Ligand substitution in a transition metal complex: a new type of photocatalytic reaction in a semiconductor

We show that a semiconductor can serve as a photocatalyst for the ligand substitution reaction in coordination compounds of chromium(III). The mechanism of the process includes stages of generation of an electron-hole pair upon action of light quanta with energy equal to (or greater than) the width of the forbidden gap in the semiconductor, capture of the electron from the conduction band by the chromium(III) complex, fast ligand substitution in the chromium(II) complex formed by a solvent molecule, and oxidation of the chromium(II) complex by the hole from the valence band. The reaction of aquation of the complex ions [CrCl₂(H₂O)₄]⁺ proceeds when using the semiconductors (CdS, GaAs), whose conduction band is located in the more negative potential region compared with the standard oxidation-reduction potential of the Cr(III)/Cr(II) pair. We show that the reaction rate is slowed down when electron acceptors (oxygen) and hole acceptors (ethanol) are introduced, which confirms the proposed mechanism.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 1, pp. 89–92, January–February, 1990.