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The iron mixed-valence complex (n-C3H7)4N-[FeIIFeIII(dto)3] exhibits a novel type of phase transition called charge-transfer phase transition (CTPT), where the thermally induced electron transfer between FeII and FeII occurs reversibly at ∼120 K, in addition to the ferromagnetic phase transition at TC = 7 K. To investigate the mechanism of the CTPT, we have synthesized a series of magnetically diluted complexes (n-C3H7)4N-[Fe1-xIIZnxIIFeIII(dto)3] (dto = C2O2S2; x = 0-1), and carried out magnetic susceptibility and dielectric constant measurements and 57Fe Mössbauer spectroscopy. With increasing ZnII concentration (x), the CTPT is gradually suppressed and disappears at x ∼ 0.13. On the other hand, the ferromagnetic transition temperature (TC) is initially enhanced from 7 K to 12 K between x = 0.00 and 0.05, despite the nonmagnetic nature of ZnII ions, and then it decreases monotonically from 12 K to 3 K with increasing ZnII concentration. This anomalous dependence of TC on ZnII concentration is related to a change in the spin configuration of the ferromagnetic state caused by the partial suppression of the CTPT. [ABSTRACT FROM AUTHOR]