Until the early seventies, tautomeric i. e. fast and reversible rearrangement reactions accompanied by migrations of carbon-centered groups - were practi- cally unknown. For a long time it was assumed that the family of tautomeric reactions was confined to proto tropic transformations only. However, the discovery in the fifties of the reversible metallotropic rearrangements showed the domain of migratory tautomerism to be substantially broader. The synthesis of the metallotropic compounds was based on the substitution of a proton in prototropic compounds by an organometallic group. This...