Metal clusters fluxionality studies

The focus of research on very mixed"-metal clusters has been on their synthesis and structure, and the limited physical measurements of these clusters have thus far been largely restricted to fluxionality and electrochemical investigations. Studies of ligand fluxionality are summarized in Section 111.A. and reports of electrochemical investigations are reviewed in Section Ill.B. The few reports of the magnetic behavior of these clusters are discussed in Section lll.C. 1., and theoretical studies are summarized in Section I1I.C.2. 

Ligand fluxionality on metal clusters has been the subject of many studies, the majority of reports focusing on carbonyl migration on homometallic tri- and... 

Table XI summarizes reports of ligand fluxionality at "very mixed -metal clusters. A number of studies (c.g.. Refs. 28, 8I, 116, 213, 214) have reported that both metal-localized and global carbonyl fluxionality occur but give little mechanistic detail. The following discussion focuses on the examples for which detailed studies have been undertaken and/or those for which mechanistic speculation is available.

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 best evidence for metal fluxionality in Cd7-MT was provided by " Cd NMR saturation transfer experiments, which established the presence of inter- and/or intramolecular metal exchange within the 3-metal cluster of the /3-domain with a half-hfe of the order of 0.5 s. The confirmation of similar processes taking place within the 4-metal cluster, but with a half-hfe of about 16 min, was afforded by metal exchange studies using the radioactive Cd isotope. In this context, it should be noted that the intersite cadmium exchange measured in the 3-metal cluster of MT-1 was found to be much faster than that in MT-2. This finding was in line with the enhanced backbone flexibility recognized in the NMR studies of the former isoform. ... 

The tetraanionic [H2O Al3(/r-OH)(/r-OR)3]" [R = C(CH2C02)2C02] moiety has been achieved by the combination of (H20)9 Al(N03)3 with citric acid [101], with the synthesis establishing the existence of polynuclear complexes of a type previously postulated [102, 103], Al NMR spectroscopic studies have established that the tris(aluminium) cluster (Section 3.2.5) is not fluxional in D2O but instead retains three distinct metal environments over a wide pH range (3.0-9.0) with acid hydrolysis occurring below pH 3.0 and the resultant spectrum revealing the presence of [(H2O), (RO)Al3] and/or [(H20) (ROH) A13] " and also [(HjOlg Al] "" [101]. 

Also rather few solid state metal NMR spectra of clusters have been reported. This type of experiment is very promising for the metal NMR of clusters, basically for two reasons. First, solid state NMR gives tensorial values of the chemical shifts, and the anistropy of coupling constants reflecting more precisely the electronic structure. The measurement of dipole-dipole interactions, where possible, should allow the determination of distances involving light nuclei such as H, Li, Be which are not easily accessible by X-ray diffraction. The second reason for interest in solid state studies is that the fluxional behaviour is often frozen out and that consequently NMR results are more amenable to correlation with X-ray studies. 

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