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.

Iron and manganese ions with terminal oxo and hydroxo ligands are discovered as key intermediates in several synthetic and biochemical catalytic cycles. Since many of these species possess vigorous catalytic abilities, they are extremely transient in nature and experiments which probe the structure and bonding on such elusive species are still rare. We present here comprehensive computational studies on eight iron and manganese oxo and hydroxo (Fe^III/IV/V-O, Fe^III-OH and Mn^III/IV/V-O, Mn^III-OH) species using dispersion corrected (B3LYP-D2) density functional method. By computing all the possible spin states for these eight species, we set out to determine the ground state S value of these species; and later on employing MO analysis, we have analysed the bonding aspects which contribute to the high reactivity of these species. Direct structural comparison to iron and manganese-oxo species are made and the observed similarity and differences among them are attributed to the intricate metal–oxygen bonding. By thoroughly probing the bonding in all these species, their reactivity towards common chemical reactions such as C–H activation and oxygen atom transfer are discussed.

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.

Actinide molecular magnets are of great interest in the area molecular magnetism as they possess strong covalency and spin-orbit coupling that their 4f congeners lack. Despite these known advantages, the actinide based molecular magnets have not been explored in detail due to the limited availability of actinidesalts. While theoretical tools are proven to be useful in lanthanide chemistry towards prediction, they are still at an infant stage in actinide chemistry. In this manuscript, we have attempted to utilise CASSCF/RASSI-SO calculations to predict suitable pseudo D_5h molecules possessing attractive magnetic properties. To begin with, we have undertaken an extensive benchmarking of the methodology employed by studying two sets of isostructural, [NdTp₃], [UTp₃], [Nd(COT^²)₂]^-, [U(COT^²)₂]^- {Tp^- = trispyrazolylborate, COT^² = bis(trimethylsilyl)cyclooctatetraenyl dianion} complexes. The method assessment reveals that the methodology employed is reliable as this has reproduced the experimental observables. With this, we have moved forward with prediction where pseudo-D_5h symmetric [L₂M(H₂O)₅][I]₃L₂.(H₂O) {M = Nd, U, Pu; L =^tBuPO(NHⁱPr)₂} systems are modelled from their Nd(III) X-ray structure. Our calculations reveal that the Uranium complex studied possess superior SIM characteristics compared to its lanthanide analogue. Plutonium complex has prolate electron density at the ground state, and hence the ligand environment isunsuitable for yielding SIM behaviour. The role of solvent molecules, counter anions and equatorial and axial ligand are explored and tantalizing prediction with a barrier height of 1403.3 and 989.5 cm^-1 are obtained for [^tBuPO(NHⁱPr)₂-U-^tBuPO(NHⁱPr)₂]³⁺ and [Pu(H₂O)₅]³⁺ models, respectively and this paves the way for synthetic chemist to target such geometries in actinide based SIMs.