Mechanistic aspects of uncatalysed and Os(VIII) catalysed oxidation of 5-flourouracil - An anticancer drug by alkaline diperiodatoargentate(III)

The oxidation of an anticancer drug 5-fluorouracil (5-FU) by diperiodatoargentate(III) (DPA) was carried out both in the absence and presence of osmium(VIII) catalyst in alkaline medium at 27 °C and a constant ionic strength of 0.20 mol dm⁻³ spectrophotometrically attached with HI-TECH SFA-12 stopped flow accessory. The oxidation products in both the cases were identified as fluoroketene and Ag(I). The stoichiometry is same in both cases, i.e., [5-FU]:[DPA] = 1:1. The reaction was of first order in both catalysed and uncatalysed cases, with respect to [DPA] and was less than unit order in [5-FU] and negative fraction in [alkali]. The order in Os(VIII) was unity. In both cases [Ag(H₃IO₆)₂]⁻ itself is the active species of DPA. The uncatalysed reaction in alkaline medium has been shown to proceed via a DPA-5-fluorouracil complex, which decomposes in a rate determining step to give the products. In catalysed reaction, it has been shown to proceed via a Os(VIII)-5-fluorouracil complex, which further reacts with one molecule of DPA in a rate determining step to give the products. The reaction constants involved in the different steps of the mechanisms were calculated for both the reactions. The catalytic constant (k_Cat.const.) was also calculated for catalysed reaction at different temperatures. The activation parameters with respect to slow step of the mechanisms were computed and discussed for both the cases. The thermodynamic quantities were also determined for both reactions.     

Kinetics and mechanism of the oxidation of ammineruthenium(II) complexes by bromine

The reactions of Ru(NH₃)₅py²⁺, Ru(NH₃)₄bpy²⁺, Ru₂(NH₃)₁₀pz⁵⁺, RuRh(NH₃)₁₀pz⁵⁺ and Ru(NH₃)₅pz²⁺ with bromine are first-order in ruthenium and first-order in bromine. The rates decrease with increasing bromide ion concentration and, except for Ru(NH₃)₅pz²⁺, are independent of hydrogen ion concentration. The reactions are postulated to proceed via outer-sphere, one-electron transfer from Ru(II) to Br₂ with the formation of Br₂⁻ as a reactive intermediate. The bromide inhibition is ascribed to the formation of Br₃⁻ which is unreactive in outer-sphere reactions because of the barrier imposed by the need to undergo reductive cleavage. The reaction of Ru(NH₃)₅pz²⁺ is inhibited by hydrogen ions. The hydrogen ion dependence shows that Ru(NH₃)₅pzH³⁺ has a pK_a of 2.49 and is at least 500 times less reactive than Ru(NH₃)₅pz²⁺. The reaction of Ru₂(NH₃)₁₀pz⁴⁺ with bromine is biphasic. The second phase has a rate identical to that of the Ru₂(NH₃)₁₀pz⁵⁺-Br₂ reaction. A detailed analysis shows that the reaction of Ru₂(NH₃)₁₀pz⁴⁺ with bromine proceeds by a sequence of one-electron steps, Br₂⁻ being produced as an intermediate. A linear free energy relationship between rate constants and equilibrium constants, obeyed for all the reactions studied, provides an estimate of 1.5 × 10² M⁻¹ s⁻¹ for the self-exchange rate constant of the Br₂/Br₂⁻ couple.     

Kinetics and mechanism of the reduction of the molybdatopentaamminecobalt(III) ion by aqueous sulfite and aqueous potassium hexacyanoferrate(II)

