Abstract: Transition metal sandwich complexes are a highly promising compounds that have attracted much attention in many fields, but currently only a small portion of these complexes can be structurally characterized from experiments. Through density functional theory, adopting the B3PW91/DZP method and M06-L/DZP method.This article investigates the geometric configuration and electronic structure of (C₄H₄)M(C₈H₈) (M=Ti to Ni), which is the isomers of (C₅H₅)M(C₇H₇) and (C₆H₆)₂M. For the structure composed of early transition metals Ti and V (C₄H₄)M(C₈H₈), regardless of how the two ring ligands are combined, the lowest spin multiplicity sandwich structure with two parallel rings is the lowest energy structure in their respective systems. For early transition metal Cr, the lowest energy (C₄H₄)Cr(C₈H₈) is not a singlet structure with parallel rings, but a triplet structure composed of a hexhapto η⁶-C₈H₈ ligand and a tetrahapto η⁴-C₄H₄ ligand. For (C₄H₄)Mn(C₈H₈), it is predicted that the lowest energy state is the quadruplet state (η⁴-C₄H₄)Mn(η⁵-C₈H₈), where the C₈H₈ ring is a pentahapto. For (C₄H₄)Fe(C₈H₈) system, the predicted structure with the lowest energy is a singlet structure with a stable 18-electron configuration. For (C₄H₄)Co(C₈H₈) system and (C₄H₄)Ni(C₈H₈) system, the C₈H₈ ring for the lowest energy becomes a tetrahapto η⁴-C₈H₈, giving the Co and Ni atoms the favored 17- and 18-electron configurations, respectively.