The reaction of monoenergetic I2Cl6 molecules with the Si(111)-7 x 7 surface is stereoselective for iodine and surface-site selective for center silicon adatoms. The adsorption starts by first forming a partial bond between an I atom of an I2Cl6 and a Si adatom on the surface so that iodine from I2Cl6 is selectively abstracted by the surface and chlorine is ejected back into the gas phase. The interaction between the asymmetric HOMO of an I2Cl6 and a partially filled Si adatom dangling bond governs the adsorption dynamics. The highly asymmetric HOMO orbital and zero dipole moment associated with I2Cl6 makes it a perfect molecular species for clarifying the role of asymmetric molecular bonding versus dipole moment in the iodine-selective abstraction of ICI by the Si(111)-7 x 7 surface.