Which factors determine the geometry of metal complexes?

Metal ions in solution are surrounded by an array of ions or molecules called ligands to form coordination complexes. Those of main group metal ions such as Na(I), Hg(II) or Al(III) (and Cu(I)), which have no lone-pair electrons, have the high symmetry shapes that minimise the ligand-ligand repulsions and at the same time, in the covalent picture, the repulsions between the bonding pairs of electrons. Complexes of main group ions such as Sn(II), which do not have lone-pair electrons in the valence shell, typically assume geometries consistent with valence shell electron pair repulsion (VSEPR) theory.