Unraveling the Mysterious Square Planar Complexes

Unless forced by a locked ring system, symmetry considerations, steric factors, or valence shell electron pair repulsions, four mono-dentate ligands will naturally settle into the tetrahedral configuration thereby allowing for longer ligand-ligand distances compared to the square planar configuration. Driven by crystal field stabilization energies, there are, however, four-ligand complexes which prefer the square planar arrangement. We have conducted a systematic study of the conditions leading to such configurations and find it instructive to study the crystal field stabilization energy differences vs. d configurations. Not only can the structures of existing four-ligand complexes be understood, but those of as yet unknown species may be predicted as well.