Structural and fluxional isomerism

For simplicity, almost all of the forms of isomerism discussed above concerned classical octahedral complexes. It was implicitly assumed that each complex has a single structure and that this structure does not change with time. Other forms of isomerism are recognized if we remove one or both of these restrictions. For instance, in Section 3.2.3, we met the fact that the anion [Ni(CN)5] can exist in two different geometries, trigonal bipyramidal and square pyramidal (a phenomenon which is usually classified under the heading of structural isomerism). A rather more extreme example is provided by Co2(CO)g of which at least two forms coexist in solution  

27 Potential energy profiles in -C5H5Mn(C0)3. Consider a microscopic probe, shown at the right hand side of the dotted circle surrounding the manganese atom. If the molecule is held rigid and the probe rotated around the dotted circle, the top end of the probe will experience five bumps per circuit and the bottom end will experience three. That is, the and Mn(C0)3 units generate 

So too will each of the other four carbon atoms in the C5H5 ring. But the geometry is such that, although all of these bumps will be equal, none will coincide. That is, in a complete circuit there will be a total of 5 x 3 = 15 bumps, a 15-fold potential. 

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