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