The order of reactivity (X = F < Cl < Br < I) determined for the reaction of the model nucleophile pinenylpotassium with halotrimethylsilanes XSi(CH3)(3) in hexane and with halotriisopropylsilanes XSi(iC(3)H(7))(3) in tetrahydrofuran supports a single-step S(N)2(Si) mechanism over the formation of a pentavalent intermediate. However, no leaving group effect is found (X = F approximate to Cl approximate to Br approximate to I) when the condensation of pinenylpotassium with halotrimethylsilanes is accomplished in tetrahydrofuran. Under such conditions the crucial event appears to be diffusion-controlled precomplex formation followed by rapid metal halide ejection.