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The constant of electron exchange rate (k ex .) in the system of sym. octamethylferrocene/sym. octamethylferricinium hexafluorophosphate [(СН 3) 4 С 5 Н] 2 Fe/(СН 3) 4 С 5 Н] 2 Fe + PF 6  ] (hereinafter are denoted as Ме 8 Fс +/0) was determined by method of 1 H NMR line broadening. The appearance of eight methyl groups in each reagent of the redox-system of Ме 8 Fс +/0 increases the constant of exchange rate approximately 4.8 times, in comparison with the unsubstituted ferrocene-ferricinium system [(С 5 Н 5) 2 Fe/(С 5 Н 5) 2 Fe + PF 6  ] (Fс +/0). The increase of exchange rate in series of Fс +/0 Ме 6 Fc +/0 Ме 8 Fс +/0 Ме 10 Fс +/0 , though it corresponds to the electron donor property of the methyl group, is interpreted by us as the result of reduction of reorganization energy of solvent molecule (acetone) by increasing the reagent volumes of the redox couples of Ме n Fс +/0 (n = 6, 8, 10).

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Two new redox systems, sym. 1,2,4,1¢,2¢,4¢-hexamethylferrocene/cation sym. 1,2,4,1¢,2¢,4¢-hexamethyl ferricinium and sym. octamethylferrocene/cation sym. Octamethyl­ferricinium (MenFc/MenFc+, n = 6, 8) were studied by the cyclic voltammetry method. The observed dif­ference between potentials of anodic and cathodic peaks of 0.063 to 0.075 V, and its inde­pendence on the potential scan rate, the straight-line depen­dence of the current value of anodic (and cathodic) peak on square root of the potential scan rate, as well as  shapes of the recorded cyclic voltammograms indicate that both redox systems in acetonitrile meet the most important requirement of IUPAC regarding internal reference redox systems (IRRS) - electrochemical reversibility of electron transfer reaction. The same method under identical conditions was used to study the effect of the number of methyl groups on the redox potential of MenFc/MenFc+ systems, n = 0, 6, 8, 10. It was shown that the successive displacement of half-wave potential in the series of Fc/Fc+ - Me6Fc/Me6Fc+ - Me8Fc/Me8Fc+ -Me10Fc/Me10Fc+ towards negative potentials is attributed to the electron-donor property of methyl groups. The location of the redox potentials values of new systems [n=6 (111 mV), n=8 (23 mV)] between redox potentials of systems of n = 0 (431 mV) and n = 10 (-77 mV) means that the redox potential of the systems of MenFc/MenFc+(n = 6, 8) has an optimal position on the electrode potential scale, i.e. meets another of the IUPAC criteria for IRRS.