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. It 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.

PSB acknowledges support by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences (CSGB) Division through its Geosciences program at Pacific Northwest National Laboratory (PNNL). TV and BS acknowledge funding from the ERC Consolidator Grant 2020 under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 101003292).

1. P. A. Cox, Transition Metal Oxides: An Introduction to their Electronic Structure and Properties (Clarendon Press, Oxford, 1992).

2. J. S. Griffith, The Theory of Transition-Metal Ions (Cambridge Press, Cambridge, 1971).

3. R. S. Mulliken, J. Chem. Phys., 1955, 23, 2343-2346.

4. J. Hernandez-Trujillo and R. F. W. Bader, J. Phys. Chem. A, 2000, 104, 1779-1794.

5. A. E. Reed, R. B. Weinstock, and F. Weinhold, J. Chem. Phys., 1985, 83, 735-746.

6. P. S. Bagus, B. Schacherl, and T. Vitova, Inorg. Chem., 2021, 60, 16090.