. Even though that may not be the case (33, 72), rapid equilibration must still be concluded. Mixed metal derivatives of the same structural type, viz. Fe2Ru(CO)i2 (104, 75) and [MnFe2(CO)i2] (II), are also nonrigid (62). 

Exchange mechanisms can be postulated for these clusters involving the types of mechanisms reported for the binuclear complexes. However, it is more informative to consider the shape of the ligand polyhedron, as well as that of the circumscribed metal skeleton (71). For 

Os6(CO)i8 has a bicapped tetrahedral metal framework in the crystal (84) with three different metal environments (Fig. 10). Three terminal 

The various types of carbonyl site exchange mechanisms can be placed in three classes. 

All of these three classes can produce bridge-terminal and/or terminal-terminal exchange, depending upon the particular example in question. In comparison with binuclear complexes, the dissociation pathway of [( -C5H5)Cr(CO)3]2 has not been identified for any cluster. Since most of the dinuclear complexes studied contain cyclopentadienyl ligands which, whenever evidence can determine this, stay bound to one metal, it seems more appropriate to place their exchange processes in the third classification. 

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