The Second Reduction Stage

In contrast to the first reduction process, where the length of the tether does not influence the nature of the reduction product, the outcome of the second reduction step depends on the strain in the system. Three different cases were reported. 

Pyrene tethered by seven carbon atoms A surprising structure was found for the dianions of 44 and 48, that have the shorter bridge (seven carbon atoms). A new intramolecular cr-bond is formed, that transforms one of the benzene rings into a cyclopropano-cydopentane (bicyclo[3.1.0]) ring system [127]. The H and C NMR spectra of these dianions show that their symmetry is lower than that of neutral 44 and 48 but higher than that of the dimers 43a-48a. These unexpected products could explain all of the unexpected NMR data. The high field shift of carbon atom Cl (Jc = 33.9 and 33.7 ppm for 44b and 48b, respectively) and its one- 

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The oxychloride is volatile and enters the second reduction stage along with the pentachloride. At about 500°C it tends to be reduced to niobium oxide, which deposits with the trichloride, i.e. 

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