Biphenylene dianion has been prepared by Li reduction of the parent hydrocarbon. It is stable below -30' and was characterized by its 'H-and I3C-NMR spectra. A comparison of these data with those of the dication indicates he existence of ion pairs in the dianion case which are responsible for a different charge distribution. The iiamagnetic ring currents of both ions, however, are of comparable magnitude, and both are more diatropic than he parent hydrocarbon. Predictions of the n-charge-density effect on 'H chemical shifts are improved by :alculations that use the linear and quadratic electric-field effect equation instead of the simple Spiesecke-Schneider .elation.Cyclic conjugated n systems containing [4n]n electrons can be transformed to -4n + 2177 systems either by oxidation to the dication or by reduction to the dianion [l]. In Zeneral, such transformations should be accompanied by a gain in delocalization energy. rhus, they are of particular interest in the case of the unstable cyclobutadiene [2], the smallest member of the neutral [4n]annulene family.While experiments to generate substituted cyclobutadiene dications were successful 31, analogous attempts to synthesize substituted cyclobutadiene dianions were less rewarding [4] [S]. For example, in the case of 3,4-diphenylbenzocyclobutadiene dianion as Nell as tetraphenylcyclobutadiene dianion, it was concluded -mainly on the basis of the NMR data -that the negative charge is predominantly residing in the phenyl substitumts, thereby rendering olefinic character to the central four-membered ring [S]. Similar results seemed to hold for biphenylene (l), where 12' was well-characterized by 'H-and 'C-NMR measurements [6], while equally clear data for 12-were still missing [7]. On the 3ther hand, however, strong support for the diatropic nature of 12-came from 'Li-NMR neasurements [8], where an upfield shift of 6.1 ppm relative to external aq. LiCl was 3bserved.We now conducted a clean reduction of biphenylene with Li sand in THF at -78" by dtrasonic treatment. A quantitative conversion to the dianion, formed as a deep copperxown suspension, was achieved in less than 2 h. We could then determine the complete H-and 13C-NMR data for 12-, and these results are reported in the present communication. In addition, we compare the results for the dianion with those obtained earlier for the dication [6], which we supplemented by an analysis of the 'H-NMR-AA'XX' system.

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