Anionic alkyl and aryl chromium complexes

Dimeric phenyl chromium(II) complexes, M2[Cr2Ph ]3Et20 have been isolated [102]. Their low magnetic moment suggests a degree of spinpairing by formation of a Cr-Cr bond a possible structure of the anion is 1.20b. 

From diromium(II) chloride, the yellow tetramethyl complex [Li2[Cr(CH3)4(THF)2]]2 may be prepared [99, 100] it has a magnetic moment of 0 5 B.M. Finally, there is evidence that treatment of CrCl3(THF)3 with vinylmagnesium chloride forms an unstable vinyl-Cr complex [100a]. 

At room temperature, the benzyl cation [PhCH2Cr(H20)j]2+ in solution has a half-life of about days. Evidence for a benzyl-chromium r-bond in this complex comes from the reaction with mercuric chloride which takes place without change of pH  

Other reactions which support the formulation are acid cleavage, which gives toluene, and the decomposition product bibenzyl [112]. Bibenzyl is also formed when the solutions of [PhCH2Cr(H20)s]2+ are treated with benzyl bromide [112]. It is proposed that this reaction occurs via an intermediate such as 7.21 [112]. In a related reaction allyl bromide gives C6HsCH2CH2CH=CH2 [113]. 

The trifluoromethyl complex is stable in solution at room temperature for several weeks [114a]. Related studies include the homogeneous reduction of alkyl halides, RI, by chromium(II) ions in aqueous dioxan, The products are R2 and RH and presumably the reactions involve RCr + intermediates [113]. 

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Fig. 58. Some preparations and reactions of anionic alkyl and aryl chromium complexes...

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