Stabilities, Electronic Structures, and Bonding Properties of 20-Electron Transition Metal Complexes (Cp)<sub>2</sub>TMO and their One-Dimensional Sandwich Molecular Wires (Cp = C<sub>5</sub>H<sub>5</sub>, C<sub>5</sub>(CH<sub>3</sub>)H<sub>4</sub>, C<sub>5</sub>(CH<sub>3</sub>)<sub>5</sub>; TM = Cr, Mo, W)