MONOBROMOIRON(3) ION . EQUILIBRIUM STUDIES ON COMPLEX AND ON ION PAIR AND KINETIC STUDIES ON SUBSTITUTION AND REDUCTION REACTIONS

Studies on iron(III)-bromide complexes reveal two species-an inner complex FeBr2+ and an ion pair Fe3+,Br-whose stabilities have been separately evaluated. At 1.6°, μ = 1.00 M, Qi - 0.034 ± 0.003 M-1, and Q0 = 0.20 M-1. The substitution rate constants for FeBr2+at 1.6° and [H+] = 1.00 M are: aquation, 10.7 sec-1; formation, 0.34 ± 0.03 M-1 sec-1. The reduction reactions of FeBr2+ and Fe3+,Br- by Cr2+ and Eu2+ have been studied. The values of kFeBr are ≥2 × 107 M-1 sec-1 for Cr2+ and 1.3 × 106 M-1 sec-1 for Eu2+ at 1.6° and n = 1.00 M. The third-order rate constants under the same conditions are 3.9 × 103 M-2 sec-1 for Cr2+ and 5.8 × 103 M-2 sec-1 for Eu2+. The mechanisms of the various reactions are discussed. The third-order reactions are best understood as reactions of the ion pair and the reducing agents. © 1969, American Chemical Society. All rights reserved.