Possible reaction mechanisms for C-C coupling and cleavage reactions involving [(eta(5)-C5H5)M-(mu-CCR)(2)M(eta(5)-C5H5)(2)] and [(eta(5)-C5H5)M(mu-RCC-CCR)M(eta(5)-C5H5)(2)] (M =:Ti, Zr) complexes are discussed on the basis of theoretical studies. A series of realistic and simplified model complexes, [L2M(mu-CCR)(2)ML2] 8 and [L2M(mu-RCC-CCR)ML2] 9 (M = Ti, Zr; L = eta(5)-C5H5, Cl, H; R = H, F, CN) were calculated using ab; initio MO and density functional methods. The formation of the bisacetylide bridging complex starting from either metal acetylides or butadiyne is explained by a mechanistic scheme involving metallacyclocumulenes (5), binuclear complexed metallacyclocumulenes (6), and the M+2-M+4 complexes (7). The latter rather than 6 led to the C-C coupled products 9 via 8. The experimentally observed unusual C-C coupling in the dimeric bis(eta(5)-cyclopentadienyl)phenylethynyltitanium complex and the lack of it in the corresponding zirconium complex are influenced by substituents on the ethynyl group. Theoretical studies demonstrate that the electron-withdrawing substituent R = F drives the thermodynamic equilibrium distinctively towards the C-C coupled product, 9 both for M = Ti and Zr. An unusually shea Zr-Zr interatomic distance in [(eta(5)-C5H5)Zr(mu-CCR)(2)Zr(eta(5)-C5H5)(2)] compared to Ti-Ti distance in the corresponding Ti complex is observed theoretically and from X-ray crystal structures of related complexes. Substitution of cyclopentadienyls by Cl and even Il to obtain computationally viable models does not change the relative energies of the ground and transition states of the binuclear Zr complexes. The Ti complexes are affected somewhat more by these changes but the trends in relative energies between Ti and Zr isomers are retained. Geometric parameters and relative energies obtained at the B3LYP/LANL2DZ level are found to be better than those at the HF level.