Organotransition metal chemistry electronic structures

The 19-electron complexes [Fe Cp(T] -arene)] have a modest stability (up to -10°C) if the arene is not CsRe (R = Me or Et) or l,3,5-C6H3t-Bu3. For instance, with C6Me6.nHn (n < 6), they dimerize in pentane around 0°C via an unsubstituted arene carbon, which is taken into account by the mesomer 18-electron structure below. As often in organotransition-metal chemistry, the frontier between mesomerism and tautomerism is fuzzy. From the point of view of the strict organic definition, the different forms should be tautomers, because carbon atoms are changing place. In reality, however, there may be only one potential well for the three forms with a too low conversion barrier, and the three forms are probably best viewed as mesomers since they are not observable as distinct species at the infrared time scale (10 /s) ... 

Electronic Structure of 2,2/-Bipyiidine Organotransition-Metal Complexes. Establishing the Ligand Oxidation Level by Density Functional Theoretical Calculations, C. C. Scarborough, K. Wieghardt, K. Inorganic Chemistry 2011, 50, 9773. 

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