Biphasic oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) by N-bromosuccinimide and the formation of chromium(IV) and chromium(V)

The kinetics of oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) complex, [Cr^III(phen)(H₂O)₂Cl₂]⁺, by N-bromosuccinimide (NBS) is biphasic. The first faster step involves the oxidation of Cr(III) to Cr(IV). The second slower step is due to the oxidation of Cr(IV) to Cr(V). The reaction product is isolated and characterized by electron spin resonance (ESR), IR, and elemental analysis. The chromium(V) product is consistent with the formula [Cr^V(phen)Cl₂(O)]Br. The rate constants k_f and k_s, for the faster and the slower steps respectively, were obtained using an Origin 9.0 software program. Values of both k_f and k_s, varied linearly with [NBS] at constant reaction conditions. The effect of pH on the reaction rate is investigated over the pH (4.11–6.01) range at 25.0°C. The rate constants k_f and k_s increased with increasing pH. This is consistent with hydroxo forms of the chromium species being more reactive than the aqua forms. Chromium(III) complexes, more often than not, are inert. The oxidation of the Cr(III) complex to Cr(IV), most likely, proceeds by an outer sphere mechanism. Since chromium(IV) is labile the mechanism of its oxidation to chromium(V) is not certain.     

Structural and Electron Density Studies on a Chromium(IV)-Oxo Complex [CrIV(O)(TMP)]

The electron density distribution of a chromium(IV)-oxo complex, [Cr^IV(O)(TMP)] (TMP = 5,10,15,20-tetrakis-p-methoxyphenyl porphyrin), is investigated by molecular orbital calculation. The molecular and crystal structure of the compound is studied by x-ray diffraction. It belongs to the space group 1 2, Z = 2, a = 14.979(4) Å, b = 9.752(3), c = 15.605(3) Å, β = 100.97(2)°, V = 2238(1) ų, Mo Kα radiation λ = 0.7107 Å, R = 4.9%, Rw = 3.5% for 3575 observed reflections. Cr is five-coordinated in a square pyramidal fashion with the Cr atom located 0.42 Å toward the oxo-ligand. Deformation density maps are derived from the single point molecular orbital calculation on the basis of HF and DFT(density functional theory) calculations. The accumulation of deformation density along the C-H, C-C, C-N and C-O bonds in the porphyrin ligand is well represented. The asphericity in electron density around the Cr ion is clearly demonstrated. Natural bond orbital analysis (NBO) reveals that the Cr-O_oxo is actually a triple-bond character (σ²π⁴) and the four N of pyrrole serves as a σ-donor to Cr. The Cr-N_pyrrole bond is essentially a dative bond d-Orbital populations of Cr derived from both calculations are in good agreement with each other. Planar d_π-orbital is the most populated, which is in accord with the prediction from crystal field theory. Detail bond characterization of the Cr-L, multiple bond is discussed.     

Synthesis and Kinetic Study on the Chromium(III) Complex [Cr(ASA)(en)2]Cl·2H2O

In order to explore the transfer mechanism of chromium(III) in mammals, a novel complex [Cr(ASA)(en)₂]Cl· 2H₂O, bis(ethylenediamine‐ κ ² N,N′)(4‐aminosalicylic acid‐ κ ² O,O′) chromium(III) monochloride dihydrate was synthesized (4‐aminosalicylic acid=H₂ASA, ethylenediamine=en). The crystal structure belongs to orthorhombic system with the space group P2₁2₁2₁ by means of X‐ray diffraction. The characteristic for transfer of Cr³⁺ from the compound to the low‐molecular‐mass chelator EDTA and the iron‐binding protein apoovotransferrin (apoOTf) was followed by UV‐visible (UV‐Vis) and fluorescence spectra in 0.01 mol·L^?1 Hepes at pH 7.4. The second order rate constants were calculated. Those spectra in conjunction were used to obtain more accurate information about the interaction of chromium complex with apoOTf. The experimental results indicate that Cr³⁺ can be transferred from the complex to apoOTf with the retention of the 4‐aminosalicylic acid acting as a synergistic anion.     

