A remarkable pattern of structural diversity has been observed for the conformation of octachloro-1,10-penanthroline (ocp) in its free form and in its d(10) metal complexes [M(ocp)(2)](+), M = Cu, Ag, and Cl2Hg(ocp). Ocp : C12Cl8N2, monoclinic, space group P2(1)/c, a = 5.6390(10) Angstrom, b = 24.970(3) Angstrom, c = 10.4660(10) Angstrom, beta = 101.33 degrees, Z = 4; [Ag(ocp)(2)](PF6). CH2Cl2, C25H2AgCl18F6N4P, monoclinic, space group P2/n, a = 13.185(2) Angstrom, b = 9.4740(10) Angstrom, c = 16.106(2) Angstrom, beta = 92.05 degrees, Z = 2; Cl2Hg(ocp). 2.5C(6)H(6) : C27H15Cl10HgN2, triclinic, space group , a = 10.391(2) Angstrom, b = 12.911(3) Angstrom, c = 13.647(3) Angstrom, alpha = 103.53(3)degrees, beta = 111.85(3)degrees, gamma = 103.69(3)degrees, Z = 2. While ocp is virtually planar in the previously characterized ion [Cu(ocp)(2)](+), the analogous silver(I) complex contains slightly twisted ocp ligands. In contrast, both the free, noncoordinated ligand and its dichloromercury(II) complex exhibit distincly saddle-shaped ocp. Cl2Hg(ocp) in particular contains a rather distorted chelate ligand. Ab initio calculations at the 6-31G** level show a flat energy hypersurface for ocp with an absolute minimum at a saddle-shaped conformation and a second energy minimum, 2.7 kcal/mol higher, for the coplanar arrangement. Nonbonded interactions between the chlorine atoms are responsible for the structural flexibility and limited coordination ability of ocp; electronically, ocp is a good acceptor ligand with two stabilized and close-lying unoccupied pi* orbitals but much reduced sigma donor capacity.