text: In the study of the structure of mercury b- , lead- and organotin derivatives of nitrosophenols, Nesmeyanov discovered the phenomenon of metallotropy, that is, a special tautomerism in which a reversible transfer of an organometallic group occurs. Joint studies by A. N. Nesmeyanov and I. F. Lutsenko discovered heteroatomic tautomerism (between carbon and oxygen atoms) in keto-enol systems of tin , o- and germanium compounds. Nesmeyanov, together with Yu. A. and N. A. Ustynyuk, discovered a new type of metallotropy: it was found that in fluorenylchromium tricarbonyl anions, η 6 -complexes are equilibrium and reversibly isomerized into η 5 -complexes.