Reduction potentials of pentaammine complexes of cobalt(III), rhodium(III) and iridium(III): physical correlations

Abstract

The reduction potentials of a large range of [M(NH₃)₅L]^π+ complexes (M = Co^III, Rh^III, Ir^III) have been determined in aqueous solution (μ 0.1 M, NaClO₄). Irreversible reductions span the range from + 0.22 to -0.47 V for cobalt(III) complexes and from -0.86 to -1.45 V for rhodium(III),and occur near -1.7 V for iridium(^III), determined against the standard (KCl) calomel electrode. Despite several reports to the contrary in the literature, no correlations exist of E_½ with absorption spectra (Co and Rh), nuclear magnetic resonance spectra [δ(¹HN), Co, Rh, Ir, and δ (⁵⁹Co)], vibrational spectra [v(Co-N)], or rates of aquation and base hydrolysis (Co, Rh, Ir). Complexes which undergo outer-sphere [e.g., with Ru(NH₃)₆²⁺] and predominantly inner-sphere (e.g., with V^Z+_aq) homogeneous electron transfer can be distinguished by the presence or absence of a correlation of In k_ex with E_½ respectively, consistent with the expectations of the Marcus-Hush theory of outer-sphere electron transfer.