We present results of the atomic and electronic structure of icosahedral (Formula presented), Ge and Sn) clusters using the ab initio molecular-dynamics method within the local density functional theory. Substitutional doping of a (Formula presented) cluster by a tetravalent atom leads to a substantial gain in energy in all the cases studied. Tin is found to have a lower energy at a vertex site in contrast to the central site for Si and Ge, leading to surface segregation of Sn in these clusters. Also in the case of a (Formula presented) cluster, Si occupies the central site of a capped icosahedral structure. These results when interpreted in terms of the interaction of closed shell clusters with atoms leads to a relatively strong interaction of (Formula presented) with (Formula presented) as compared to the weak interaction of rare gas atoms with other elements. {\textcopyright} 1998 The American Physical Society.