A Dimeric Chromium(II) Pincer as an Electron Shuttle for N=N Bond Scission

Reduction of the bis-pyrazolyl pyridine complex [CrL]₂ with 4 KC₈, followed by addition of one azobenzene (overall mole ratio 1:4:1), PhNNPh, transfers reducing equivalents to three azobenzenes, to form [K₃Cr(PhNNPh)₃]. This has three κ² PhNNPh²⁻ ligands and K⁺ bound to nitrogen atoms of azobenzene. When the stoichiometry is modified to 1:4:3, the product is changed to [K₂CrL(PhNNPh)₂], which has C₂ symmetry except for the intimate ion pairing of two K⁺ ions to reduced azobenzene nitrogen atoms, and to pyrazolate and phenyl rings. The origin of the observed delivery of reducing equivalents to several, not to a single N=N bond, is traced to the resistance of the one-electron-reduced substrate to receiving a second electron, and is thus a general phenomenon. [CrL]₂ alone is shown to be a two-electron reductant towards benzo[c]cinnoline (BCC) resulting in a product of formula [Cr₂L₂(BCC)], in which the reducing equivalents originate purely from Cr^II. An analogous study of the reaction of [CrL]₂ with azobenzene yields [Cr₂L₂(PhNNPh)(THF)], an adduct in which one THF has displaced one of four hydrazide nitrogen/Cr bonds. Together these illustrate different modes for the Cr₂L₂ unit to bind and reduce the N=N bond. Collectively, these results show that two divalent Cr, without added K⁰, have the ability to reduce the N=N bond. Further KC₈ reduction of preformed Cr₂L₂(RNNR) inevitably gives products in which K⁺ stabilizes the charge in the increasingly electron-rich nitrogen atoms, in a phenomenon which mimics proton coupled electron transfer: K⁺ performs the role of H⁺. A least-squares fit of the two singly reduced DFT structures shows that the only major change is a re-orientation of one of the two phenyl rings in order to avoid repulsion with potassium but to still allow interaction of that phenyl π system with K⁺. This shows both the impact of K⁺, being modest to nitrogen/chromium interactions, but nevertheless accommodating some π donation of phenyl to potassium. Finally, delivering increasing equivalents of KC₈ leads to complete cleavage of the N=N bond, and both N bind to three Cr^II. The varied impacts of the K⁺ electrophile on NN multiple bond reduction is discussed.     

Kinetics and mechanism of oxidation of (aqua-2-aminomethyl-pyridine) chromium(III) by N-bromosuccinimide

Summary The kinetics of oxidation of (aqua-2-aminomethyl-pyridine) Cr^III by N-bromosuccinimide (NBS) in aqueous solution to yield chromium(VI) has been studied spectrophotometrically over the 25–40 °C range. The reaction rate is first order with respect to both [NBS] and [Cr^III], and increases with increasing pH between 7.6 and 8.6. The thermodynamic activation parameters were calculated. The experimental rate law is consistent with a mechanism in which the deprotonated [Cr(L)₂(OH)]²⁺ was considered to be the most reactive form compared to its conjugate acid. It is assumed that electron transfer takes place via an inner-sphere mechanism.     

Optimized structures of Cr2(CO)6+ with three different symmetries by density functional calculation

Density functional calculations have been made on a binuclear metal carbonyl ion Cr₂(CO)₆⁺ found in our laser ablation–molecular beam (LAMB) experiment. Optimized structures are calculated for three different conformations: T33 of D_3d symmetry with three terminal carbonyl groups on each chromium atom, B2T22 of D_2h symmetry with two bridging carbonyl groups and two terminal carbonyl groups on each chromium atom, and B4T11 of D_4h symmetry with four bridging carbonyl groups and one terminal carbonyl group on each chromium atom. The most stable conformation is T33 which is 36.76 and 286.44 kJ mol⁻¹ lower in energy than B2T22 and B4T11, respectively. The difference of conformation exerts a significant influence on the internuclear distance between chromium and the carbon of terminal CO, but hardly on the Cr–Cr bond length. For B2T22 and B4T11, longer C–O distances for bridging carbonyls compared with those for terminal ones indicate effective π*-back donation from the chromium atom to the bridging carbonyl groups. Furthermore, the relative abundance of Cr₂(CO)_n⁺ (n = 0–6) observed in our previous experimental study can be explained qualitatively by comparison of the excess energy produced in the formation of a Cr⁺–Cr bond with the CO dissociation energy of Cr₂(CO)₆⁺. © 1998 John Wiley & Sons, Ltd.     

