Photochemistry of coordination compounds

Chromium(iii) photochemistry and photophysics

Structure and reactivity of the metal-centered transition metal excited states

Direct analysis of thin-layer chromatography spots by diffuse reflectance Fourier transform infrared spectrometry

Irradiation of the [Cr(C2O4)3]3− complex or the chromate(VI)–oxalate mixture, or the ternary system composed of Cr(III), Cr(VI) and oxalate, leads to chromium photoreductions in consequence of the ligand to metal charge transfer (LMCT) excitations induced by artificial solar radiation. In the case of the Cr(III) complex, the photoreduction involves the innersphere electron transfer leading to the formation of the Cr(II) species and the C2O4 − radicals. On reacting with molecular oxygen, Cr(II) is oxidised to Cr(III) catalysing thereby the oxalate substitution reaction. Moreover, under specific conditions, Cr(II) can be also oxidised to Cr(VI). Chromate(VI) is not photoreducible, but in the presence of oxalate, or other sacrificial electron donor, the outersphere photoinduced electron transfer (PET) produces Cr(V) species and the C2O4 − radicals. This initiates a series of thermal reactions leading to the formation of Cr(III) and oxidized oxalate (CO2). In the system composed of [Cr(C2O4)3]3− and chromate(VI), the acidic medium and anoxic conditions favour the Cr(VI) photoreduction, whereas alkaline oxygenated solutions assist the Cr(VI) photoproduction. When an approximately neutral solution equilibrated with the ambient air is irradiated intermittently, Cr(VI) is consumed and/or produced, accordingly to the time sequence of exposure and dark periods. The oscillations of Cr(VI) concentrations are accompanied by continuous oxidation of oxalate, playing the role of the sacrificial electron donor. The effects of solution pH, molecular oxygen, concentrations of reagents and cations on the reaction rates were investigated. The results of this paper revealed that the Cr(III)/Cr(VI) system under environmental conditions behaves as the photocatalytic one catalysing the oxidation of oxalate or other organic matter by molecular oxygen, contributing thereby to the abatement of pollution.

Photoredox processes in the Cr(VI)–Cr(III)–oxalate system and their environmental relevance

The photoreactions of cis- and trans-[CrCl2(en)2]+ have been investigated in acidic aqueous solution at pH 3 and 0 °C. On photolysis in the ligand field bands the cis isomer was found to undergo ma...

Photochemistry of the cis- and trans-Dichlorobisethylenediamine Chromium(III) Cations in Acidic Aqueous Solutions. Observation of a Stereochemical Change in Photoaquation

Separation, identification and estimation of Cr3 complex ions. Thin-layer chromatography and diffuse reflectance spectroscopy.

The photolysis reactions of the complex ions [Cr(en)2ox]+ and [Cren(ox)2]− have been studied in the pH range 1 → 3 and at temperatures from 0 to 25 °C. It has been found that both ions photoaquate to a monodentate protonated ethylenediamine complex which then undergoes further aquation thermally with loss of the ethylenediamine ligand. Quantum yields for these aquations were measured for irradiation into the ligand field bands, the values obtained being 0.18 and 0.02, respectively. The results are discussed in terms of known empirical correlations for Cr(III) complexes. Assuming these reactions may occur from the 4T2g state as precursor, arguments are presented that the correlation between quantum yield and ΔE may depend on mixing of the 4T2g and 2Eg states, and its effect on the intersystem crossing rate constant.

The Photolysis in Acidic Aqueous Solutions of the Monooxalato-bisethylenediamine Chromium(III) Cation and the Bisoxalatomonoethylenediamine Chromate(III) Anion: Photolysis Modes and Quantum Yields

K. C. Moss

Papers

Photochemistry of the cis- and trans-Dichlorobisethylenediamine Chromium(III) Cations in Acidic Aqueous Solutions. Observation of a Stereochemical Change in Photoaquation

Separation, identification and estimation of Cr3 complex ions. Thin-layer chromatography and diffuse reflectance spectroscopy.

The Photolysis in Acidic Aqueous Solutions of the Monooxalato-bisethylenediamine Chromium(III) Cation and the Bisoxalatomonoethylenediamine Chromate(III) Anion: Photolysis Modes and Quantum Yields