Structural features

Metal difluorides, MF₂, where M is a Group2 or d-metal, generally adopt the CaF₂ or rutile structures and are described well by the ionic model. In contrast, whereas the Group 2 dichlorides, dibromides, and diiodides may be described by the ionic model, the d-metal analogues adopt the CdI₂ or CdCl₂ layer structures and their bonding is not described well by either the ionic or covalent models. Many metal trifluorides have three-dimensional ionic structures but the trichlorides, tribromides, and triodides have layered structures. The compounds NbF₃ and FeF₃ (at high temperature) adopt the ReO3 structure type (Section 3.6) and many other metal trifluorides (including AlF₃, ScF₃, and CoF₃) have a slightly distorted variant of this structure type.

As the oxidation number of the metal atom increases, the halides become more covalent. Thus, all metal hexahalides such as MoF6 and WCl6, are molecular covalent compounds. For inter mediate oxidation states (such as MF₄ and MF₅) the structures normally consist of linked MF6 polyhedra. Titanium tetrafluoride has a structure based on columns of triangular Ti₃ F₁₅ units formed from three TiF₆ octahedra (8) whereas NbF₅ is built from four NbF6 octahedra forming a square unit of composition Nb₄F₂₀ (9).

Although not as important in applications as complex ox ides, complex solid fluorides and chlorides such as the ternary phases MMF_n and MM’Cl_n , and the quaternary compounds MM’M’’F_n, have structures similar to their oxide counterparts. As F has an oxidation number of 1 compared to 2 for O₂ -, the compositionally equivalent fluorides or chlorides generally contain d metals in lower oxidation states than the equivalent oxide. Thus, ternary fluorides of stoichiometry ABF₃ with, for example, A=K, Rb, and Cs and M a dipositive d-metal ion, adopt the perovskite structure (Section 3.9). One example is KMnF₃, which precipitates when potassium fluoride is added to Mn (II) solutions. Molten cryolite, Na3 AlF₆, is used to dissolve aluminium oxide in electrochemical extraction of aluminium. Its structure is related to that of perovskite (ABO₃) with Na in the A sites and a mixture of Na and Al in the B sites: the formula Na (Al½ Na½) F₃, which is equivalent to Na₃AlF₆. Mixed-anion compounds containing halides are also well characterized and include the superconducting cuprate Sr_2-x Na_x CuO₂F₂.

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Work, heat, and energy

Mixtures of gases

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Imaging agents

Anti-arthritis drugs

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