Complex of general formula I <IMAGE> in which R1, R2 and R3, which may be identical or different, represent separately a linear or branched alkyl radical of 1 to 5 carbon atoms, or, together with the nitrogen atom to which they are attached, a heterocyclic radical having 5 to 7 carbon atoms; "alk" represents a linear or branched alkylene radical having 1 to 5 carbon atoms; R4 to R8, which may be identical or different, represent hydrogen, a hydroxyl, alkoxy or aryloxy group, a primary, secondary or tertiary amino group, or the group -alk-N<+)<R1R2R3) in which "alk", R1, R2 and R 3 have the above meanings; M represents a transition metal such as Fe, Ru or Os; X represents an anion and n represents an integer corresponding to the number of positive charges of the complex.