Electrochemical and surface science investigations of PtCr alloy electrodes

Electrodes of supported Pt, modified with Cr, have shown an increase in electrochemical activity for oxygen reduction in phosphoric acid fuel cells over supported Pt only electrodes. To clarify the role of chromium and its chemical nature at the electrode surface, we have characterized a series of Pt_xCr(_1-x) bulk alloys (x = 0.9, 0.65, 0.5, 0.2) by electrochemical and ex-situ surface science methods. In this paper we report the surface characterization of native and post-electrochemical electrodes by XPS, cyclic voltammetry in 0.05 M H₂SO₄ and 85% H₃PO₄, and analysis of 0.05 M H₂SO₄ electrolyte following electrochemical treatment. The surface Cr(1 to 2 nm) was oxidized to Cr³⁺ oxide for surfaces at open circuit and those exposed to potentials < + 1.3 V vs DHE in 0.05 M H₂SO₄ and < + 1.55 V vs. DHE in 85% H₂PO₄. In 0.05 M H₂SO₄ the Cr component was electrooxidized to solube Cr⁶⁺ species at potentials > +1.3 V with the extent of Cr dissolution dependent on initial alloy stoichiometry. Alloys with Cr content 0.5 are capable of producing (dependent on time spent at potentials above +1.3 V in 0.05 M H₂SO₄) very porous Pt-rich surfaces. Loss of Cr was also observed in 85% H₃PO₄ for the alloys with Cr content 0.5, although at the more positive potential limit of +1.55 V. For the Pt_0.2Cr_0.8, treatment in 85% H₃PO₄ at +1.4 V and above led to the appearance of Pt⁴⁺ and Cr⁶⁺ species, apparently stabilized in a porous phosphate overlayer up to 5 nm thick (dependent on time spent at potentials above this limit). The enhancement reported for supported Pt+Cr oxygen cathodes is discussed in the light of these results.     

Kinetics and mechanism of the oxidation of chromium(III)–dipicolinic acid complex by N-bromosuccinimide

The kinetics of oxidation of the chromium(III)–dipicolinic acid complex [Cr^III(DPA)₂(H₂O)₂]^– by N-bromosuccinimide (NBS) in aqueous solution to Cr^VI have been studied spectrophotometrically over the 20–40 °C range. The reaction is first order with respect to both [NBS] and [Cr^III], and increases with pH over the 5.92–6.93 range. Thermodynamic activation parameters were calculated. It is proposed that electron transfer proceeds through an inner-sphere mechanism via coordination of [NBS] to chromium(III).     

Cu(Cu0.44Cr4.56)Ge2O12: a close‐packed oxide with CuO4 tetra­hedra

The structure of copper(I,II) penta­chromium(III) germanate, Cu(Cu_0.44Cr_4.56)Ge₂O₁₂, contains one Cu position (m2m), one Ge position (m) and three Cr positions (2/m, m and 2). The close‐packed structure is described in terms of slabs of edge‐sharing Cr³⁺O₆ octa­hedra and isolated CuO₄ and GeO₄ tetra­hedra. These slabs are aligned parallel to the bc plane and are separated from each other by GeO₄ tetra­hedra along a. The tetra­hedral coordination observed for the Cu⁺/Cu²⁺ ions represents an unusual feature of the structure. The Cr—O and Cu—O bond lengths are compared with literature data.     

Spin Crossover and Long-Lived Excited States in a Reduced Molecular Ruby

The chromium(III) complex [Cr^III(ddpd)₂]³⁺ (molecular ruby; ddpd=N,N′-dimethyl-N,N′-dipyridine-2-yl-pyridine-2,6-diamine) is reduced to the genuine chromium(II) complex [Cr^II(ddpd)₂]²⁺ with d⁴ electron configuration. This reduced molecular ruby represents one of the very few chromium(II) complexes showing spin crossover (SCO). The reversible SCO is gradual with T_1/2 around room temperature. The low-spin and high-spin chromium(II) isomers exhibit distinct spectroscopic and structural properties (UV/Vis/NIR, IR, EPR spectroscopies, single-crystal XRD). Excitation of [Cr^II(ddpd)₂]²⁺ with UV light at 20 and 290 K generates electronically excited states with microsecond lifetimes. This initial study on the unique reduced molecular ruby paves the way for thermally and photochemically switchable magnetic systems based on chromium complexes complementing the well-established iron(II) SCO systems.     

