Scanning tunneling microscopy (STM), Fourier-transform infrared spectroscopy and semiempirical quantum cluster calculations have been used to investigate the chemisorption of benzene on the Si(100)(2X1) surface. Room temperature adsorption results in the occupation of multiple bonding configurations including on-top of a single Si dimer and two different bridging geometries involving interaction with two Si dimers. While the single dimer state is populated preferentially upon adsorption, it is observed to be metastable with respect to the bridging configuration. The single dimer to bridge conversion is activated, with a barrier of 0.94 eV. The single dimer geometry can be repopulated via a process assisted by the STM tip.