The nature of the chemisorption bond between alkali metals, Li to Cs, on the Si(l 11) surface has been studied by means of the ab initio Hartree-Fock cluster-model approach. A comparative and systematic study has been carried out for a variety of cluster models simulating the high-symmetry sites of this sur­face. In all cases we found the bond highly ionic with a small participation of covalent effects to the in­teraction energy, which ranges from ,:::,,20% for chemisorbed Li to less than 10% for Rb and Cs above the different active sites. This result is consistent with several analyses of the interaction focused on the interaction energy, the final Hartree-Fock wave function, the analysis of the dipole moment, and of its variation with the adsorbate-surface distance. We show that the dipole moment for chemisorbed alkali metals is smaller than the one expected from an ionic bond beca use of the substrate polarization. Conse­quently we argue that changes in the measured work function are not adequate to extract information about the ionicity of a given interaction. This is in agreement with previous works considering a metal substrate. Here we show that the same mechanism holds for semiconductor surfaces as well.