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.