First-principles calculations have been carried out for the 20-electron transition metal complexes (Cp)2TMO and their molecular wires (Cp = C5H5, C5(CH3)H4, C5(CH3)5; TM = Cr, Mo, W). The calculation results at the BP86/def2-TZVPP level reveal that the ground state is singlet and the optimized geometries are in good agreement with the experimental values. The analysis of frontier molecular orbitals shows that two electrons in the highest occupied molecular orbital HOMO-1 are mainly localized on cyclopentadienyl and oxygen ligands. Furthermore, the nature of the TM-O bond was investigated with the energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV). The attraction term in the intrinsic interaction energies ΔEint is mainly composed of two important parts, including electrostatic interaction (about 52% of the total attractive interactions ΔEelstat + ΔEorb) and orbital interaction, which might be the major determinant of the stability of these (Cp)2TMO complexes. All of the TM-O bonds should be described as electron-sharing σ single bonds [(Cp)2TM]+-[O]- with the contribution of 53-57% of ΔEorb and two π backdonations from the occupied p orbitals of oxygen ligands into vacant π* MOs of the [(Cp)2TM]+ fragments, which are 35-40% of ΔEorb. The results of bond order and interaction energy from EDA-NOCV calculations suggest the influence of the radius of TM and methyl in the interactions between TM and O in (Cp)2TMO. Additionally, the relativistic effects slightly amplify the strength of bonding with increasing ΔEorb for the EDA-NOCV calculations on three metal complexes (C5H5)2TMO. Finally, the geometries, electronic structures, and magnetics of infinitely extended systems, [(C5H5)TMO]∞, have also been explored. The results of the density of states (DOS) and band structure revealed that [(C5H5)CrO]∞ and [(C5H5)WO]∞ are semiconductors with the narrow bands, whereas [(C5H5)MoO]∞ behaves as metal.