A rigid and linear bridging ligand containing two 4,5-diazafluorene chelating units separated by an adamantane spacer (diazf-a-diazf) has been synthesized and its dinuclear complexes [(bpy) 2 Ru(diazf-a-diazf)Ru(bpy) 2 ] 4 + (Ru I I . FAF . Ru I I ), [(bpy) 2 Os(diazf-a-diazf)Os(bpy) 2 ] 4 + (Os I I . FAF . Os I I ), and [(bpy) 2 Ru(diazf-a-diazf)Os(bpy) 2 ] 4 + (Ru I I . FAF . Os I I ) have been prepared as PF - 6 salts. In these novel compounds, each Ru-based and Os-based unit displays its own absorption spectrum and electrochemical properties, regardless of the presence of a second metal-based unit. The luminescence properties have also been investigated and it has been shown that electronic energy transfer takes place in the mixed-metal Ru I I . FAF . Os I I species at 77K from the Ru-based to the Os-based unit with rate constant 2.6 . 10 8 s - 1 . At room temperature the intrinsic decay of the Ru-based unit is too fast (3.3 . 10 9 s - 1 ) to allow the occurrence of energy transfer. Partial oxidation of the binuclear compounds Ru I I . FAF . Os I I and Os I I . FAF . Os I I by Ce I V in acetonitrile-water solutions leads to the mixed-valence Ru I I . FAF . Os I I I and Os I I . FAF . Os I I I species where the oxidized metal-based unit quenches, by electron transfer, the luminescent excited state of the unit that is not oxidized. At room temperature, the rate constants for the excited state * Ru I I . FAF . Os I I I -> Ru I I I . FAF . Os I I and * Os I I FAF . Os I I I -> Os I I I . FAF . Os I I processes are 8.3 . 10 8 s - 1 and 3.9 . 10 8 s - 1 , respectively, and the rate constant for the back-electron-transfer processRu I I I . FAF . Os I I -> Ru I I . FAF . Os I I I is 2.9 . 10 7 s - 1 .