Metal clusters stereochemical 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. 

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... 

Melnik and Parish (10) correlated 197Au Mossbauer and X-ray structural data for gold clusters in 1986, and Salter (11) has recently published a review of the stereochemical nonrigidity exhibited in solution by the metal skeletons of some mixed-metal clusters containing copper, silver, and gold. 

Summary of the Heteronu clear Clusters with Stereochemically Nonrigid Metal Frameworks Which Are Discussed in Detail in Ref. 11... 

Figure 8. Possible pathway for the complete scrambling of metal vertices in the monocapped square antiprismatic Rh, core (containing an encapsulated P atom) of the [Rh,P(CO)2i] dianion. The stereochemical nonrigidity of the rhodium polyhedron involves elongation-contraction sequences of the Rh—Rh bonds corresponding to cluster-breathing movements. ...

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