The effect of zinc(II) on the formation of 2,2'- and 2,3'-bipyridine complexes of [Ru-2(II)(ttha)](2-)(ttha(6-) equals triethylenetetraminehexa-acetate)

[Ru-2(II)(ttha)(H2O)(2)](2-)(ttha(6-) = triethylenetetraminehexaacetate), prepared by the reduction of the ruthenium(III) precursor, reacts with 2,2'-bipyridine (2,2'-bpy) in a multi-step fashion. The first 2,2'-bpy equivalent (1:1) adds with bidentate chelation at one ruthenium(II) site as revealed by separate ruthenium(II)/(III) waves at 0.03 and 0.54 V vs. n.h.e. A second equivalent of 2,2'-bpy (1:2) is initially stored and retained as the [Zn(2,2'-bpy)](2+) complex. Further addition of 2,2'-bpy initiates coordination at the second ruthenium(II) site. [Ru-2(ttha)-(2,2'-bpy)(H2O)](2-) forms a strong ion-pair with zinc(II) that is in rapid equilibrium with the Zn(H2O)(6)(2+)/ [Zn(2, 2'-bpy)](2+) pool. The solubility of the ion-pair is low. The ion-pair exhibits a shifted ruthenium(II)/(III) wave at 0.60 V. Higher amounts of 2,2'-bpy recomplex the zinc(II), solubilizing the complex and returning the E-1/2 value to 0.54 V. Other ligands which either have a higher affinity for ruthenium(II) centres than for zinc(II) as bidentate donors (1,10-phenanthroline), or ligands that cannot form bidentate zinc(II) complexes [(2-methylpyrazine, 4,4'-bipyridine (4,4'-bpy), and 2,3'-bipyridine (2,3'-bpy)] do not exhibit the unusual competition by zinc(II). These ligands all add statistically to the ruthenium(II) centres forming 1:2 complexes with 1:2 stoichiometries. H-1-n.m.r. studies of the Ru(II)polyaminopolycarboxylate complexes [Ru-II(hedta)(H2O)](-) complex, and [Ru-2(ttha)(H2O)(2)](2-) itself, reveal that substitution of 2,3'-bpy at ruthenium(II) sites occurs with an initial kinetic split between the pyridyl rings of the 3'- less-hindered and 2-more-hindered ring. A slower rearrangement occurs, producing the isomer of the more-hindered 2-substituted ring. A process is driven by forming a more pi-accepting system when ruthenium(II) binds to the 2-ring of 2,3'-bpy. Understanding the unusual influence of zinc(II) on the substitution of 2,2'-bpy with [Ru-2(ttha)(H2O)(2)](2-) clarifies the nature of the 1:1 complex - namely that the 2,2'-bpy becomes bidentate at one ruthenium(II) centre rather than serving as a trans-bridging ligand between both ruthenium(II) centres within one [Ru-2(ttha)](2-) unit.