The n-alkyl halides, RX, were oxidatively added to the platina(II)cyclopentane complexes [Pt{(CH₂)₄}(NN)], in which NN = bpy (2,2′-bipyridyl) or phen (1,10-phenanthroline), to give the platinum(IV) complexes [PtRX{(CH₂)₄}(NN)], R = Et and X = Br or I; R = ⁿBu and X = I, 1–3. The same reactions with the analogous dimethyl complex [PtMe₂(bpy)] gave the expected platinum(IV) complexes [PtRXMe₂(bpy)], R = Et or ⁿPr and X = Br or I; R = ⁿBu and X = I, 4–8. Kinetics of the reactions in benzene and acetone was studied using UV-vis spectrophotometery and a common S_N2 mechanism was suggested for each case. The platina(II)cyclopentane complexes reacted faster than the corresponding dimethyl analogs by a factor of 2–3. This is described as being due to a lower positive charge, calculated by density functional theory (DFT), on the platinum atom of [Pt{(CH)₂)₄}(bpy)] compared with that on the platinum atom of the dimethyl analog [PtMe₂(bpy)]. The values of ΔΔS^‡ = ΔS^‡_acetone − ΔS^‡_benzene were found to be either positive or negative in different reactions and this is related to the solvation of the corresponding alkyl halide. It is suggested that in these reactions of RX reagents, for a given X, the electronic effects of the R group are mainly responsible for the change in the rates of the reactions and the bulkiness of the group is far less important.