Left-click in empty space

This will paste the structure at the desired spot. In most cases, one of the atoms in the structure will be selected. The selected atom, defined as the origin of the structure, is the same atom that will replace the atom that is double-clicked. When a molecule is already present, a potential bond will appear so that you can connect the structure to the molecule. If no atom in selected, it means that the 'replacing atom' is not defined, as is the case with the metal complexes.

If you press spacebar, the last structure tool is selected again. You can then directly use it again.