The electrochemical and UV-visible spectroscopic properties of Rh(III) and Ir(III) complexes of the ortho-metalating (NC) ligands, 2-phenylpyridine (ppy) and benzo[A]quinone (bzq), have been studied. Cyclic voltammetric studies of several of the dimeric species, [M(NC)2C1]2, indicate metal-centered oxidation occurs at moderate potentials. Cationic monomers of the type M(NC)2(NN)+ where (NN) = 2,2'-bipyridine or 1,10-phenanthroline have been prepared by reaction of the chelating ligands with the parent dimers. Cyclic voltammetric studies of these monomers indicate that several reversible ligand-centered reductions are generally observed and that the chelating ligand is more easily reduced than is the ortho-metalating ligand. Spectroscopic studies of the mixed ligand monomers indicate that dual emissions from MLCT states associated with the ortho-metalating and chelating ligands occur in the Ir(III) complexes whereas a single emission from a ligand-localized excited state is observed in the Rh(III) complexes. These results are discussed in terms of electronic and nuclear coupling factors analogous to those encountered in descriptions of bimolecular energy and electron-transfer processes.
Voltammetric data have been obtained for 2,2'-dipyridyl (dip) complexes of rhodium in acetonitrile. A reaction sequence is proposed based on analysis of electrochemical and luminescence data. For [Rh(dip)3I3+, the first one-electron reduction is followed by moderately fast elimination of one dip ligand. A second, apparently reversible, one-electron transfer which produces a Rh(1) species is also followed by a chemical reaction. Two more stepwise, reversible one-electron transfers yield [Rh(dip)z]O and [Rh(dip)z] -successively. Reactions for [Rh(dip)zClz]+ exactly parallel those found for the tris dip complex except chloride is eliminated following the first electron transfer. Preparation and cyclic voltammetry of (dipH)[Rh(dip)Cla(OH)].H20, [Rh(dip)z]C104.3HzO, and [Rh(dip)&l]2(C104)2.4Hz0 confirm the above sequence.
50) The question of "What is the radius of an ion In solution?", is a vexed one; and not only because of the difficulty of precise definition and the uncertainty of the exact meaning of any value calculated from experimental data, or otherwise. There is also the problem of selecting the value that is most appropriate for a particular application. In the present context, the distance of closest approach of a nonpolar nonelectrolyte to an ion seems best defined In terms of the size of an ion plus the very small number of tightly bound water molecules which could be described as "having lost their own degrees of translation freedom and have those of the Ion".45 Bockris and Saluja42•45 define this number of water molecules of the solvation number of an ion. A set of radii of hydrated ions approximately corresponding to this extent of hydration have been used.
further metal-localized oxidations and/or DPA'-localized oxidations. For 3 only irreversible reduction waves are seen at very negative potentials. Consistent with the other results is the appearance of an additional oxidation wave for 3 ( = 2). For 1 visible charge-transfer bands indicating transitions from Ru(II) to both the bpy :r* orbitals (~450 nm) and the HDPA pyridyl it* orbitals (350 nm) are observed. The emission remains dir* from bpy as in the = 0 complex. For 2 ( = 1 and 2) very low energy charge-transfer transitions (558 and 605 nm, respectively) assigned as dir* (bpy) are seen.The energies of these bands correlate well with the electrochemically predicted values. These results confirm that substantial changes occur in coordinated HDPA on deprotonation, and the results for 2 suggest that a metal-ligand interaction unique to Ru imine complexes exists.
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