Monolayers of the dimeric complex [p0p Os(bpy)(2) 4-tet Os(bpy)(2) Cl](3+), where p0p is 4,4 ' -bipyridyl, bpy is 2,2 ' -bipyridyl, and 4-tet is 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine, have been formed by spontaneous adsorption onto platinum microelectrodes. These monolayers are stable, and both metal centers exhibit well-defined voltammetric responses for the Os2+/3+ redox reaction. Adsorption isotherms reveal that the areas of occupation for the dimer and a model monomer, [p0p Os(bpy)(2) 4-tet](2+), are identical at 158 +/- 14 Angstrom2. This result is consistent with the dimer adopting an extended configuration rather than lying coplanar with the electrode surface. High-speed chronoamperometry reveals that the standard heterogeneous electron-transfer rate constants, k, for the "inner" [p0p Os(bpy)(2) 4-tet](2+) and "outer" [4-tet Os(bpy)(2)Cl](+) moieties are (1.3 +/- 0.2) x 10(6) and (1.1 +/- 0.1) x 10(4) s(-1), respectively. The reorganization energy is at least 0.6 +/- 0.1 eV. The relatively small decrease in the heterogeneous electron-transfer rate constant across the 14 Angstrom 4-tet bridge is interpreted in terms of electron superexchange. Solution phase transient emission measurements reveal that the rate of photoinduced electron transfer (PET) between the two metal centers is (1.6 +/- 0.1) x 10(7) s(-1). This rate constant is a factor of approximately 400 smaller than the ground-state electron-transfer rate constant for monomeric [4-tet Os(bpy)(2) Cl](+) monolayers when the driving forces are identical. This significant difference is interpreted in terms of the energy separation between the ground or excited states and the bridge. These data also reveal that the strength of electronic coupling across the tetrazine bridge is significantly greater for two metal centers than for a metal electrode and a remote redox moiety.