Ring whizzing

Metal migration about a jt-complexedpolyene ligand ( ring-whizzing )... 

Because the tropylium cation contains one uncoordinated olefinic bond, yet displays a single proton NMR signal in solution, rapid valence tautomerism or ring whizzing is proposed (18S). 

Coordination modes of reduced hapticity (rf 2v, v = 1, 2, 3) may be observed, especially when the maximum hapticity would lead to >18VE. In such cases the metal may migrate between possible donor sites ( ring-whizzing ) in solution (NMR evidence), although this is frozen out in the solid state (IR, crystallography). 

The papers reviewed here can be divided into two broad categories (i) those that are fairly non-specific in their choice of data, studying basically all complexes with a common stoichiometry ML , where M = metal, L = a coordinated ligand atom, and rt = 3, 4, 5,..., 12, and (ii) those that set out to examine specific reaction paths - such as metal cluster rearrangements, geared rotations, ring whizzing, etc. - and which are fairly selective in data retrieval. We shall discuss the papers in the above order. 

---