Oxidation of Coordinated Ammonia to Nitrosyl in the Reaction of Aqueous Chlorine with cis-[Ru(bpy)₂(NH₃)₂]²⁺

Abstract

Rapid-scan stopped-flow kinetic studies show that the multielectron oxidation of coordinated ammonia in cis-[Ru(bpy)2(NH3)2]2+ by acidic (pH 0.5−1.3) aqueous chlorine provides the nitrosyl complex cis-[Ru(bpy)2(NH3)(NO)]3+ as the final product. The first step in this process involves the metal-centered oxidation of cis-[Ru(bpy)2(NH3)2]2+ to cis-[Ru(bpy)2(NH3)2]3+, and the rate law for this process is −d[Ru(II)]/dt = 2k 1[Ru(II)][Cl2] with the second order rate constant, k 1, as (1.1 ± 0.1) × 103 M-1 s-1. Independent studies conducted on the cis-[Ru(bpy)2(NH3)2]3+ complex show that conversion to the nitrosyl complex follows an A → B → C type consecutive pathway with k slow and k fast components, respectively. In the pH range of 0.5−2.8, the k slow process follows a competitive pathway where both Cl2 and HOCl react with deprotonated coordinated ammonia. The rate law for the k slow process has the form k slow = {(k 2[H+][Cl-] + k 3 K Cl)/(K Cl + [H+][Cl-])}([Cl2]tot/[H+]), where K Cl corresponds to the equilibrium constant for the hydrolysis of Cl2. The rate constant k 2, corresponding to the Cl2 term, is 6.5 ± 0.3 s-1 while the rate constant k 3, corresponding to the HOCl reaction, is 2.0 ± 0.2 s-1. The k fast process involves further oxidation of intermediate B by Cl2. The intermediate of this reaction is speculated as either a nitrene, nitrido, or chloroamine complex. The kinetic studies indicate that the conversion of this unidentified intermediate to the final nitrosyl complex proceeds through a fast preequilibrium, involving deprotonation of the intermediate, followed by a direct attack by Cl2. The rate law corresponding to the k fast process has the form k fast = [k 4 K int[Cl2]tot[H+][Cl-]]/[{K int + [H+]}{[H+][Cl-] + K Cl}], the equilibrium constant K int for the deprotonation process is 0.11 ± 0.04 M, and the rate constant k 4 is (7.8 ± 1.5) × 102 M-1 s-1.

Selective Photoinduced Ligand Exchange in a New Tris–Heteroleptic Ru(II) Complex

“…All solvents used were purchased from commercial sources and used without further purification unless otherwise specified. The complexes Ru(phen)Cl 4 , 42 [Ru(phen)(CH 3 CN) 4 ]-(PF 6 ) 2 , 40 and Ru(phen) 2 Cl 2 43 were prepared according to literature procedures.…”

Chemical pathways for the formation of ammonia in Hanford wastes

“…Assefa and Stanbury (1997), for example, studied the oxidation of ammonia ligands to nitrosyl in coordination complexes with ruthenium. Analogous complexes may be present in Hanford wastes, which contain certain noble metals, iron, chromium, nickel, and other transition metals.…”

“…Via similar complexes, ammonia has been oxidized to coordinated nitrosyl in aqueous solution (Assefa and Stanbury 1997). Further, dinitrogen compound formation was observed from reactions of coordinated nitrosyl.…”

“…While there is relatively little information available on chemical pathways for the conversion of nitrite ion into the nitrogenous gases in homogeneous solution free of potential inorganic catalysts, the interconversion reactions of nitrogen-containing ligands coordinated to transition metals has received considerable attention (Assefa and Stanbury 1997;MacNeil et al 1997). This information provides an additional guide for the discussion of the chemistry that sequentially converts nitrite ion through the mawy available oxidation states eventually leading to ammonia.…”

“…Coordination of various nitrogen oxide fragments to transition metals have been studied extensively (Cotton and Wilkinson 1980). Compounds with coordinated nitrosyl ligands are in acid-base equilibrium with corresponding compounds with coordinated nitro ligands (Assefa and Stanbury 1997;MacNeil et al 1997). The very active field of research has identified many rapid reactions involving organometallic catalysts as illustrated in Reaction 5.9:…”