Ligand Substitution Studies of Aquo (phosphine) ruthenium (II) Complexes

Ligand substitutions for a series of complexes of the form cir-[Ru_II(OH₂)(bpy)₂(PR₃)]₂₊ (where bpy = 2,2’-bipyridine and PR₃ = tertiary phosphine ligand) have been examined in nonaqueous solution. The second-order rate constants for the substitution of the water ligand by acetonitrile, 4-acetylpyridine, and chloride depend on the steric and electronic nature of the coordinated phosphine ligand. By use of tris(para-substituted phenyl)phosphines, the electronic effect of the coordinated ligand on the rate constant for substitution is investigated while a constant steric environment is maintained. The natural logarithms of the second-order rate constants for the substitution of water by acetonitrile decrease as a linear function of the Hammett parameters of the para substituents in the following order: N(CH₃)₂ > OCH₃ > CH₃ > H > F > CF₃. Similarly, by use of various trialkylphosphines, the steric effect of the ligand can be examined while the E_1/2 values of the Ru_III/Ru_II couples remain constant. In this case, increases in the ligand cone angles result in a linear increase in the natural logarithms of the second-order rate constants for the substitution of water by acetonitrile.