Metal core rearrangements

Isomerizations involving metal core rearrangements that are not fast on the NMR time scale have also been observed. Couture and Farrar (127) have... 

A number of mixed-metal clusters which undergo metal core rearrangements have been reported since 1986, and these species are listed in Table XVII. The effect of the monodentate phosphine or phosphite ligands... 

The analogous cluster which contains Ph2AsCH2AsPh2 probably also undergoes an intramolecular metal core rearrangement in solution, but it is not... 

B. Metal-Core Rearrangements in Several Tetrametal Clusters... 

As stated in the previous section, dynamic behaviour involving intramolecular rearrangements of the metal skeletons of Group 11 metal heteronuclear clusters is relatively common, in marked contrast to the situation observed for almost all other transition metal clusters, which have metal frameworks that are stereochemically rigid in solution. The mechanisms of these metal core rearrangements are, therefore, of considerable interest. 

The skeletal rearrangement mechanism proposed for (l)-(3) (Fig. 4) would also explain the single methylene proton environments observed in the low-temperature H NMR spectra of each of the clusters [Au2Ru4(/r3-H)(/r-H)(//-Ph2ECH2E Ph2)-(CO)i2l (E = E = As or P E = As, E = P) (see Sec. 1.27.2.1.1). It is interesting that the proposed metal core rearrangement in each of the above clusters must have a very much smaller value of AG than those observed for (l)-(3), since a H NMR spectrum consistent with the ground-state structure could not be obtained for any of the hexanuclear clusters, even at -90... 

Figure 6. The restricted Berry pseudo-rotation mechanism proposed for the intramolecular metal core rearrangements observed in solution for the clusters [M2Ru4H2(CO)i2L2] (M = Cu, Ag or Au L = a variety of monodentate phosphine ligands or L2 = a variety of bidentate diphosphine ligands). The mechanism exchanges the two coinage metals in sites M(l) and M(2) of the trigonal bipyramidal M2RU3 unit in the metal skeletons of the clusters via a square-based pyramidal intermediate (reprinted by permission of the Royal Society of Chemistry from ref. 36).

All of the values of AS in Table 2 are between -20 and -1-20 J mol , which supports the proposed intramolecular nature of the dppf ligand fluxionality and the metal core rearrangements. 

No NMR spectra consistent with the ground-state structure of [Au2Ru3(yU3-S)(//-dppf)(CO)9] (8) could be obtained, even at low temperatures. Therefore, no value of AG for the intramolecular metal core rearrangement of (8) could be obtained. 

The AG values calculated for the dppf ligand fluxion (Table 2) clearly show that this process and the metal core rearrangement are independent for each of the clusters, (3) and (5)-(8), which is in marked contrast to the two mutually dependent processes reported " for [Au2Ru4(/ -dppf )(CO)i2BH]. 

For Os6 clusters the metal core rearrangement is accompanied by a change of the ligand positions but the number of metal-metal bonds is retained. Thus, the bridged-square pyramidal cluster [Os6(CO)i6(yU4-S)(/43-S)] (34) partially isomerizes upon heating to 125 °C to give a basket skeleton (also with ten Os Os bonds) in which the two sulfur atoms are in a /I3 mode (35). ... 

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