Chemical hardness and the adaptive coordination behavior of the d ⁰ transition metal oxide fluoride anions

Five new isostructural d⁰ transition metal oxide fluoride compounds Rb₃Na(NbOF₅)₂ H₂O and A₃Na(MO₂F₄)₂ H₂O (A = K, Rb and M = Mo, W) have been synthesized by hydrothermal methods and their original structures determined by single-crystal X-ray diffraction. In these compounds, the sodium ions preferentially engage in strong electrostatic interactions with the least polar-izable fluoride ions and concurrently the Nb⁵⁺, M⁶⁺ (M = Mo, W) metal centers displace toward the oxide ion(s) located trans to the fluorides in order to maintain atomic valences. Consequently, the oxide and fluoride sites retain unequal charge and the most negatively charged ions form the most/strongest interactions with the cations. The electrostatic potentials and the chemical hardness differences of the oxide and fluoride ions thus determine the anion connectivities: the cis-oxo MO₂F₄^2- anions (M = Mo, W) engage in the most/strongest electrostatic interactions with the alkali counter-cations through the two fluorides trans to the cis oxides. The NbOF₅ ^2- anion forms its most/strongest electrostatic interactions with the cations through the one fluoride position trans to the oxide.