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Metal halides are compounds between
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
s and
halogen The halogens () are a group in the periodic table consisting of five or six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts). In the modern IUPAC nomenclature, this group is ...
s. Some, such as
sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...
are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as
uranium hexafluoride Uranium hexafluoride (), (sometimes called "hex") is an inorganic compound with the formula UF6. Uranium hexafluoride is a volatile white solid that reacts with water, releasing corrosive hydrofluoric acid. The compound reacts mildly with alumin ...
, but most adopt polymeric structures, such as
palladium chloride Palladium(II) chloride, also known as palladium dichloride and palladous chloride, are the chemical compounds with the formula PdCl2. PdCl2 is a common starting material in palladium chemistry – palladium-based catalysts are of particular value ...
. File:NaCl polyhedra.png, Sodium chloride crystal structure File:Uranium-hexafluoride-unit-cell-3D-balls.png, Discrete UF6 molecules File:Alpha-palladium(II)-chloride-xtal-3D-balls.png, Infinite chains of one form of palladium chloride


Preparation

The halogens can all react with metals to form metal halides according to the following equation: :2M + nX2 → 2MXn where M is the metal, X is the halogen, and MXn is the metal halide. In practice, this type of reaction may be very exothermic, hence impractical as a preparative technique. Additionally, many
transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
s can adopt multiple oxidation states, which complicates matters. As the halogens are strong oxidizers, direct combination of the elements usually leads to a highly oxidized metal halide. For example,
ferric chloride Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The col ...
can be prepared thus, but
ferrous chloride Iron(II) chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a paramagnetic solid with a high melting point. The compound is white, but typical samples are often off-white. FeCl2 crystallizes from water as ...
cannot. Heating the higher halides may produce the lower halides; this occurs by
thermal decomposition Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is req ...
or by
disproportionation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can b ...
. For example, gold(III) chloride to gold(I) chloride: :AuCl3 → AuCl + Cl2 at 160°C Metal halides are also prepared by the neutralization of a metal oxide, hydroxide, or carbonate with the appropriate halogen acid. For example, with
sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...
: :NaOH + HCl → NaCl + H2O Water can sometimes be removed by heat, vacuum, or the presence of anhydrous hydrohalic acid. Anhydrous metal chlorides suitable for preparing other coordination compounds may be dehydrated by treatment with
thionyl chloride Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year bein ...
: :MCl''n''·''x''H2O + ''x'' SOCl2 → MCl''n'' + x SO2 + 2''x'' HCl The silver and thallium(I) cations have a great affinity for halide anions in solution, and the metal halide quantitatively precipitates from aqueous solution. This reaction is so reliable that silver nitrate is used to test for the presence and quantity of halide anions. The reaction of silver cations with bromide anions: :Ag+ (aq) + Br (aq) → AgBr (s) Some metal halides may be prepared by reacting oxides with halogens in the presence of carbon (
carbothermal reduction Carbothermic reactions involve the reduction of substances, often metal oxides (O^2-), using carbon as the reducing agent. These chemical reactions are usually conducted at temperatures of several hundred degrees Celsius. Such processes are appl ...
): :


Structure and reactivity

"Ionic" metal halides (predominantly of the
alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
and
alkali earth metal The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are all ...
s) 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 Nickel(II) chloride (or just nickel chloride) is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. Nickel(II) chloride, in various forms, is the most important source of nickel for chemi ...
, and
cupric chloride Copper(II) chloride is the chemical compound with the chemical formula CuCl2. The anhydrous form is yellowish brown but slowly absorbs moisture to form a blue-green dihydrate. Both the anhydrous and the dihydrate forms occur naturally as the ver ...
. Metal cations with a high oxidation state tend to undergo hydrolysis instead, e.g.
ferric chloride Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The col ...
,
aluminium chloride Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms hexahydrate with the formula , containing six water molecules of hydration. Both are colourless crystals, but samples are often contam ...
, and
titanium tetrachloride Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds o ...
. 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 Acetonitrile, often abbreviated MeCN (methyl cyanide), is the chemical compound with the formula and structure . This colourless liquid is the simplest organic nitrile (hydrogen cyanide is a simpler nitrile, but the cyanide anion is not clas ...
and
benzonitrile Benzonitrile is the chemical compound with the formula , abbreviated PhCN. This aromatic organic compound is a colorless liquid with a sweet bitter almond odour. It is mainly used as a precursor to the resin benzoguanamine. Production It is p ...
: : dCl2sub>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 Antimony pentafluoride is the inorganic compound with the formula Sb F5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, formed when mixing liquid HF with liquid SbF5 in a 2:1 ratio. ...
is a strong Lewis acid. It gives
fluoroantimonic acid Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony pentafluoride, containing various cations and anions (the simplest being and ). This substance is a superacid that can be over a billion times stronger than 100% pure sulfuri ...
, the strongest known acid, with
hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...
. 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 In chemistry a donor number (DN) is a quantitative measure of Lewis basicity. A donor number is defined as the negative enthalpy value for the 1:1 adduct formation between a Lewis base and the standard Lewis acid SbCl5 (antimony pentachloride), i ...
.


