Metal Halides - Structure and Reactivity

Structure and Reactivity

"Ionic" metal halides (predominantly of the alkali and alkali earth metals) tend to have very high melting and boiling points. They freely dissolve in water, and some are deliquescent. They are generally poorly soluble in organic solvents.

Some low-oxidation state transition metals have halides which dissolve well in water, such as ferrous chloride, nickelous chloride, and cupric chloride. Metal cations with a high oxidation state tend to undergo hydrolysis instead, e.g. ferric chloride, aluminium chloride, and titanium tetrachloride.

Discrete metal halides have lower melting and boiling points. For example, titanium tetrachloride melts at −25 °C and boils at 135 °C, making it a liquid at room temperature. They are usually insoluble in water, but soluble in organic solvent.

Polymeric metal halides generally have melting and boiling points that are higher than monomeric metal halides, but lower than ionic metal halides. They are soluble only in the presence of a ligand which liberates discrete units. For example, palladium chloride is quite insoluble in water, but it dissolves well in concentrated sodium chloride solution:

PdCl₂ (s) + 2 Cl− (aq) → PdCl₄2− (aq)

Palladium chloride is insoluble in most organic solvents, but it forms soluble monomeric units with acetonitrile and benzonitrile:

_n + 2n CH₃CN → n PdCl₂(CH₃CN)₂

The tetrahedral tetrahalides of the first-row transition metals are prepared by addition of a quaternary ammonium chloride to the metal halide in a similar manner:

MCl₂ + 2 Et₄NCl → (Et₄N)₂MCl₄ (M = Mn, Fe, Co, Ni, Cu)

Antimony pentafluoride is a strong Lewis acid. It gives fluoroantimonic acid, the strongest known acid, with hydrogen fluoride. Antimony pentafluoride as the prototypical Lewis acid, used to compare different compounds' Lewis basicities. This measure of basicity is known as the Gutmann donor number.