A combination of electrochemistry, spectroelectrochemistry, and H-1 NMR has been used to study the reduction and solution speciation in acetonitrile of two mononuclear Ru complexes containing the redox-active 9,11,20,22-tetraazatetrapyrido [3,2-a:2',3'-c:3 '',2 ''-1:2'",3'"-n]pentacene (tatpp) ligand. These complexes, [(bpy)(2)Ru-(tatpp)] [PF6](2) (1[PF6](2)), and [(phen)(2)Ru(tatpp)] [PF6](2) (2[PF6](2)) (where bpy is 2,2'-bipyridine and phen is 1,10-phenanthroline), form pi-pi stacked dimers (e.g., pi-{1}(2)(4+) and pi-{1}(2)(4+)) in solution as determined by H-1 NMR studies in an extended concentration range (90 - 5000 mu M) as well as via simulation of the electrochemical data. The dimerization constant for 1(2+) in acetonitrile is 2 x 10(4) M-1 as determined from the NMR data. Slightly higher dimerization constants (8 x 10(4) M-1) were obtained via simulation of the electrochemical data and are attributed to the presence of the supporting eletrolyte. Electrochemical and spectroelectrochemical data show that the pi-pi stacked dimers are electroreduced in two consecutive steps at -0.31 and -0.47 V vs Ag/AgCl, which is assigned to the uptake of one electron by each tatpp ligand in pi-{1}(2)(4+) to give first pi-{1}(2)(3+) and then pi-{1}(2)(2+). At potentials negative of -0.6 V, the electrochemical data reveal two different reaction pathways depending on the complex concentration in solution. At low concentrations (<= 20 mu M), the next electroreduction occurs on a monomeric species (e.g., [(bpy)(2)Ru(tatpp)](+/0)) showing that the doubly reduced pi-pi dimer (pi-{1}(2)(2+) and pi-{2}(2)(2+) dissociates into monomers. At high concentrations (>= 100 mu M), reduction of pi-{1}(2)(2+) or pi-{2}(2)(2+) induces another dimerization reaction, which we attribute to the formation of a or-bond between the radical tatpp ligands and is accompanied by the appearance of a new peak in the absorption spectrum at 535 nm. This new or-dimer can undergo one additional tatpp based reduction to form sigma-{1}(2)(0) or sigma-{2}(2)(0), in which the.tatpp-bridged assembly is the site of all four reductions. Finally, potentials negative of -1.2 V result in the electroreduction of the bpy or phen ligands for complexes 1(2+) or 2(2+), respectively. For the latter complex 2(2+), this process is accompanied by the formation of an electrode adsorbed species.