A detailed investigation on the oxidation of aqueous sulfite and aqueous potassium hexacyanoferrate(II) by the title complex ion has been carried out using the stopped-flow technique over the ranges, 0.01≤[S(IV)]_T≤0.05 mol dm⁻³, 4.47≤pH≤5.12, and 24.9≤θ≤37.6 °C and at ionic strength 1.0 mol dm⁻³ (NaNO₃) for aqueous sulfite and 0.01≤[Fe(CN)₆ ⁴⁻]≤0.11 mol dm⁻³, 4.54≤pH≤5.63, and 25.0≤θ≤35.3 °C and at ionic strength 1.0 or 3.0 mol dm⁻³ (NaNO₃) for the hexacyanoferrate(II) ion. Both redox processes are dependent on pH and reductant concentration in a complex manner, that is, for the reaction with aqueous sulfite, k_obs={(k₁K₁K₂K₃+k₂K₁K₄[H⁺])[S(IV)]_T]/([H⁺]²+K₁[H⁺]+K₁K₂) and for the hexacyanoferrate(II) ion, k_obs={(k₁K₃K₄K₅+k₂K₃K₆[H⁺])[Fe(CN)₆ ⁴⁻]_T)/([H⁺]²+K₃[H⁺]+K₃K₄). At 25.0 °C, the value of k₁′ (the composite of k₁K₃) is 0.77±0.07 mol⁻¹ dm³ s⁻¹, while the value of k₂′ (the composite of k₂K₄) is (3.78±0.17)×10⁻² mol⁻¹ dm³ s⁻¹ for aqueous sulfite. For the hexacyanoferrate(II) ion, k₁′ (the composite of k₁K₅) is 1.13±0.01 mol⁻¹ dm³ s⁻¹, while the value of k₂′ (the composite of k₂K₆) is 2.36±0.05 mol⁻¹ dm³ s⁻¹ at 25.0 °C. In both cases there was reduction of the cobalt(III) centre to cobalt(II), but there was no reduction of the molybdenum(VI) centre. k₂₂, the self-exchange rate constant, for aqueous sulfite (as SO₃ ²⁻) was calculated to be 5.37×10⁻¹² mol⁻¹ dm³ s⁻¹, while for Fe(CN)₆ ⁴⁻, it was calculated to be 1.10×10⁹ mol⁻¹ dm³ s⁻¹ from the Marcus equations.     

Characterization and oxidation of chromium(III) by sodium hypochlorite in alkaline solutions

Chromium exists in nuclear waste sludges and is a problematic element in the vitrification process of high-level nuclear wastes. It is therefore necessary to treat the waste sludges to remove chromium prior to vitrification, by caustic leaching or oxidation of Cr(III) to Cr(VI). The objective of this study is to investigate the effect of oligomerization of Cr(III) on its oxidation by hypochlorite in alkaline solutions.Monomeric, dimeric and trimeric Cr(III) species in solution were separated by ion exchange. The kinetics of the oxidation of the separated species by hypochlorite in alkaline solutions was studied by UV/Vis absorption spectroscopy, and compared with the oxidation by hydrogen peroxide previously studied. Results indicate that hypochlorite can oxidize Cr(III) to Cr(VI) in alkaline solutions, but the rate of oxidation by hypochlorite is slower than that by hydrogen peroxide at the same alkalinity and concentrations of oxidants. The rate of oxidation of Cr(III) by both oxidants decreases as the concentration of sodium hydroxide is increased, but the oxidation by hypochlorite seems less affected by the degree of oligomerization of Cr(III) than that by peroxide. Compared with the oxidation by hydrogen peroxide where the major reaction pathway has an inverse order with respect to C_NaOH, the oxidation by hypochlorite has a significant reaction pathway independent of [OH⁻].     

Kinetics and mechanism of the Ir(III)-catalyzed oxidation of xylose and maltose by potassium iodate in aqueous alkaline medium

For the first time, the Ir(III) catalysis of the iodate oxidation of xylose and maltose in aqueous alkaline medium has been investigated. The reactions exhibit first-order kinetics with respect to lower [IO(3)(-)] and [OH(-)] and show zero-order kinetics at their higher concentrations. Unity order at low concentrations of maltose becomes zero order at its higher concentrations, whereas zero-order kinetics with respect to [xylose] was observed throughout its variation. The reaction rate is found to be directly proportional to [Ir(III)] in the oxidation of both reducing sugars. Negligible effect of [Cl(-)] and nil effect of ionic strength (mu) on the rate of oxidation have also been noted. The species, [IrCl(3)(H(2)O)(2)OH](-) was ascertained as the reactive species of Ir(III) chloride for both the redox systems. Various activation parameters have been calculated. Formic acid and arabinonic acid for maltose and formic acid and threonic acid for xylose were identified as the main oxidation products of the reactions. Mechanisms consistent with the observed kinetic data and spectral evidence have been proposed for the oxidation of xylose and maltose.     

