Trivalent lanthanide ions play a special role in magnetism, thanks to their large magnetic moments and magnetic anisotropy. At the root of their enhanced magnetic properties are the rather weak interactions between the 4f electron shell and its coordination environment, mainly as a result of the 4f-shell radially compact nature, and its consequent efficient shielding from the surrounding electrostatic and chemical environment by the outer 5s 2 5p 6 orbital shells . The simplest and earliest approach to describe the splitting of a ground (2J + 1)-fold degenerate ground multiplet produced by the surrounding chemical environment is called School of Chemistry, The University of Melbourne, Australia. ⇤ E-mail: asoncini@unimelb.edu.au ‡ These authors contributed equally to this work. † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 00.0000/00000000. Crystal Field Theory (CFT) . CFT describes the splitting of the 4f many-electron wavefunctions associated to the (2J+1) ground spin-orbit multiplet of the pristine atomic 4f-shell as operated solely by the static electric fields induced by the charge distribution on the ligands. CFT entails a chemical bonding description which is purely ionic, and it is in fact referred to also as the ionic model.