In this work, we systematically investigated the ligand effects on spontaneous electron transfer (ET) between alkanethiolate-protected metal clusters in solution. The donor and acceptor clusters used were [PtAu24(SCnH2n+1)18]2- (8e(Cn)) and [PtAu24(SCmH2m+1)18]0 (6e(Cm)) (n, m = 2-16), which have icosahedral Pt@Au12 cores with eight and six valence electrons, respectively. The ET rate constant (kET) from 8e(Cn) to 6e(Cm) in benzene exhibited a novel turnover behavior as a function of the total chain length n + m: the kET decreased with n + m in the range of 4-12, whereas it monotonically increased with n + m in the range of 12-32. Electrospray ionization mass spectrometry of the mixture of 8e(Cn) and 6e(Cm) detected the dimer complex 8e(Cn)·6e(Cm), the relative population of which increased with n + m. The activation energy (Ea), determined based on the Arrhenius plots for n = m, monotonically decreased with n (≥ 6). Based on these results, we proposed that the promotion of ET by longer alkanethiolates was ascribed to two effects on the key intermediate 8e(Cn)·6e(Cm): (1) elongation of the lifetime and (2) the contraction of the distance between 8e(Cn) and 6e(Cm) due to the stronger van der Waals interaction between the longer alkyl chains. Such alkyl-chain-promoted ET is specific to ultrasmall clusters in solution because a nonuniform ligand layer could be formed due to the large curvature of the cluster core.