THE EFFECT OF ZINC(II) ON THE FORMATION OF 2,2'-BIPYRIDINE 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-) = triethylenetetraminehexaacetat e), 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 additio n 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 exhibi ts 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 t he E-1/2 value to 0.54 V. Other ligands which either have a higher aff inity 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'-bip yridine (2,3'-bpy)] do not exhibit the unusual competition by zinc(II) . These ligands all add statistically to the ruthenium(II) centres for ming 1:2 complexes with 1:2 stoichiometries. H-1-n.m.r. studies of the Ru(II)polyaminopolycarboxylate complexes [Ru-II(hedta)(H2O)](-) compl ex, 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 be tween the pyridyl rings of the 3'- less-hindered and 2-more-hindered r ing. A slower rearrangement occurs, producing the isomer of the more-h indered 2-substituted ring. A process is driven by forming a more pi-a ccepting system when ruthenium(II) binds to the 2-ring of 2,3'-bpy. Un derstanding 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.