, 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.