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

The reduction of the M^III/M^II 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 M^III/M^II 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 M^III(β-diketonato)₃ complexes, show that the ease of reduction of the M^III/M^II couple increases with decreasing acidic strength (pK_a) 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, (χ_R + χ_R'), as well as with an increase in the total Hammett sigma meta constants (σ_R + σ_R'), and also with increasing value of the Lever ligand electronic parameter (E_L) of ligand (RCOCHCOR')⁻.