Head-head coupling of a dimethylfulvene ligand occures in of 8a and the corresponding bis(ferrocene) 9a have been dethe reaction of 6,6-dimethylpentafulvene (la) with termined. The electrochemical behaviour of 8a and 9a shows [(C,H,)Co(C,H,),] (7) to give (1,1,2,2-tetramethyl-1,2-etha-8a+, 8a-, and 9af to be localised mixed-valence species. Monediyl)bis(cobaltocene) (8a). In contrast, the mononuclear q4-lecular mechanics calculations have been performed on the fulvene complexes 3b, c are obtained from 6-ethyl-6-methyl-title complex and related derivatives. (lb) and 6,6-diethylfulvene (lc) and 7. The crystal structures 6,6-Diaryl derivatives of pentafulvenes 1 have been reported to react with 14-valence electron (VE) organometallic fragments to give mononuclear complexes with q4-coordinated fulvene ligands. For example, 2l1x2l, 3[3-51, 4c4', and 5C21 have been prepared and structurally characterised by X-ray crystallography. When fulvenes with small substituents on C-6 are treated with iron carbonyls, a variety of mono-and dinuclear complexes are f~rmed [~z~]. A key step in these reactions was thought to be loss of carbon monoxide o M e Me b Me Et