Theoretical (HF+DFT) investigations of the adsorption of chlorobenzene (ClPh), 1,2- and 1,4-dichlorobenzene (1,2-diClPh and 1,4-diClPh) on a silicon (100) surface are reported for the first time, and are compared with one another and with benzene. Binding energies for various structures with the molecules attached on-top and in-between the surface dimer-rows are correlated with the STM experimental data. Novel structures with the molecules linking two dimer rows, stabilised by detachment of Cl (or H) atoms forming Cl-Si (or H-Si) bonds, are described. For 1,4 and 1,2 binding, these linking structures are predicted to attach the phenyl ring parallel or perpendicular to the Si surface, respectively, while preserving its aromaticity. The potential-energy barriers between several different structures are evaluated, and compared with available experimental evidence. For 1,4-diClPh it is shown that the potential energy barrier for the second Cl transfer is significantly lower than for the first one in contrast to the gas-phase, and comparable to the barrier for lifting the Bz-ring into a vertical position and forming a singly-bonded `displaced' structure. The predicted barrier-heights are consistent with the experimentally-observed relative occurrence of the on-top, linking, and displaced structures.