Dummy ("Xx") atoms are treated a little different when used in structures. A dummy atom will not replace an existing atom when it is defined as the 'replacing atom'. Instead, the double-clicked atom will remain and will accept the bonds that the dummy atom had in the structure.
Thus, the dummy atom in the structure will be replaced instead of the atom that is double-clicked.
This behavior is utilized in the multidentate ligands, which can be added to a bare metal center. The dummy atom disappears and the ligand is bonded to the metal via the bonds that were previously located on the dummy atom.
