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Reaction of chromocene with chloroform yields [Cp2Cr]+[CpCrCl3]- (2) which is transformed to trans-[CpCrCl2]2 (3 a) on heating and to a mixture of cis/trans-[CpCrCl2]2 (3 b/a) on standing in chloroform. 2 and 3a/b have been characterized by paramagnetic 1H and 13C NMR yielding the first spectra of the chromocenium cation and the halfsandwich type [CpCrX2L]n-. For comparison, CpCrCl2(pyridine) (4) has been studied; the NMR data of 4 and [CpCrC3]- indicate that both are electronically similar. An X-ray analysis shows one isomer of 3 to be the trans-compound. Its chromium and bridging chlorine atoms constitute a square with a metal-metal distance of 336.2 pm. Magnetic exchange is established for 3 a by temperature dependent NMR studies. The X-ray results suggest superexchange to be the dominating mechanism.
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Köhler et al. (1983) studied this question.
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