Structures of monoadducts of tris(β-diketonato) lanthanoid shift reagents

Abstract

Ligand–ligand repulsion energies have been calculated for [M (bidentate)₃(unidentate)]^x± complexes using a ‘normalised bite’ of 1·22, which is the appropriate value for bidentate β-diketonate ligands attached to a lanthanoid element. (‘Normalised bite’ is defined as the distance between the two donor atoms of the same bidentate ligand divided by the metal atom–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 (C₁ 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 → lanthanoid bonds.