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　?Eint　is　mainly　composed　of　two　important　parts,　including　electrostatic　interaction　(about　52%　of　the　total　attractive　interactions　?Eelstat　+　?Eorb)　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　?Eorb　and　two　πbackdonations　from　the　occupied　p　orbitals　of　oxygen　ligands　into　vacant　?∗　MOs　of　the　[(Cp)2TM]+　fragments,　which　are　35-40%　of　?Eorb.　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　?Eorb　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.　©