The compounds [Ru(NH3)5DTDP](PF6)2,K3[Ru(CN)5DTDP].3H2O,Na3[Fe(CN)5DTDP].4H2O, [{Ru(NH3)5}2DTDP](PF6)4, and Na6[{Fe(CN)5}2DTDP].6H2O (DTDP = 4,4’-dithiodipyridine) have been synthesized and characterized by infrared, electronic, and nuclear magnetic resonance spectroscopies, microanalyses, and cyclic voltammetry. The extent of back-bonding M(II)-->DTDP in the complexes was determined to be Ru(NH3)5DTDP]2+>[Ru(CN)5DTDp]3->[Fe(CN)5DTDP]3-, based on pK(a), electrochemical, and spectroscopic data. The pK(a) for the acids [Ru(NH3)5DTDPH]3+, [Ru(CN)5DTDPH]2-, and [Fe(CN)5DTDPH]2- are 5.25, 4.70, and 4.40 (25.0 +/- 0.2-degrees-C, mu = 0.10 M (NaCF3COO)), respectively. The rates of aquation of the DTDP ligand from the complexes [Fe(CN)5DTDP]3-, [Ru(CN)5DTDP]3-, and [Ru(NH3)5DTDP]2+ were determined to be 1.1 X 10(-3), 1.2 X 10(-4), and 4.5 X 10(-5) s-1, respectively. Electrochemical and near-infrared data for the binuclear complexes [{Ru(NH3)5}2DTDP](PF6)4, Na6[{Fe(CN)5}2DTDP], and their respective mixed-valence derivatives indicate intense electron delocalization between the two ruthenium centers.