Kinetics and mechanism of Ru(III) and Hg(II) co-catalyzed oxidation of D-galactose and D-ribose by N-bromoacetamide in perchloric acid

Kinetics of oxidation of reducing sugars D-galactose (Gal) and D-ribose (Rib) by N-bromoacetamide (NBA) in the presence of ruthenium(III) chloride as a homogeneous catalyst and in perchloric acid medium, using mercuric acetate as a scavenger for Br(minus sign) ions, as well as a co-catalyst, have been investigated. The kinetic results indicate that the first-order kinetics in NBA at lower concentrations tend towards zero order at its higher concentrations. The reactions follow identical kinetics, being first order in the [sugar] and [Ru(III)]. Inverse fractional order in [H(+)] and [acetamide] were observed. A positive effect of [Hg(OAc)(2)] and [Cl(minus sign)] was found, whereas a change in ionic strength (mu) has no effect on oxidation velocity. Formic acid and D-lyxonic acid (for Gal) and formic acid and L-erythronic acid (for Rib) were identified as main oxidation products of reactions. The various activation parameters have been computed and recorded. A suitable mechanism consistent with experimental findings has been proposed.     

Mechanistic aspects of uncatalyzed and ruthenium(III) catalyzed oxidation of dl-ornithine monohydrochloride by silver(III) periodate complex in aqueous alkaline medium

The oxidation of an amino acid, dl-ornithine monohydrochloride (OMH) by diperiodatoargentate(III) (DPA) was carried out both in the absence and presence of ruthenium(III) catalyst in alkaline medium at 25 °C and a constant ionic strength of 0.10 mol dm⁻³ spectrophotometrically. The reaction was of first order in both catalyzed and uncatalyzed cases, with respect to [DPA] and was less than unit order in [OMH] and negative fraction in [alkali]. The order with respect to [OMH] changes from first order to zero order as the [OMH] increases. The order with respect to Ru(III) was unity. The uncatalyzed reaction in alkaline medium has been shown to proceed via a DPA-OMH complex, which decomposes in a rate determining step to give the products. Where as in catalyzed reaction, it has been shown to proceed via a Ru(III)-OMH complex, which further reacts with two molecules of DPA in a rate determining step to give the products. The reaction constants involved in the different steps of the mechanisms were calculated for both the reactions. The catalytic constant (K_Cat.const.) was also calculated for catalyzed reaction at different temperatures. The activation parameters with respect to slow step of the mechanism and also the thermodynamic quantities were determined.     

Kinetics and mechanism of the axial substitution reactions of (ligand)bis(4,5-dichloro-1,2-benzo semiquinonediiminato)cobalt(III) complexes

Square-pyramidal (Ph₃X)bis(4,5-dichloro-1,2-benzosemiquinonediiminato)cobalt(III) complexes (X = As, Sb or P) have been synthesized. The kinetics of axial substitution for the triphenylantimony complex have been studied for 10 entering ligands (L*). The reaction is of reversible second-order in both directions for all complexes. Labile behavior is indicated by the rate constants in the range from 6.33 × 10³ (for L* = Ph₃P in MeOH) to 5.4 (L* = py in CH₂Cl₂) M⁻¹ s⁻¹. The kinetics is consistent with an I_a mechanism. The log of the second-order rate constant for axial substitution is a linear function of nucleophilic reactivity n_Pt°, which is due to the trans-labilizing effect of the entering ligand in the six-coordinate transition state.     

Polymerization study of o-Si(OH)4 and complexation with Am(III), Eu(III) and Cm(III)

The stability constants of Am⁺³, Cm³⁺ and Eu³⁺ with ortho silicate, were measured at pH 3.50 and in ionic strengths of 0.20-1.00 M (NaClO₄) by the solvent extraction method. The Am⁺³, Cm³⁺ and Eu³⁺ forms 1:1 complex with ortho silicate ion at pH 3.60 with the stability constant (log β₁) value of 8.02 ± 0.10, 7.78 ± 0.08 and 7.81 ± 0.11, respectively. The stability of these metal ions decrease with increased ionic strength from 0.20 to 1.00 M (NaClO₄) for silicic acid concentrations of 0.002-0.020 M. Increasing silicic acid concentration above 0.02 M increased the amount of M³⁺ extracted into the organic phase, contrary to the trend usually observed for increased ligand concentration in solvent extraction. This reversed trend is likely due to the extraction of cationic species of silicic acid by HDEHP. Aging time (60-300 min) had no effect on the stability constant of these metal ions for 0.002-0.020 M silicic acid at pH 3.50 and I = 0.20 M (NaClO₄).The fraction of polymeric silicic acid present in solutions of 0.20-4.50 M NaClO₄ solutions at pH 3.0-10.0, T = 0-60 °C and aging time = 5-300 min was measured for determination of the silicomolybdate reaction to ascertain the proper conditions to study metal-silicate complexation.     

