WATER LIGAND-EXCHANGE IN TETRAAMMINEIRIDIUM(III) COMPLEXES - KINETIC TRANS-EFFECT IN OCTAHEDRAL SUBSTITUTION-REACTIONS

Water ligand exchange in trans-tetraamminediaqua- and in trans-tetraammineaquachloroiridium(III) has been investigated using an acidic 1 M perchlorate medium. Rate constants for the exchange of one water ligand and activation energies are: trans-[Ir(NH3)4(OH2)2]3+: k(90-degrees-C) = 1.73(7) x 10(-6) s-1, E(a) = 165(4) kJ mol-1; trans-[Ir(NH3),(OH)(OH2)]2+: k(90-degrees-C) = 7.6(7) x 10(-3) s-1, E(a) = 96(3) kJ mol-1, and trans-Ir(NH3)4(OH2)Cl]2+: k(60-degrees-C) = 5.30(8) x 10(-5) s-1, E(a) = 113.0(13) kJ mol-1. These data, in combination with literature data for water ligand exchange in pentaammineaquairidium(III), establish the kinetic trans-effect order, for a fixed set of cis ligands, as: OH2 << NH3 < Cl- for substitution reactions at 25-degrees-C at an iridium(III) centre. The variation in the kinetic trans-effect for iridium(III) as compared to rhodium(III) and chromium(III) for the same trans-ligand has been rationalized in terms of differences in ligand field stabilization energies.