The invention relates to a method for producing homogeneous solutions of metal complexes of general formula (2): [MaLxXn]Ym, where M is a metal from the group comprising Mn(II), Mn(III), Mn(IV), Fe(III), Fe(III) or Fe(IV), X is a coordinated compound selected from mono- bi- or tri-charged anions or neutral molecules which are able to coordinate with a metal mono- bi- or tri-dentate, Y is a non-coordinated counter-ion which ensures charge equalization of the complex, L is a ligand of general formula (1) or the protonized or de-protonized form thereof, and a, x, n, m, R, R1, R2, R3 and z are as specified in claim 1, in diols or polyols, the monoethers or mixtures of said substances. Said method is characterized in that a ligand of the formula (1) is reacted with an iron or manganese salt in a heterogeneous reaction in the diol or polyol, the monoethers or mixtures of these substances.