STEREOCHEMICALLY NONRIGID ORGANOMETALLIC MOLECULES .22. A FLUXIONAL INDENYLMERCURY COMPOUND

The compound bis(indenyl)mercury has been prepared and its pmr spectrum studied as a function of temperature from –41 to +68°. At the lowest temperatures a spectrum indicative of a bis(1-monohaptoindenyl)-mercury molecule is observed. As the temperature is raised this spectrum changes in a manner indicative of increasingly rapid occurrence of some process which exchanges the environments of the 1 and 3 protons of the indenyl groups. Concentration independence of the spectrum in the temperature range of intermediate exchange rate rules out a second-order intermolecular exchange. It is therefore suggested that the process is an intramolecular one with an activated state (or very short-lived intermediate) in which the mercury atom lies over the face of the five-membered ring forming a kind of delocalized bond to three or more of the carbon atoms. Mercury is able to participate in such a transition state by utilizing additional 6p orbitals of its valence shell, whereas the iron atom of (h5-C5H5)(h1-C9H7)(CO)2Fe has no comparable orbitals available and, therefore, this latter molecule is not fluxional, as previously reported. Data reported here on chemical shifts for 1-indenyl compounds vitiate the argument used to assign the spectrum of (C2H5)3PCu(h1-C5H5), and hence the question of whether 1,2 or 1,3 shifts predominate in that case is very much open. Similarly, the proposal that 1,2 shifts predominate in (C5H5)2Hg is also shown to lack proof. © 1969, American Chemical Society. All rights reserved.