On the origin of the differences in structure directing properties of polar metal oxyfluoride [MO_xF_6-x]^2- (x = 1, 2) building units

In oxyfluoride chemistry, the [MO_xF_6-x]^2- anions (M = transition metal) are interesting polar building units that may be used to design polar materials, but their polar vs antipolar orientations in the solid state, which directly depend on the interactions between O^2-/F^- ligands and the extended structure, remain difficult to control. To improve this control, these interactions were assessed through crystallization of five related [MO_xF_6-x]^2- (M = Ti⁴⁺, V⁵⁺, Mo⁶⁺, W⁶⁺) anions with organic molecules. The hybrid organic-inorganic compounds, (4,4′-bpyH₂)TiF₆ (1), (enH₂)MoO₂F₄ (2), (4-hpyH)₂MoO₂F₄·H₂O (3), (4,4′-bpyH₂)WO₂F₄ (4), and (4,4′-bpyH₂)VOF₅ (5), exhibit isolated [MO_xF_6-x]^2- anions in a hydrogen bond network. The analysis of these crystal structures in combination with DFT calculations elucidate how differences in structure directing properties of these anions arise when π-overlap between O 2p orbitals and M d orbitals is weak and significantly affected by an increase of the energy of the d orbitals from 3d to 5d.