Cycloaddition reactions have been utilized widely in organic chemistry and are now emerging as a method for attachment of organic molecules to silicon(001) surfaces in a controlled way. Scanning tunneling microscopy (STM), Fourier transform infrared spectroscopy (FTIR), and ab initio molecular orbital calculations have been used to investigate the structure and bonding of 1,3,5,7-cyclooctatetraene (COT), an antiaromatic molecule, on the Si(001)-2 x 1 surface. STM images show that the COT molecules spontaneously order on Si(001) at room temperature. The individual molecules are separated by 7.7 Angstrom, equal to twice the separation between Si=Si dimers on the underlying reconstructed Si(001)-2 x 1 surface. FTIR measurements in conjunction with STM experiments show that the COT molecules bond to Si(001) via the direct interaction of two C=C units with two adjacent Si=Si dimers within a single dimer row. Ab initio molecular orbital calculations show that the lowest-energy geometry bonding of two C=C groups with two Si=Si dimers is a di-sigma configuration via a double [2 + 2] cycloaddition reaction. The result of this bonding is a monolayer organic film that terminates in an ordered array of alkene groups exposed to the vacuum.