General mechanisms of ligand exchange

Despite the above mentioned problems in obtaining detailed experimental information about the pathways of nuclear permutations, several general empirical models for the fluxional behavior in clusters have been proposed. Several general classes of fluxional exchange have been observed in clusters, e.g.  

The metal clusters of the iron triad M3(CO)i2, 4 M = Fe, 5 M = Ru, and 6 M = Os, despite being some of the simplest carbonyl clusters known, are fascinating molecules, which still present interesting structural and dynamic problems. Because of their simplicity, they have been, and indeed remain, archetypal molecules for discussions on fluxional mechanisms. The two heavier congeners 5 and 6 have a clearer and less complicated history. In the solid state they are isostructural, and possess 

For Fe3(CO)i2 (4) the situation is considerably more complex and considerably more interesting. The oft-repeated saga of the numerous structural and spectroscopic studies on 4 is well known and does not bear repeating here - the interested reader is directed elsewhere for a recent resume. In a nutshell, cluster 4 in the solid phase at room temperature has a disordered structure. The carbonyl ligand polytope corresponds to a distorted icosahedron, in contrast with the anti-cubeoctahedron found for 5 and 6, and the metal triangle is statistically disordered over two sites, as required by the crystallographic inversion center in the space group P2 /n. This is the well known Star of David disorder, which has been observed in a number of other related clusters. It arises because the distorted 

Some evidence for possible pathways is afforded by the variable-temperature structures of the closely related molecules Fe2M(CO)i2 7M = and 8M = 

The ruthenium analog 7 is isomorphous and isostructural with 8, and has almost identical behavior. The high-temperature phase change occurs at a slightly lower temperature, between 313 and 323 K. This centrosymmetric phase is isomorphous 

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