Stabilization of Ta-Ta bonding in an oxide across a shared octahedral-site edge of a Ta-2 dimer is not known. Investigation of Li insertion into the trirutile structure of MTa2O6 with M = Mg, Cr, Fe, Co, and Ni indicates that Ta-Ta bonding across the shared octahedral-site edge of the dimer can be stabilized by a reversible electrochemical reduction of Ta-V to Ta-IV for M = Cr, Fe, Co, and Ni but not for M = Mg. Chemical reduction of MTa2O6 by n-butyl lithium only reduced NiTa2O6 to any significant extent. With M = Fe, Co, or Ni, electrochemical formation of the Ta-Ta bonds is accompanied by a partial reduction of the Fe-II, Co-II, or Ni-II to Fe-0, Co-0, or Ni-0. For M = Cr, two Li per formula unit can be inserted reversibly with no displacement of Cr-0. For M = Mg, no Mg-II are displaced by Li insertion, but a solid-electrolyte interphase (SEI) layer is formed on the oxide with no evidence of Ta-Ta bonding. Stabilization of Ta-Ta bonding across a shared octahedral-site edge in a dimer appears to require significant hybridization of the Ta-V 5d(0) and M 4s(0) states.