Theoretical studies have been carried out at different levels of theory to verify the hydrogen adsorption characteristics of pyridine-lithium ion (1:1) complexes. The nature of interactions associated with the bonding between pyridine and lithium as well as that between lithium and adsorbed molecular hydrogen is studied through the calculation of electron density and electron-density-based reactivity descriptors. The pyridine-lithium ion complex has been hydrogenated systematically around the lithium site, and each lithium site is found to adsorb a maximum of four hydrogen molecules with an interaction energy of ∼-4.0 kcal/mol per molecule of H2. The fate of the hydrogen adsorbed in a pyridine-lithium ion complex (corresponding to the maximum adsorption) is studied in the course of a 2 ps time evolution through ab initio molecular dynamics simulation at different temperatures. The results reveal that the complex can hold a maximum of four hydrogen molecules at a temperature of 77 K, whereas it can hold only two molecules of hydrogen at 298 K.

The need for efficient macrocyclic ligands that can sequester U(VI) has gained immense importance due to the increased applications of U(VI) in various sectors, including but not limited to nuclear energy. Structural attributes such as number and type of donor centers ("hard" and "soft") of ligands are essentially the key components for providing the adequate bonding scenario for uranyl. Beside hard or soft-donor-based binding cavity, the mixeddonor ligands are also finding popularity for achieving optimized performances. However, many aspects are still unknown about how and at what extent the ratio of hard-to-soft donor centers tune the bonding attributes with uranyl. Moreover, a consensus is yet to be reached on the nature and role of underlying covalent interaction between U and donors upon complexation, particularly in the mixed-donor ligand environment. In this work, using relativistic density functional theory (DFT), we attempted to address these important issues by systematically investigating the impact on the bonding characteristics of uranyl ion and an expanded porphyrin, viz. sapphyrin with increasing number of 'O' substitution at the cavity. Our results suggest that in the O-substituted sapphyrin variants, UO 2+ 2 prefers to bind N over O donor sites, and decrease in N donor sites at the cavity prompts UO 2+ 2 to have better interaction with the rest of N donor centers. Extended transition state (ETS)with natural orbital for chemical valences (NOCV) analysis shows that at equatorial plane N acts as better σ donor to uranyl than O donor. Molecular orbital (MO) and density of states (DOS) analysis shows favorable bonding-interaction between U(d) and donor's p orbitals, the participation of U(f)-orbitals in bonding are of low-extent but non-negligible. Energy decomposition analysis (EDA), natural population analysis (NPA) along with thermodynamic analyses confirms the dominance of electrostatic interaction on the thermodynamic stability of the complexes. However, the U-N/O bonds at the equatorial plane do carry appreciable amount of covalent character. Analysis of quantum theory of atoms in molecules (QTAIM) descriptors in conjugation with MO analysis and overlap integral calculations confirms its nature as near-degeneracy driven type. Statistics of mixed-orbitals and overlap integral fur-

Chemistry of f-element gains importance in several elds due to the extensive scope of their applications.The aim of this study is to understand the subtle differences in bonding of theexceptionally similar actinides and their homologous (isovalent) lanthanides with several donors, which may lead to their covalency mediated separation. Several experimental and theoretical studies have been reported to address this aspect. However, to the best of our knowledge, systematic study on the variations in the bonding patterns of the isovalent 'Ln' and 'An' pairs encompassing the effect of valence f-orbitals participation was not encountered. In this study, the minute differences in covalent interactions of these isovalent f-element pairs with chalcogenide ions of varying softness via the metal (n-2)f orbital participation was probed using relativistic density functional theory (DFT). The f-electronic con gurations of the metal ions were observed to play an important role in the f-orbital participation. f 0 , f 7 (half lled)[AnX] + / [LnX] + pairs expresses resistance to f-orbital directed bonding, unlike the f 3 , f 5 , f 6 pairs. QTAIM study indicated that the f-orbital directed covalent bonding is pronounced for the [AnX] + systems of the latter set than the corresponding [LnX] + s, and it is near degeneracy driven. The extent of near degeneracy driven covalency was found to increase with the subsequent increase in the softness of the donor centres i.e. oxides through sulphides to selenides, although energetic stabilisation was not rendered. These results are anticipated to provide deeper understanding on exclusive differences in bonding of the homologous An 3+ /Ln 3+ ions with the chalcogenide donors.

Chemistry of f-element gains importance in several fields due to the extensive scope of their applications. The aim of this study is to understand the subtle differences in bonding of theexceptionally similar actinides and their homologous (isovalent) lanthanides with several donors, which may lead to their covalency mediated separation. Several experimental and theoretical studies have been reported to address this aspect. However, to the best of our knowledge, systematic study on the variations in the bonding patterns of the isovalent ‘Ln’ and ‘An’ pairs encompassing the effect of valence f-orbitals participation was not encountered. In this study, the minute differences in covalent interactions of these isovalent f-element pairs with chalcogenide ions of varying softness via the metal (n-2)f orbital participation was probed using relativistic density functional theory (DFT). The f-electronic configurations of the metal ions were observed to play an important role in the f-orbital participation. f0, f7 (half filled) [AnX]+/ [LnX]+ pairs expresses resistance to f-orbital directed bonding, unlike the f3, f5, f6 pairs. QTAIM study indicated that the f-orbital directed covalent bonding is pronounced for the [AnX]+ systems of the latter set than the corresponding [LnX]+s, and it is near degeneracy driven. The extent of near degeneracy driven covalency was found to increase with the subsequent increase in the softness of the donor centres i.e. oxides through sulphides to selenides, although energetic stabilisation was not rendered. These results are anticipated to provide deeper understanding on exclusive differences in bonding of the homologous An3+/Ln3+ ions with the chalcogenide donors.