Kinetics and mechanism of permanganate oxidation of iota- and lambda-carrageenan polysaccharides as sulfated carbohydrates in acid perchlorate solutions

The kinetics of oxidation of iota- and lambda-carrageenan as sulfated carbohydrates by permanganate ion in aqueous perchlorate solutions at a constant ionic strength of 2.0 mol dm⁻³ have been investigated spectrophotometrically. The pseudo-first-order plots were found to be of inverted S-shape throughout the entire courses of reactions. The initial rates were found to be relatively slow in the early stages, followed by an increase in the oxidation rates over longer time periods. The experimental observations showed first-order dependences in permanganate and fractional first-order kinetics with respect to both carrageenans concentration for both the induction and autoacceleration periods. The results obtained at various hydrogen ion concentrations showed that the oxidation processes in these redox systems are acid-catalyzed throughout the two stages of oxidation reactions. The added salts lead to the prediction that Mn^III is the reactive species throughout the autoacceleration periods. Kinetic evidence for the formation of 1:1 intermediate complexes was revealed. The kinetic parameters have been evaluated and tentative reaction mechanisms in good agreement with the kinetic results are discussed.     

Kinetics and mechanism of the reduction of hexacyanoferrate(III) by myoglobin in aqueous solution

 The kinetics of the reduction of hexacyanoferrate(III) by myoglobin was studied as a function of temperature and pressure. The results of the study show that both oxy- and deoxymyoglobin are redox active species. The rate and activation parameters underline the operation of an outer-sphere electron transfer mechanism for the studied system. Received: 9 December 1996 / Accepted: 16 June 1997     

Kinetics and mechanism of the complex formation of [Pd(NNN)Cl] with pyridines in methanol: synthesis and crystal structure of [Pd(terpy)(py)](ClO4)2

The kinetics of the complex-formation reactions between monofunctional palladium(II) complexes [Pd(NNN)Cl]⁺, where NNN is 2,2^′:6^′,2″-terpyridine (terpy), diethylenetriamine (dien) or bis(2-pyridylmethyl)amine (bpma), with pyridine, 4-methylpyridine, 4-acetylpyridine, 4-cyanopyridine and 4-aminopyridine, have been studied in methanol at 25 °C using stopped-flow spectrophotometry. The highest reactivity was observed for the [Pd(terpy)Cl]⁺ complex, whereas 4-aminopyridine is the strongest nucleophile. The results, compared with those previously published on the [Pt(NNN)Cl]⁺ complexes, are discussed in terms of reactivity and discrimination ability of the reaction centre. The crystal structure of [Pd(terpy)(py)](ClO₄)₂ has been determined by X-ray diffraction. Crystals are triclinic, space group , and consist of distorted square planar [Pd(terpy)(py)]²⁺ cations and perchlorate anions. The Pd-N bond length to the central atom of terpy ligand is well below 2.0 Å and significantly shorter than any of the other M-N distances. The pyridine plane forms a dihedral angle of 61.9(2)° with the coordination N4 donors.     

Kinetics and mechanism of the oxidation of D-fructose by vanadium(V) in H2SO4 medium

The oxidative degradation of D-fructose by vanadium(V) in the presence of H(2)SO(4) has an induction period followed by autoacceleration. The kinetics and mechanism of the induction period have been studied at constant ionic strength. The reaction was followed spectrophotometrically by measuring the changes in absorbance at 350 nm. Evidence of induced polymerization of acrylonitrile and of reduction of mercuric chloride indicates that a free-radical mechanism operates during the course of reaction. Vanadium(V) is only reduced to vanadium(IV). The reaction is first and fractional order in [V(V)] and [D-fructose], respectively; but dependence on [H+] is complex, that is, [equation: see text]. At constant [H2SO4], sodium hydrogensulfate accelerates the reaction. The effect of added sodium sulfate on the H2SO4 and HSO4-catalyzed reaction is also reported. The activation parameters Ea=118 kJ mol(-1), DeltaH#=116 kJ mol(-1), DeltaS#=-301 J K(-1) mol(-1), and DeltaG#=213 kJ mol(-1) are calculated and discussed. Reaction products are also examined, and it is concluded that oxidation of D-fructose by vanadium(V) involves consecutive one-electron abstraction steps.     

