All the heavy atoms that have so far been found to provide good derivative crystals do so by forming direct coordination bonds to either the backbone or the bases of nucleic acids in an environment where the coordination shell can be further stabilized by several hydrogen bonds. A summary of coordination sites is shown in Fig. 5 and listed below: Lanthanide ions such as Sm(III), Lu(III), Pr(III), Eu(III), Tb(III), Dy(III), Gd(III) form coordination bonds to oxygen atoms of two adjacent phosphates or to phosphates from different parts of the chain. The N-7 position of guanine is the most common site for heavy atoms. N-7 can become a ligand to many metal ions such as Os(VI) from OsO3 X (Py)2, Pt(II) from square-planar cis- or trans-dichlorodiammine complexes, Co(II), and Mn(II). The O-4 position of uracil can be a binding site for the Hg atom of hydroxymercuryhydroquinone-O-O-diacetate. The N-7 of adenine and the N-4 of cytosine in the base sequence A-C can be a binding site for an octahedral platinum(IV) from K2PtCl6.