The interactions of cyanoacetylene and diacetylene with a Si(I 1 l)-7 x 7 surface have been studied as model systems to mechanistically understand the chemical binding of unsaturated organic molecules to diradical-like silicon dangling bonds. Vibrational studies show that cyanoacetylene mainly binds to the surface through a diradical reaction involving both cyano and C&3bond; C groups with an adjacent adatom-rest atom pair at 110 K, resulting in an intermediate containing triple cumulative double bonds (C=C=C=N). On the other hand, diacetylene was shown to the covalently attached to Si(111)-7 x 7 only through one of its C&3bond; C groups, forming an enynic-like structure with a C=C-C&3bond; C skeleton. These chemisorbed species containing triple cumulative double bonds (C=C=C=N) and C=C-C&3bond; C may be employed as precursors (or templates) for further construction of bilayer organic films on the semiconductor surfaces.