Kinetics of anation of hexaaquachromium(III) by thiocyanates – specific salt effects

Abstract

The kinetics of anation of hexaaquachromium(III) by thiocyanates follows the rate law:The specific salt effect has been studied for five media: NaCl, NaBr, NaClO 4 , KCl and CsCl. The series of chloride (Na + , K + and Cs + ) salts show a negligible effect on the anation rate. On the contrary, the series of sodium salts (Cl ) , Br ) and ClO 4 ) ) reveal a marked difference in the reaction rate. The anation rate decreases sharply with the ionic strength increase (I ¼ 0.2-2.0 M, NaCl). The results were interpreted within the frame of fast equilibria of ion-pair formation followed by an interchange mechanism step. The difference of reaction rate is a result of competition between anions (thiocyanates and supporting electrolyte anions) to the complex cation at an ion-pair formation process.

Ultrafast Photochemistry of the [Cr(NCS)6]3– Complex in Dimethyl Sulfoxide and Dimethylformamide upon Excitation into Ligand-Field Electronic State

“…Previous studies have demonstrated that photoexcitation of chromium­(III) complexes can result in ligand oxidation, ligand elimination, and ligand substitution. Typically, photolysis of chromium­(III) complexes results in ligand elimination ([Cr(NH 3 ) 2 (NCS) 4 ] − ) and ligand substitution ([Cr(CO) 4 (phen)], [Cr­(H 2 O) 5 (NCS)] 2+ ). ,, In some cases, the quantum yield of photoactivated ligand release was reported to be greater than 1. For example, Wegner and Adamson observed a photoactivated isothiocyanate release process for the K[Cr(NH 3 ) 2 (NCS) 4 ] complex with a quantum yield of 1.1 .…”

“…Theoretical studies of the Cr­(acac) 3 system performed by Ando et al demonstrated a stepwise relaxation mechanism including ultrafast (less than 100 fs) ISC from 4 T 2 to the second doublet 2 T 1 LF excited state followed by internal conversion to the 2 E LF state . Upon excitation, chromium­(III) complexes are also able to undergo photochemical reactions, typically ligand elimination and substitution. − However, the complete photochemical mechanism of the chromium­(III) complex reaction remains unclear due to its complexity.…”

Mechanisms of reactions in solution

“…Rate constants for anation of [Cr(H 2 O) 6 ] 3+ by NCS À show a marked decrease with increasing ionic strength. 88 A mechanism is proposed involving ion-pair formation and an interchange step.…”