Kinetics and thermodynamics of nitric oxide binding to transition metal complexes. Relationship to dioxygen binding

Abstract

The kinetics and activation parameters for ˙NO dissociation from L(H₂O)Co(NO)²⁺ (L = L¹ = cyclam and L² = meso-Me₆-cyclam), L²(H₂O)Rh(NO)²⁺, and Cr(H₂O)₅NO²⁺ were determined in aqueous solution in the presence of IrCl₆²⁻, IrBr₆²⁻, or O₂ as scavengers for ˙NO and/or metal(II) complexes. The rate constants k_–NO at 25 °C are (5.7 ± 0.1) × 10⁻³ s⁻¹ for L²(H₂O)Co(NO)²⁺, (1.0 ± 0.1) × 10⁻⁴ for L¹(H₂O)Co(NO)²⁺, (5.9 ± 2.0) × 10⁻⁸ for Cr(H₂O)₅NO²⁺ and (3.5 ± 1.8) × 10⁻⁹ s⁻¹ for L²(H₂O)Rh(NO)²⁺. The kinetics of the reverse reaction were determined by laser flash photolysis at 25 °C, k_NO = (1.7 ± 0.2) × 10⁷ M⁻¹ s⁻¹ for L²Co(H₂O)₂²⁺ and (1.8 ± 0.1) × 10⁸ M⁻¹ s⁻¹ for L²(H₂O)Rh²⁺. The rate constants k_NO and k_–NO obtained here and those available in the literature were used to calculate the equilibrium binding constants K_NO for a series of nitrosyl metal complexes. A good correlation was found between log K_NO and log K_O2, the latter corresponding to O₂ binding. The correlation includes complexes of four different transition metals as well as hydrogen atom, and covers a range of about 3 V in reduction potentials. The implications of these results are discussed.