Catalytic oxidation of alkenes by manganese(III) porphyrin-encapsulated Al, V, Si-mesoporous molecular sieves

Cationic manganese-porphyrin, [meso-tetrakis(4-trimethylammoniophenyl)porphyrinato]manganese(III) pentachloride (MnTAPP), has been prepared and encapsulated into mesoporous molecular sieves Al-MCM-41 and V-MCM-41, containing different amounts of Al and V, respectively. The catalytic activities of these heterogeneous materials were tested in the liquid phase oxidation of cyclohexene and styrene in acetonitrile with iodosylbenzene (PhIO) as oxygen source. Both types of catalysts were active in the oxidation reaction. MnTAPP encapsulated in Al-MCM-41 produces allylic oxidation products alone and no epoxide with styrene was found. However, it produces both epoxide and allylic oxidation products with cyclohexene. At the same time, MnTAPP encapsulated in V-MCM-41 produces epoxide as major product and little allylic oxidation product with styrene, while both epoxide and allylic oxidation products were obtained with cyclohexene. It is suggested that the regioselective effect is due to relatively more acidic Al-MCM-41 than V-MCM-41 which could make the CC bond unreactive towards epoxidation and produces allylic oxidation product. With increasing Al or V content in the support, the porphyrin loading was found to increase, which in turn increases the catalytic activity of the heterogeneous systems. The heterogeneous catalysts were reused for three times. The selectivity of these heterogeneous catalysts does not change appreciably even after three times of reusing, but their catalytic activity decreases marginally. This may be attributed to catalyst leaching and/or decomposition of MnTAPP complex under the reaction conditions.     

Sclerotinia sclerotiorum growth and oxalic acid production on selected culture media

Abstract Two isolates of Sclerotinia sclerotiorum , the highly aggressive (B24) and the weakly aggressive (SS41), were grown on liquid media containing one of the following carbon sources: purified cell walls obtained from onion or sunflower, pectin, polygalacturonic acid, carboxymethylcellulose, xylan or arabinogalactan. Isolates were equally able to utilize these substrates for mycelial growth but differed in their ability to utilize them for oxalate production. B24 produces oxalic acid always to a substantial extent, SS41 only in traces. The poor ability to produce oxalic acid by SS41 seems to be due to a lower efficiency in the synthetic pathway.     

Secondary ligand-metal interactions in rhodium(III) and iridium(III) phosphoramidite complexes

The synthesis, characterization, and application in asymmetric catalytic cyclopropanation of Rh(III) and Ir(III) complexes containing (S_a,R_C,R_C)-O,O′-[1,1′-binaphthyl-2,2′-diyl]-N,N′-bis[1-phenyl-ethyl]phosphoramidite (1) are reported. The X-ray structures of the half-sandwich complexes [MCl₂(C₅Me₅)(1,κP)] (M = Rh, 2a; M = Ir, 2b) show that the metal-phosphoramidite bond is significantly shorter in the Ir(III) analog. Chloride abstraction from 2a (with CF₃SO₃SiMe₃ or with CF₃SO₃Me) and from 2b (with AgSbF₆) gives the cationic species [MCl(C₅Me₅)(1,2-η-1,κP)]⁺ (M = Rh, 3a; M = Ir, 3b), which display a secondary interaction between the metal and a dangling phenethyl group (NCH(CH₃)Ph) of the phosphoramidite ligand, as indicated by NMR spectroscopic studies. Complexes 3a and 3b slowly decompose in solution. In the case of 3b, the binuclear species [Ir₂Cl₃(C₅Me₅)₂]⁺ is slowly formed, as indicated by an X-ray study. Preliminary catalytic tests showed that 3a cyclopropanates styrene with moderate yield (35%) and diastereoselectivity (70:30 trans:cis ratio) and with 32% ee (for the trans isomer).     

