The chemisorption of maleic anhydride on Si(100)-2x1 at room temperature has been studied with scanning tunneling microscopy (STM). We observe that, at low coverages below 0.12 monolayer, the unsaturated organic compound adsorbs almost exclusively on inter-dimer sites and not through a direct [2+2] cycloaddition. The STM images provide evidence that the adsorption of a second maleic anhydride molecule on an adjacent inter-dimer site results in the cleavage of the silicon dimer bond located between the two molecules. The chemisorption of the organic molecules induces a local rearrangement of unsaturated silicon dimers.