Alkyl and aryl complexes of chromium

The formation of unstable organochromium complexes was first observed in 1903 [85], but it was not until 1919 that Hein successfully isolated a polyphenyl du-omhun complex , firom the reaction between phenyl-magnesium bromide and dhromium(III) chloride in anhydrous ether [86] 

Subsequent studies by Zeiss and co-workers have shown that the formation of these Ji-arene complexes proceeds via chromium- r-aryl complexes which they have isolated in a few cases. 

Triphenylchromium has been isolated as the deep red tris-tetrahydro-furanate, (C6Hs)3Cr(THF)3, by treatment of anhydrous chromiui n) chloride in tetrahydrofuran with phenylmagnesium bromide in an exact 1 3 mole ratio at —20° [93, 94]. Evidence for the presence of r-bonded phenyl groups is shown by its reaction with mercuric chloride, giving phenylmercury(II) chloride in quantitative yield. 

The mesitylene complex (Mes)3Cr(THF)3 is more stable than the phenyl analogue and does not rearrange when the tetrahydrofuran is removed. Indeed, the monotetrahydrofuranate (Mes)3Cr(THF) has been isolated [95a]. It appears that washing the complexes (p-RC6H4)3Cr(THF)3 with diethylether or benzene gives r-arene complexes only when R = Ph or CH3 [95h]. 

Both the triphenyl- and trimesityl Cr(III) complexes react with olefins with addition of the aryl group to C=C tystems. Thus the products formed from norbomadiene are shown below together with a postulated intermediate [96], 

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