New insights in the bonding regime and ligand field in Wernerian complexes. A density functional study

Abstract

We advance challenging new observations about the bonding regime and ligand field (LF) effects in the Wernerian complexes, with the help of several computer experiments realized exploiting the specific leverages of the ADF (Amsterdam density functional) code. Assembling the molecular systems from preliminarily prepared metal ion and individual ligand fragments and analyzing the components of the total bonding energy (Pauli repulsion, electrostatic interactions and orbital part), we noticed interesting correlations between the orbital part and the simple formulas of the qualitatively defined ligand field stabilization energy (LFSE). The facts are beyond the obvious or predictable comprehension, since the orbital components incorporate the subtle balance of intra- and inter-fragment density rearrangements, charge transfer and orbital mixing. The energy decomposition analysis was performed considering metal ions and ligand sets in their nominal oxidation states, or with fractional charges, resulted after a preliminary step of electronegativity equalization driven charge transfer. We worked on series of prototypic simple octahedral units: [MqF6]q−6 , [Mq(CN)6]q−6 and [Mq(H2O)6]q complexes with MII and MIII ions selected from the M=Cr to Cu 3d series. Besides, as illustration of observed regularities in the case of more complex ligands and intermediate symmetries, we considered [Mq(bpca)2]q−2 complexes (Hbpca=bis(2-pyridilcarbonyl)amine) with D2d symmetry. Using the ADF facilities of orbital population control and fractional occupation schemes, we devised two simple ways to estimate 10Dq parameters by numerical experiments that simulate the effects of Ligand Field stabilization and electron promotion. To the best of our knowledge, though simple, such considerations on bonding regime or methodological procedures designed to meet the phenomenological demands of LF models have not been discussed before. Interesting new aspects can yet be discovered exploring the subtle details of coordination bonding as intermediate between ionic and covalent bonding regime.