Fluxional Metal Frameworks

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... 

Some use has been made of severe reaction conditions in which a metal salt or complex is reduced in the presence of a main group element compound [Eqs. (229)-(233)]83 375,378,384,385 to give extremely robust compounds. Unfortunately, little of the subsequent chemistry of these high-nuclearity species has been reported, except for spectroscopic investigations of the rhodium compounds that have shown the metal frameworks to be fluxional as well as the ligand complement. 

It should also be noted that both the Rhn- and Rhi4-metal frameworks remain intact whereas less well-packed polyhedral metal skeletons containing a main group interstitial atom (see Sections 3.3.6 and 3.3.7) are fluxional. 

VT-multinuclear NMR studies have allowed one to establish unequivocally whether the M-coordinated-C02 framework is a rigid structure or not To this end, it is necessary that other nuclei (such as the P-donor atoms of phosphanes) are bonded to the metal centre so that, when the metal favours the coupling, it is possible to observe whether the solid state stmcture is maintained in solution or the framework exhibits some fluxionality. Complex 1 has been extensively investigated and the fluxional behaviour has been well explained by coupling IR and NMR studies [6, 40],... 

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