Factors affecting the equilibrium constant of homolysis of complexes with metal-carbon Q bonds in aqueous solutions. Pulse radiolysis studies

bstract. Pulse-Radiolysis is a powerful technique for the Getermination of the equilibrium constants of the homolytic cleavage of metal-carbon o bonds in aqueous solutions. In most systems studied the observed reaction is: L,,,-,M("++')-R + L L-----direct determination of the metal-carbon bond dissociation energies. The results obtained indicate that these equilibrium constants are not directly related to the redox potential of either LmM(") or of .R, or to the activation energies for the homolytic cleavage of a family of similarly substituted ethanes. ML,,,@) + .R. Therefore the results do not. enable, a The measurement of the dissociation energies of metal-carbon d bonds and the study of the factors affecting these energies is of importance in the framework of the research of many organometallic, biochemical and catalytic systems (ref. 1). The most common method used to measure these dissociation energies is the kinetic. technique. In this technique the activation energy, A H * , of reaction the bond dissociation energy equals A H * (ref. 1,2). This assumption is based on the observation that the reverse reaction: is very fast, often approaching the diffusion controlled limit, and therefore it is assumed that the activation energy for it is negligible (ref. 1,2). This assumption introduces an error of several kjoule/mole into the bond dissociation energy. The specific rates of reaction It should be pointed out that this technique is applicable only for complexes with relatively stable metal-carbon d bonds, i,e. usuallf only to systems where the metal-carbon bond is not formed in situ. his imitation has several important implications

What Are the Oxidizing Intermediates in the Fenton and Fenton-like Reactions? A Perspective

The Fenton and Fenton-like reactions are of major importance due to their role as a source of oxidative stress in all living systems and due to their use in advanced oxidation technologies. For many years, there has been a debate whether the reaction of FeII(H2O)62+ with H2O2 yields OH• radicals or FeIV=Oaq. It is now known that this reaction proceeds via the formation of the intermediate complex (H2O)5FeII(O2H)+/(H2O)5FeII(O2H2)2+ that decomposes to form either OH• radicals or FeIV=Oaq, depending on the pH of the medium. The intermediate complex might also directly oxidize a substrate present in the medium. In the presence of FeIIIaq, the complex FeIII(OOH)aq is formed. This complex reacts via FeII(H2O)62+ + FeIII(OOH)aq → FeIV=Oaq + FeIIIaq. In the presence of ligands, the process often observed is Ln(H2O)5−nFeII(O2H) → L•+ + Ln−1FeIIIaq. Thus, in the presence of small concentrations of HCO3− i.e., in biological systems and in advanced oxidation processes—the oxidizing radical formed is CO3•−. It is evident that, in the presence of other transition metal complexes and/or other ligands, other radicals might be formed. In complexes of the type Ln(H2O)5−nMIII/II(O2H−), the peroxide might oxidize the ligand L without oxidizing the central cation M. OH• radicals are evidently not often formed in Fenton or Fenton-like reactions

On the Mechanism of Heterogeneous Water Oxidation Catalysis: A Theoretical Perspective

Earth abundant transition metal oxides are low-cost promising catalysts for the oxygen evolution reaction (OER). Many transition metal oxides have shown higher OER activity than the noble metal oxides (RuO2 and IrO2). Many experimental and theoretical studies have been performed to understand the mechanism of OER. In this review article we have considered four earth abundant transition metal oxides, namely, titanium oxide (TiO2), manganese oxide/hydroxide (MnOx/MnOOH), cobalt oxide/hydroxide (CoOx/CoOOH), and nickel oxide/hydroxide (NiOx/NiOOH). The OER mechanism on three polymorphs of TiO2: TiO2 rutile (110), anatase (101), and brookite (210) are summarized. It is discussed that the surface peroxo O* intermediates formation required a smaller activation barrier compared to the dangling O* intermediates. Manganese-based oxide material CaMn4O5 is the active site of photosystem II where OER takes place in nature. The commonly known polymorphs of MnO2; α-(tetragonal), β-(tetragonal), and δ-(triclinic) are discussed for their OER activity. The electrochemical activity of electrochemically synthesized induced layer δ-MnO2 (EI-δ-MnO2) materials is discussed in comparison to precious metal oxides (Ir/RuOx). Hydrothermally synthesized α-MnO2 shows higher activity than δ-MnO2. The OER activity of different bulk oxide phases: (a) Mn3O4(001), (b) Mn2O3(110), and (c) MnO2(110) are comparatively discussed. Different crystalline phases of CoOOH and NiOOH are discussed considering different surfaces for the catalytic activity. In some cases, the effects of doping with other metals (e.g., doping of Fe to NiOOH) are discussed

On the Mechanism of Heterogeneous Water Oxidation Catalysis: A Theoretical Perspective

Earth abundant transition metal oxides are low-cost promising catalysts for the oxygen evolution reaction (OER). Many transition metal oxides have shown higher OER activity than the noble metal oxides (RuO2 and IrO2). Many experimental and theoretical studies have been performed to understand the mechanism of OER. In this review article we have considered four earth abundant transition metal oxides, namely, titanium oxide (TiO2), manganese oxide/hydroxide (MnOx/MnOOH), cobalt oxide/hydroxide (CoOx/CoOOH), and nickel oxide/hydroxide (NiOx/NiOOH). The OER mechanism on three polymorphs of TiO2: TiO2 rutile (110), anatase (101), and brookite (210) are summarized. It is discussed that the surface peroxo O* intermediates formation required a smaller activation barrier compared to the dangling O* intermediates. Manganese-based oxide material CaMn4O5 is the active site of photosystem II where OER takes place in nature. The commonly known polymorphs of MnO2; α-(tetragonal), β-(tetragonal), and δ-(triclinic) are discussed for their OER activity. The electrochemical activity of electrochemically synthesized induced layer δ-MnO2 (EI-δ-MnO2) materials is discussed in comparison to precious metal oxides (Ir/RuOx). Hydrothermally synthesized α-MnO2 shows higher activity than δ-MnO2. The OER activity of different bulk oxide phases: (a) Mn3O4(001), (b) Mn2O3(110), and (c) MnO2(110) are comparatively discussed. Different crystalline phases of CoOOH and NiOOH are discussed considering different surfaces for the catalytic activity. In some cases, the effects of doping with other metals (e.g., doping of Fe to NiOOH) are discussed

Properties of (1,4,7,10,13-pentaazacyclohexadecane)nickel(III) in aqueous solutions: a pulse radiolytic study

The oxidn. of (1,4,7,10,13-pentaazacyclohexadecane)nickel(II) by OH• in aq. solns. was studied, and the properties of the resulting Ni(III) complex were compared with those of [NiL]3+ (L = tetraazacyclotetradecane). The difference in the properties is attributed to the 5th N donor atom which decreases the activity of the central tervalent Ni