The room-temperature adsorption of pyrrolidine and its methyl-protected analogue, N-methylpyrrolidine, on the Si(100)-2 x 1 surface has been investigated using multiple internal reflection Fourier transform infrared spectroscopy and ab initio quantum chemistry calculations. For both compounds, initial adsorption occurs by barrierless formation of a dative bond between the nitrogen lone pair and the electrophilic atom of the Si dimer. However, while pyrrolidine proceeds to chemisorb dissociatively through N-H bond cleavage, methylpyrrolidine is shown to be trapped in its dative-bonded precursor state at room temperature due to a substantial barrier for N-CH3 cleavage. Additionally, the saturation coverage of methylpyrrolidine on Si is seen to be significantly less than that of pyrrolidine, due likely to both steric factors and charge-transfer effects.