COMPARISON BETWEEN A DIARYLFERROCENYLMETHYLIUM ION AND ITS ISOLOBAL COBALT SPECIES - SIMILARITIES AND DIFFERENCES

Reaction of }-{eta(5)-(methoxycarbonyl)cyclopentadienyl}cobalt (9) wit h either 2 mol of arylmagnesium bromide or aryllithium and subsequent hydrolysis yields the alcohols ta(5)-(diarylhydroxymethyl)cyclopentadi enyl}cobalt (13a-c). The treatment of the yellow colored alcohols 13a- c with HBF4 in ether affords the tetrafluoroborates of the violet colo red carbocations 14a-c. Investigations of single crystals of 14b by me ans of the X-ray technique show a displacement of the C-1-C-exo bond b y alpha = 10.8 degrees toward the metal and a pronounced bond alternat ion in the fulvene moiety. The bending of C-1-C-exo is considerably la rger than in the corresponding alcohol 13b (alpha = 2.5 degrees). The interaction between the exo-carbon and the metal in 14b is further sub stantiated by comparison of the C-13-NMR chemical shift of 14b with 13 b (Delta delta = -65 ppm). The comparison of the data obtained for 14b with those of diphenylferrocenylcarbenium ion Ib suggests a stronger bonding between metal and fulvene moiety in 1b as compared to 14b. The se results are substantiated by the results of ab initio calculations on 1b and 4a which show a smaller positive charge at the exo-carbon in 4a than in 1b and a stronger bond index between the exo-carbon and th e metal in Ib as compared to 4a.