Electron transfer. 138. Potentials involving chelated chromium(V)

The bis(chelated) complex of Cr^V(0) derived from the dianion (L^{2 −}) of 2-ethyl-2-hydroxybutanoic acid is readily reduced to a bis(chelate of Cr^III, featuring the monoanion (LH⁻) [Cr ^V(0)(L²⁻)₂]⁻+4H⁺+H₂O+2e⁻→[Cr^III(OH₂)₂(LH⁻ ₂]⁺ of this acid. Potentials estimated by Ghosh in 1993 for this 2e⁻ change, E⁰ (pH 0) 1.32 V, E^eff (pH 3.3) 0.93 V, are in accord with the nearly irreversible reductions of the Cr(V) species (in 1∶1 ligand buffer) by Fe²⁺, V0²⁺, IrCl₆ ^{3 −} and I⁻, whereas lower values reported by Bose in 1996, E⁰ (pH 0) 0.84 V, E^eff (pH 3.3) 0.45 V, are potentiometrically inconsistent with these conversions. A similar discrepancy is noted for potentials for Cr(V,IV) estimated in 1996, E⁰ (pH 0) 0.84 V, E^eff (pH 3.3) 0.46 V, which, wholly contrary to observation, predict that the reductions of excess Cr(V) to CR(IV) by Fe²⁺, V0²⁺, and I⁻ are thermodynamically disfavored.     

Microcalorimetric Study on Effect of Chromium(III) and Chromium(VI) Species on the Growth of Escherichia coli

The effects of Cr(III) and Cr(VI) species (Cr₂O₇^2?, CrO₄^2? and Cr³⁺) on the growth of Escherichia coli (E. coli) have been investigated in detail by microcalorimetry at 37 °C. Parameters including the growth rate constant (k), inhibitory ratio (I), half‐inhibitory concentration (IC₅₀), total heat output (Q_total), time of the maximum heat production (t_log) in the log phase have been obtained. The results showed that Cr(VI) and Cr(III) had the inhibition effect on the growth of E. coli in aquatic environment; however, the inhibitory ratio of Cr(III) to E. coli was smaller than that of Cr(VI). The k values of E. coli in the presence of Cr(VI) and at high concentrations of Cr(III) were decreased with increasing the concentrations of these chromium species. Among the three chromium species investigated, Cr₂O₇^2? was found to be the most poisonous species against E. coli with an IC₅₀ value of 35.52 µg·mL^?1. CrO₄^2? exhibited moderate toxicity on E. coli with an IC₅₀ of 50.24 µg·mL^?1, and Cr³⁺ had the lowest toxicity with an IC₅₀ of 84.30 µg·mL^?1. Microcalorimetry can provide a convenient, sensitive and reliable method to study the effect of various metal species on the growth of bacteria or other microorganisms.     

Photolyse der Chrom(III)-azido-Komplexe und Darstellung des Chrom(V)-nitrido-Komplexes mit N,N′-Ethylen-bis(salicylaldimin)

Photochemical Reactions of Chromium(III) Azido Complexes. Preparation of Nitrido-N, N′-ethylene-bis(salicylideniminato) Chromium(V) Complex The azide to nitride ligand conversion in the coordination sphere of chromium(III) complexes under light of 313 nm excitation has been observed. This photochemical reaction has been used to prepare the nitrido-N, N′-ethylene-bis(salicylideniminato)-chromium(V) complex characterized by an elementar analysis, electron, absorption, IR, and ESR spectra. The results of previous authors that the 313 nm photolysis of Cr(NH₃)₅N₃²⁺ yields Cr²⁺ or coordinated nitrenes are reinterpreted solely by formation of chromium(V) nitrido species.     

Effect of Mn(II) and Ce(IV) Ions on the Oxidation of Lactic Acid by Chromic Acid

The kinetics and mechanism of lactic acid oxidation in the presence of Mn(II) and Ce(IV) ions by chromic acid were studied spectrophotometrically. The oxidation of lactic acid by Cr(VI) was found to proceed in two measurable steps, both of which gave pyruvic acid as the primary product in the absence of Mn(II). 2Cr(VI)+2CH₃CHOHCOOH → 2CH₃COCOOH+Cr(V)+Cr(III) Cr(V)+CH₃CHOHCOOH → Cr(III)+CH₃COCOOHThe observed kinetics was explained due to the catalytic and inhibitory effects of Mn(II) and Ce(IV) on the lactic acid oxidation by Cr(VI). The reactivity of lactic acid depends upon the experimental conditions. It acts as a two-or three-equivalent reducing agent in the absence or presence of Mn(II). It was examined that Cr(III) products resulting from the direct reduction of Cr(VI) by three-equivalent reducing agents. The oxidation of lactic acid follows the complex order kinetics with respect to [lactic acid]. The activation parameters E_a, ΔH^#, and ΔS^# were calculated and discussed.     

