Electronic Influence of Different β-Diketonato Ligands on the Electrochemical Behaviour of Tris(β-Diketonato)M(III) Complexes, M = Cr, Mn and Fe

Abstract

The reduction of the MIII/MII metal couple of complexes Cr (β-diketonato)3, Fe (β-diketonato)3 and Mn (β-diketonato)3 is reviewed and compared. The ease of reduction of the MIII/MII couple of M(β-diketonato)3 complexes increases according to the metal sequence Cr (β-diketonato)3 < Fe (β-diketonato)3 < Mn (β-diketonato)3 (with the most positive reduction potential). Good linear relationships obtained between the reduction potential and different electronic parameters related to the β-diketonato ligand on these MIII(β-diketonato)3 complexes, show that the ease of reduction of the MIII/MII couple increases with decreasing acidic strength (pKa) of the respective β-diketone ligands. It also increases with increasing total group electronegativity of the R and R' groups on the respective β-diketonato ligand (RCOCHCOR')− of the M(β-diketonato)3 complexes, (χR + χR'), as well as with an increase in the total Hammett sigma meta constants (σR + σR'), and also with increasing value of the Lever ligand electronic parameter (EL) of ligand (RCOCHCOR')−.