Metal carbonyls nonrigidity

One aspect of metal carbonyl chemistry that should be mentioned in surveying the more commonly found modes of CO coordination is the stereochemical nonrigidity of carbonyl clusters. This aspect has received considerable attention over the past decade, especially as 13C nmr instrumentation has become more readily available. In many carbonyl clusters, terminal and bridging carbonyls as established by x-ray structural studies are equilibrated on the nmr time scale (37, 39-41). The manner of equilibration takes place in a concerted way in order that each metal center maintains a constant electron count. For example, bridge terminal interconversion, (1), proceeds via complementary unsymmetrical CO bridges. 

Stereochemical Nonrigidity in Metal Carbonyls and Their Derivatives... 

A common feature of metal clusters is their stereochemical nonrigidity, in which carbonyl and hydride ligands exchange their coordination sites. Mixed-metal clusters are ideally suited for studies of the fluxional processes in clusters because of the low symmetry inherent in their metal framework. In such clusters, the majority of the ligands are in chemically nonequivalent positions and should thus be distinguishable by NMR... 

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). 

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