Synthesis and molecular structure of Cr(salen)(μ‐N)RhCl(COD): the first example of a heterobimetallic nitride‐bridged complex containing chromium

Five‐coordinate Cr(N)(salen) {salen is 2,2′‐[ethane‐1,2‐diylbis(nitrilomethylidyne)]diphenolate} reacts with [RhCl(COD)]₂ (COD is 1,5‐cyclooctadiene) to yield the heterobimetallic nitride‐bridged title compound, namely chlorido‐2κCl‐[2(η⁴)‐1,5‐cyclooctadiene]{2,2′‐[ethane‐1,2‐diylbis(nitrilomethylidyne)]diphenolato‐1κ⁴O,N,N′,O′}‐μ‐nitrido‐1:2κ²N:N‐chromium(V)rhodium(I), [CrRh(C₁₆H₁₄N₂O₂)ClN(C₈H₁₂)]. The Cr—N bond of 1.5936 (14) Å is elongated by only 0.035 Å compared to the terminal Cr—N bond in the precursor. The nitride bridge is close to being linear [173.03 (9)°] and the Rh—N bond of 1.9594 (14) Å is very short for a monodentate nitrogen‐donor ligand, indicating significant π‐acceptor character of the Cr[triple‐bond]N group.     

Polymerization of aniline derivatives by K2Cr2O7 and production of Cr2O3 nanoparticles

Oxidative polymerization of aniline, anthranilic acid, and aniline‐co‐anthranilic acid by potassium dichromate Cr(VI) as an oxidant in acidic medium was investigated. In this study, the polymerization process of aniline, o‐anthranilic acid as well as aniline/o‐anthranlic acid using K₂Cr₂O₇ produced, coordinated Cr(III)/polyaniline (PANI), Cr(III)/polyanthranilic acid (PAA) and Cr(III)/poly aniline‐co‐anthranilic acid (PANAA). The mechanism of polymerization reaction in the presence of dichromate was hypothesized. The precursor chromium doped polymers were characterized by TGA, FT‐IR, UV‐visible, XRD analyses. Cr₂O₃ nanoparticles size were determined using TEM analysis. The calcinations process of synthesized chromium doped PANI, PAA and PANAA yields Cr₂O₃ nanoparticles 26%, 31%, and 34% wt. respectively. Rhombohedral phase of Cr₂O₃ particles in the range from 33 to 61 nm was produced from chromium/polyanthranilic acid (PAA) and chromium/poly(aniline‐co‐anthranilic acid) PANAA. UV‐ visible analysis showed that optical band gaps (E_g) of doped poly aniline and its derivatives are in the range from1.55 to 1.80 using Tacu's law. The band gap values reveal that the doped chromium emeraldine base can be used as semiconductor materials. Copyright © 2016 John Wiley & Sons, Ltd.     

Factors controlling electron transfer reactions and stabilisation of uncommon oxidation states of chromium

Some experimental approaches to seek semi-quantitative understanding of factors controlling outer sphere electron transfer reactions of some transition metal complexes have been made. The relative importance of nuclear and electronic factors to outer sphere processes has been examined. By the manipulation of Franck-Condon or nuclear factors, it has now been possible to gain access into the chemistry of chromium in unusual oxidation states. An example of a reorganisation controlled electron transfer reaction involving Cr(IV)-Cr(III) system has been demonstrated. The bimolecular rate of reduction of diperoxoaquaethylenediamine chromium(IV) and diperoxodiethylenetriamine chromium(IV) is independent of the nature of th reductant employed viz. Fe²⁺ or VO²⁺ indicating that the generation of6 coordinate Cr(IV) species from7 coordinate of diperoxochromium(IV) reactant may be rate limiting. Similarly by increasing the barrier for the6 coordinate to4 coordinate structures through equatorial coordination of macrocyclic ligands, it has now been possible to detect through cyclic voltommogram the formation of relatively stable Cr(IV) species in the electrochemical oxidation of Cr(Me₄[14] tetraene)(H₂O) ₂ ³⁺ in aqueous sulphuric acid media. The kinetics and mechanism of the cerium(IV) and iodosyl benzene oxidation of Cr(salen)(H₂O) ₂ ⁺ and Cr(salprn)(H₂O) ₂ ⁺ have been investigated and kinetic and spectroscopic evidence for the formation of Cr(IV) transients and stable Cr(V) products has been presented. The relative importance of Franck-Condon factors in the oxidation of Cr(III) to Cr(IV) and Cr(V) states in different macrocyclic and multidentate ligand environments has been discussed.     