Kinetics and mechanism of the oxidation of L-ascorbic acid by trisoxalatocobaltate(III) in basic aqueous solution

The kinetics and mechanism of the oxidation of L- ascorbic acid by trisoxalatocobaltate(III) were studied as a function of pH, ascorbate concentration, ionic strength and temperature in a weakly basic aqueous solution. The pH dependence of the process can be ascribed to the oxidation of the doubly deprotonated ascorbate ion for which k = 20 M⁻¹ s⁻¹ at 25 °C, ΔH^# = 34 ± 2 kJ mol⁻¹ and ΔS^# = −108 ± 7 J K⁻¹ mol⁻¹. The results are discussed in reference to literature data for this reaction in weakly acidic medium and for the oxidation by a series of other oxidants.     

Further evidence for detection of short-lived transient hypomanganate(V) and manganate(VI) intermediates during oxidation of some sulfated polysaccharides by alkaline permanganate using conventional spectrophotometric techniques

Spectrophotometric evidence for the formation of hypomanganate(V), [CAR-], and manganate(VI), [CAR-], intermediate complexes has been confirmed during the oxidation of iota- and lambda-carrageenan-sulfated polysaccharides (CAR) by alkaline permanganate at pHs ?12 using a conventional spectrophotometer. These short-lived intermediate complexes were identified and characterized. A reaction mechanism in good consistence with the experimental results is suggested.     

Solvent exchange, solvent interchange, aquation and isomerisation reactions of cis- and trans-[Co(tmen)2(NCMe)2] in water, Me2SO and MeCN: kinetics and stereochemistry

The synthesis and characterisation of cis- and trans-[Co(tmen)₂(NCCH₃)₂](ClO₄)₃ are described. Solvolysis rates have been measured by both ¹H NMR spectroscopy and UV-Vis spectrophotometry in dimethyl sulfoxide at 298.2 K. The cis isomer undergoes solvolysis by consecutive first-order reactions, k₁=5.61 × 10⁻⁴ and k₂=5.35 × 10⁻⁴ s⁻¹, each with steric retention. The measured solvolysis rate (single step reaction) for the trans isomer is k=1.54 × 10⁻⁵ s⁻¹. The solvent exchange rates have been measured by ¹H NMR spectroscopy in CD₃CN at 298.2 K: k_ex(cis)=k_ct + k_cc=2.0 × 10⁻⁵ and k_ex(trans)=k_tc + k_tt=4.56 × 10⁻⁶ s⁻¹. From these data, the measured cis-trans isomerisation rate (1.71 × 10⁻⁶ s⁻¹) and equilibrium position in CH₃CN (17% trans), the steric course for substitution in the exchange processes has been determined: trans reactant - 69% trans product; cis reactant - 99% cis product. Aquation rates for cis- and trans-[Co(tmen)₂(NCCH₃)₂](ClO₄)₃ have also been determined spectrophotometrically and by NMR; k_cis=1.3 × 10⁻⁴ and k_trans=2.7 × 10⁻⁵ s⁻¹. In both cases the steric course for the primary aquation step is indeterminate because the subsequent steps are faster. Where data are available, the [Co(tmen)₂X₂]ⁿ⁺ complexes are found to be consistently much more reactive than their [Co(en)₂X₂]ⁿ⁺ analogues.     

The kinetics of the complex formation between iron(III)-ethylenediaminetetraacetate and hydrogen peroxide in aqueous solution

The kinetics of the formation of the purple complex [Fe^III(EDTA)O₂]³⁻, between Fe^III-EDTA and hydrogen peroxide was studied as a function of pH (8.22-11.44) and temperature (10-40 °C) in aqueous solutions using a stopped-flow method. The reaction was first-order with respect to both reactants. The observed second-order rate constants decrease with an increase in pH and appear to be related to deprotonation of Fe^III-EDTA ([Fe(EDTA)H₂O]⁻ ⇔ Fe(EDTA)OH]²⁻ + H⁺). The rate law for the formation of the complex was found to be d[Fe^IIIEDTAO₂]³⁻/dt=[(k₄[H⁺]/([H⁺] + K₁)][Fe^III-EDTA][H₂O₂], where k₄=8.15±0.05×10⁴ M⁻¹ s⁻¹ and pK₁=7.3. The steps involved in the formation of [Fe(EDTA)O₂]³⁻ are briefly discussed.