Ligandâ ligand repulsion energies have been calculated for [M (bidentate)3(unidentate)]^x± complexes using a â normalised biteâ of 1·22, which is the appropriate value for bidentate β-diketonate ligands attached to a lanthanoid element. (â Normalised biteâ is defined as the distance between the two donor atoms of the same bidentate ligand divided by the metal atomâ donor atom distance.) Three minima of closely similar energies appear on the potential-energy surfaces. The first two correspond to a capped octahedron (C 3v symmetry) and an irregular polyhedron (C 1 symmetry), and are the structures observed for compounds whose structures have been determined, and where the unidentate ligand is water or a carbonyl compound. The third minima (C s symmetry) corresponds to a stereochemistry intermediate between a pentagonal bipyramid and a capped trigonal prism, and it is predicted that it will be formed only with unidentate ligands which form particularly stable ligand â lanthanoid bonds.