Effect of mono- (Cr) and bication (Cr, V) substitution on LiMn2O4 spinel cathodes

A study on the structural and electrochemical properties of LiCr_0.2Mn_1.8O₄ and LiV_0.2Cr_0.2Mn_1.6O₄ cathodes has been made with a view to understand the effect of mono- (Cr) and bication (Cr and V) substitution on LiMn₂O₄ spinel individually. Citric acid assisted modified sol–gel method has been followed to synthesize a series of LiMn₂O₄, LiCr_0.2Mn_1.8O₄, and LiV_0.2Cr_0.2Mn_1.6O₄ cathodes, and the corresponding lattice structure, surface morphology, and site occupancy of lithium in the spinel matrix are acknowledged using X-ray diffraction, scanning electron microscopy, and magic angle spinning ⁷Li nuclear magnetic resonance results. The site occupancy of Cr³⁺ in the 16d octahedral and that of V⁵⁺ in the 16d octahedral and 8a tetrahedral positions are understood. Electrochemical cycling studies of LiCr_0.2Mn_1.8O₄ cathode demonstrate an enhanced structural stability and better capacity retention (94%) resulting from the Cr³⁺ dopant-induced co-valency of Li-O-Mn bond. On the other hand, simultaneous substitution of Cr and V in LiV_0.2Cr_0.2Mn_1.6O₄ has failed to improve the electrochemical properties of native LiMn₂O₄ spinel cathode, mainly due to vanadium-driven cation mixing and the reduced lithium diffusion kinetics. Among the candidates chosen for the study, LiCr_0.2Mn_1.8O₄ qualifies itself as a better cathode for rechargeable lithium battery applications.     

Biphasic Oxidation of cis‐Diaquabis(1,10‐phenanthroline)chromium(III) by N‐Bromosuccinimide and the Formation of Chromium(IV) and Chromium(V)

The oxidation of cis‐diaquabis(1,10‐phenanthroline)chromium(III) [cis‐Cr^III(phen)₂(H₂O)₂]³⁺ by ‐bromosuccinimide (NBS) to yield cis‐dioxobis(1,10‐phenanthroline)chromium(V) has been studied spectrophotometrically in the pH 1.57–3.56 and 5.68–6.68 ranges at 25.0°C. The reaction displayed biphasic kinetics at pH < 4.0 and a simple first order at the pH > 5.0. In the low pH range, the reaction proceeds by two successive steps; the first faster step corresponds to the oxidation of Cr(III) to Cr(IV), and the second slower one corresponds to the oxidation of Cr(IV) to Cr(V), the final product of the reaction. The formation of both Cr(IV) and Cr(V) has been detected by electron spin resonance (ESR). The ESR clearly showed the formation and decay of Cr(IV) as well as the formation of Cr(V). Each oxidation process exhibited a first‐order dependence on the initial [Cr(III)]. The pseudo–first‐order rate constants k₃₄ and k₄₅, for the faster and slower steps, respectively, were obtained by a computer program using Origin7.0. Both rate constants showed first‐order dependence on [NBS] and increased with increasing pH.     

Kinetic studies on the oxidation of [N-phenylethylethylenediaminetriacetate]chromium(III) by periodate

Summary Kinetic studies of the oxidation of [Cr^IIIZ(H₂O)](Z=N-phenylethylethylenediaminetriacetate) by periodate ion, to produce chromium(VI), were carried out in aqueous solutions. The reaction is first order with respect to both total chromium(III) and total periodate, and the rate is inversely dependent upon H⁺ in the 5.43–7.02 pH range. The reaction may follow a two-step inner-sphere electron transfer mechanism. The activated parameters are reported. Steric effects of the phenyl ring account for the smaller electron-transfer rate constants for [Cr^IIIZ(H₂O)] compared to [Cr^III(TOH)(H₂O)], (TOH=N-(2-hydroxyethyl)ethylenediaminetriacetate).