In contrast, the reports in which the f orbital itself interacts with the orbitals of other atoms (s-f, p-f, d-f interaction) have been reported for intermetallics, metal oxides, and metal salts, and superconducting behavior was demonstrated in the 1980s. Recently, Kuga et al. determined the exact model of hybridization among Yb 4f and B 2p on YbAlB4. However, recent studies have reassessed the covalency of Ln metal complexes. La Pierre et al. reported the Tb(IV) complex stabilized by metal-ligand covalency. On the other hand, Liddle et al. reported the heterometallic U-M complexes with an M-U bond (U: uranium, M: metal). Also, Hall et al. reported an unsupported Ln-Fe bond of PyCp2Ln-FeCp(CO)2 described as a dative bond. In addition, Totti et al. revealed the clear association between the covalent contribution of the coordinated water (which varied the quadrupolar potential) and the magnetic anisotropy of the Dy ion of [Dy(DOTA)(H2O)] -. Although the details of the interaction between f-orbitals and ligands for other complexes are still unclear, the effect of the heterometal bonds needs to be considered when discussing the magnetic properties of heterometallic bonded systems.