Noncovalent Bonding in d and f-Type Coordination Compounds and Lattices. A Case Study.

Using as example the [Fe(bpca)(μ-bpca)Gd(NO₃)₄]×4CH₃NO₂×CH₃OH system, where Hbpca=bis(2-pyridilcarbonyl)amine), we perform the analysis of bonding components inside the d and f coordination units and between molecular entities from crystal. Aside the nominal long-range interactions between molecular components of the crystal, we considered that the bonding inside the coordination units is also not a covalent regime. We performed Density Functional Theory (DFT) calculations, with plane-waves (PW), in band-structure mode, and with atom-centred bases, by molecular procedures. Observing that the PW-DFT frame is free of basis set superposition errors, which is an important methodological hint underlined here, we estimated various non-covalent terms. E.g. the interaction between inversion-related FeGd units amounts -394.47 kcal/mol, only about -37 kcal/mol being due to Coulomb effects, the remainder being assignable to dispersion forces. One FeGd binuclear interacts with the closest methanol molecule by -9.30 kcal/mol and by -36.57 kcal/mol with the set of four nitromethane molecules. The energy decomposition analysis of molecular calculations showed, aside the expected ionic character (about 82 % of the total cohesion energy of lanthanide ion in molecule), the important role of empty 5d orbitals. The d virtuals are contributing with 68.4 kcal/mol, out of 97.15 kcal/mol quantity estimated as ligand-to-metal donor-acceptor effects.