Halides and halide complexes

Almost all elements create thermodynamically stable halides. The general stability sequence is F>Cl>Br>I, that in covalent compounds follows the supposed order of bond strengths, and in ionic compounds which of lattice energies. The thermodynamic stability of fluorides (and the kinetic reactivity of F2) is also aided through the weak F-F bond. Several halides can be made through direct combination, but fluorinating agents like ClF₃ are sometimes employed in preference to F₂, that is very difficult to handle.

The bonding and structural trends in halides follow identical patterns to those in oxides. Several nonmetallic elements create general molecular compounds where halogen atoms each have a single bond to another element. This is right also for metals in high oxidation states (example UF₆ and TiC_l4). The compounds might be solids, gases or liquids, with volatility in the order F>Cl>Br> I as expected from the strength of van der Waals' forces. HF is exceptional due to strong hydrogen bonding, in the hydrogen halides. HF is a weak acid in water, another HX compounds being strong acids.

Covalent halides are less frequently polymeric in structure than oxides, a variation partly caused through the dissimilar stoichiometries (example SiF₄ versus SiO₂), which give a higher coordination number in the monomeric molecular halides. Though, the halides of some metals (example beryllium;) may be better considered as polymeric than ionic. A few molecular halides of both metallic and nonmetallic elements create halogen-bridged dimers and higher oligomers (example Al₂Cl₆).

Several metallic elements create solid halides with structures supposed for ionic solids. Structural variations often take place with MX₂ and MX₃, fluorides more frequently having rutile, rhenium or fluorite trioxide structures and the heavier halides layer structures. These variations reflect the additional ionic nature of fluorides, and the higher polarizability of the larger halide ions. Several halides are extremely soluble in water, but low solubilities are frequently found with fluorides of M²⁺ and M³⁺ ions (example CaF₂, AlF₃), and with heavier halides of less electropositive metals (example AgCl, TlCl). These variations are linked to lattice energy trends.

Several halides of metals and nonmetals are good Lewis acids. Such type of compounds are frequently hydrolyzed through water, and also create halide complexes (example AlCl₄^2-, PF^6-), that can make helpful counterions in solids along with large or strongly oxidizing cations. Both cationic and anionic complexes might be formed through halide transfer, for instance, in solid PCl₅ and in liquid BrF₃. Several metal ions also create halide complexes in aqueous solution. For elements' majority the fluoride complexes are more stable but softer or class b metals form stronger complexes with heavier halides.