Evaluation of Covalent Bonding In Ionic Compounds

The extent of covalent bonding in ionic compounds like oxides and halides is of considerable importance for their chemical properties; see, for example Refs. [1-2]. It is often estimated from populations analyses either based on the original Mulliken formalism [3] or on more modern variants. [4-5] However, population analyses may have artifacts and be misleading. In contrast, we will be using a set of three criteria to estimate covalent character of orbitals in cluster models of compounds. [6] The methods are based on: (1) variation of orbital energies for different symmetry frontier orbitals; (2) estimates of the size of the orbitals as measured by an effective radius; and (3) projection of atomic orbitals. We examine the changes in covalency for two sets of compounds. The first set includes three nominally Ni(II) compounds: NiO, Ni(OH)2, and NiCO3. The second set are nominally oxidation state IV actinide dioxides UO2, NpO2, and PuO2. As well as the covalency for ground state WFs, we also consider the different covalent character of cations where a core electron has been ionized or excited. The wavefunctions are obtained as fully relativistic ab initio solutions of Dirac Hartree-Fock and many body configuration interaction solutions. ... mehrIt is shown that there are surprising departures from the nominal oxidation states and that the changes are not always consistent with intuitive views of the changes in the covalent character of the different compounds.
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6. P. S. Bagus, B. Schacherl, and T. Vitova, Inorg. Chem., 2021, 60, 16090.