Ab initio CASSCF+RASSI-SO investigations on a series of lanthanide complexes [Ln^III = Dy(1), Tb(2), Ce(3), Nd(4), Pr(5) and Sm(6)] have been undertaken and in selected cases (for 1, 2, 3 and 4) coordination number (C.N.) around the Ln^III ion has been gradually varied to ascertain the effect of C.N. on the magnetic anisotropy. Our calculations reveal that complex 3 possesses the highest barrier height for reorientation of magnetisation (U_eff) and predict that 3 is likely to exhibit Single Molecule Magnet (SMM) behaviour. Complex 5 on the other hand is predicted to preclude any SMM behaviour as there is no intrinsic barrier for reorientation of magnetization. Ground state anisotropy of all the complexes show mixed behaviour ranging from pure Ising type to fully rhombic behaviour. Coordination number around the lanthanide ion is found to alter the magnetic behaviour of all the lanthanide complexes studied and this is contrary to the general belief that the lanthanide ions are inert and exert small ligand field interaction.High symmetric low-coordinate Ln^III complexes are found to yield large U_eff values and thus should be the natural targets for achieving very large blocking temperatures.

Ab initio CASSCF+RASSI-SO+SINGLE_ANISO and DFT based NBO and QTAIM investigations were carried out on a series of trigonal prismatic M(Bc^Me)₃ (M = Tb(1), Dy(2), Ho(3), Er(4), [Bc^Me]⁻ = dihydrobis (methylimidazolyl) borate) and M(Bp^Me)₃ (M = Tb(1a), Dy(2a), Ho(3a), Er(4a) [Bp^Me]⁻ = dihydrobis (methypyrazolyl) borate) complexes to ascertain the anisotropic variations of these two ligand field environments and the influence of Lanthanide-ligand bonding on the magnetic anisotropy. Among all the complexes studied, only 1 and 2 show large Ucal (computed energy barrier for magnetization reorientation) values of 256.4 and 268.5 cm⁻¹, respectively and this is in accordance with experiment. Experimentally only frequency dependent χ” tails are observed for complex 1a and our calculation predicts a large Ucalof 229.4 cm⁻¹ for this molecule. Besides these, none of the complexes (3, 4, 2a, 3a and 4a) computed to possess large energy barrier and this is affirmed by the experiments. These observed differences in the magnetic properties are correlated to the Ln-Ligand bonding. Our calculations transpire comparatively improved Single-Ion Magnet (SIM) behaviour for carbene analogues due to the more axially compressed trigonal prismatic ligand environment. Furthermore, our detailed Mulliken charge, spin density, NBO and Wiberg bond analysis implied stronger Ln...H–BH agostic interaction for pyrazole analogues. Further, QTAIM analysis reveals the physical nature of coordination, covalent, and fine details of the agostic interactions in all the eight complexes studied. Quite interestingly, for the first time, using the Laplacian density, we are able to quantify the prolate and oblate nature of the electron clouds in lanthanides and this is expected to have a far reaching outcome beyond the examples studied.