Nature of the Metal-Ligand Bonding in Bis(Cyclopentadienyl) and Bis (Cyclobutadienyl) Lutetium(III) Metallocenes

Metal-ligand bonding in bis(cyclopentadienyl) and bis(cyclobutadienyl) lutetium(III) complexes was studied in a variety of geometries using density functional theory. Based on analysis of the orbitals, the Morokuma-Ziegler-Rauk extended transition state method and natural orbitals of the chemical valence, the bonding interaction is dominantly ionic with a very important covalent contribution. The relative contribution of covalency is so large that it should not be neglected even in qualitative treatments of the electronic structure. The covalency arises from electron donation from the ligand HOMOs and HOMO-1 orbitals to the vacant 5d, 6s and 6p orbitals at the Lu3+ ion. While 5d and the 6s orbitals play the most important part, the 6p orbitals also make a comparable contribution. A similar bonding picture emerges both in realistic low-symmetry metallocenes and idealized high-symmetry geometries. The results put to question the common view of the metal-ligand bonding in lanthanide complexes as completely or almost completely of ionic character, and have implications in the design of crystal-fields in lanthanide complexes when targeting specific magnetic properties.