Halide ligands

Halides are X-type
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
s in
coordination chemistry A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
. The halides are usually good σ- and good π-donors. These ligands are usually terminal, but they might act as bridging ligands as well. For example, the chloride ligands of
aluminium chloride Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms hexahydrate with the formula , containing six water molecules of hydration. Both are colourless crystals, but samples are often contam ...
bridge two aluminium centers, thus the compound with the empirical formula AlCl3 actually has the molecular formula of Al2Cl6 under ordinary conditions. Due to their π-basicity, the halide ligands are
weak field ligand Ligand field theory (LFT) describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valen ...
s. Due to a smaller crystal field splitting energy, the halide complexes of the first transition series are all high spin when possible. These complexes are low spin for the second and third row transition series. Only rCl6sup>3− is exchange inert. Homoleptic metal halide complexes are known with several stoichiometries, but the main ones are the hexahalometallates and the tetrahalometallates. The hexahalides adopt
octahedral coordination geometry In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The oc ...
, whereas the tetrahalides are usually tetrahedral. Square planar tetrahalides are known as are examples with 2- and 3-coordination.
Alfred Werner Alfred Werner (12 December 1866 – 15 November 1919) was a Swiss chemist who was a student at ETH Zurich and a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration of ...
studied
hexamminecobalt(III) chloride Hexaamminecobalt(III) chloride is the chemical compound with the formula o(NH3)6l3. It is the chloride salt of the coordination complex o(NH3)6sup>3+, which is considered an archetypal "Werner complex", named after the pioneer of coordination c ...
, and was the first to propose the correct structures of coordination complexes.
Cisplatin Cisplatin is a chemotherapy medication used to treat a number of cancers. These include testicular cancer, ovarian cancer, cervical cancer, breast cancer, bladder cancer, head and neck cancer, esophageal cancer, lung cancer, mesothelioma, br ...
, ''cis''-Pt(NH3)2Cl2, is a
platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Platinu ...
drug bearing two chloride ligands. The two chloride ligands are easily displaced, allowing the platinum center to bind to two
guanine Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is called ...
units, thus damaging DNA. Due to the presence of filled pπ orbitals, halide ligands on transition metals are able to reinforce
π-backbonding In chemistry, π backbonding, also called π backdonation, is when electrons move from an atomic orbital on one atom to an appropriate symmetry antibonding orbital on a ''π-acceptor ligand''. It is especially common in the organometallic chemi ...
onto a π-acid. They are also known to labilize ''cis''-ligands.


Applications

The volatility of the tetrachloride and tetraiodide complexes of Ti(IV) is exploited in the purification of titanium by the
Kroll Kroll is a German, Anglo-Saxon, and Scottish surname. Notable people with the surname include: * Colin Kroll (1983/4–2018), American businessman; co-founder of Vine and HQ Trivia * Eric Kroll (born 1946), American photographer * Fredric Kr ...
and van Arkel–de Boer processes, respectively. Metal halides act as Lewis acids.
Ferric In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+. The adjective ferric or the prefix ferri- is often used to sp ...
and
aluminium chloride Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms hexahydrate with the formula , containing six water molecules of hydration. Both are colourless crystals, but samples are often contam ...
s are catalysts for the Friedel-Crafts reaction, but due to their low cost, they are often added in stoichiometric quantities.
Chloroplatinic acid Chloroplatinic acid (also known as hexachloroplatinic acid) is an inorganic compound with the formula 3Osub>2 tCl6H2O)''x'' (0 ≤ ''x'' ≤ 6). A red solid, it is an important commercial source of platinum, usually as an aqueous solution. Alth ...
(H2PtCl6) is an important catalyst for
hydrosilylation Hydrosilylation, also called catalytic hydrosilation, describes the addition of Si-H bonds across unsaturated bonds."Hydrosilylation A Comprehensive Review on Recent Advances" B. Marciniec (ed.), Advances in Silicon Science, Springer Science, 2009 ...
.


Precursor to inorganic compounds

Metal halides are often readily available precursors for other inorganic compounds. Mentioned above, the halide compounds can be made anhydrous by heat, vacuum, or treatment with thionyl chloride. Halide ligands may be abstracted by silver(I), often as the
tetrafluoroborate Tetrafluoroborate is the anion . This tetrahedral species is isoelectronic with tetrafluoroberyllate (), tetrafluoromethane (CF4), and tetrafluoroammonium () and is valence isoelectronic with many stable and important species including the perchlo ...
or the
hexafluorophosphate Hexafluorophosphate is an anion with chemical formula of . It is an octahedral species that imparts no color to its salts. is isoelectronic with sulfur hexafluoride, , and the hexafluorosilicate dianion, , and hexafluoroantimonate . In this anio ...
. In many transition metal compounds, the empty coordination site is stabilized by a coordinating solvent like
tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...
. Halide ligands may also be displaced by the alkali salt of an X-type ligand, such as a
salen-type ligand A metal salen complex is a coordination compound between a metal cation and a ligand derived from ''N'',''N''′-bis(salicylidene)ethylenediamine, commonly called salen. The classical example is salcomine, the complex with divalent cobalt , usua ...
. This reaction is formally a transmetallation, and the abstraction of the halide is driven by the precipitation of the resultant alkali halide in an organic solvent. The alkali halides generally have very high
lattice energies In chemistry, the lattice energy is the energy change upon formation of one mole of a crystalline ionic compound from its constituent ions, which are assumed to initially be in the gaseous state. It is a measure of the cohesive forces that bin ...
. For example,
sodium cyclopentadienide Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp, where Cp− is the cyclopentadienide anion. Sodium cyclopentadienide is a colorless solid, although samples often are pi ...
reacts with
ferrous chloride Iron(II) chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a paramagnetic solid with a high melting point. The compound is white, but typical samples are often off-white. FeCl2 crystallizes from water as ...
to yield
ferrocene Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, a ...
: :2 NaC5H5 + FeCl2 → Fe(C5H5)2 + 2 NaCl While inorganic compounds used for catalysis may be prepared and isolated, they may at times be generated ''in situ'' by addition of the metal halide and the desired ligand. For example, palladium chloride and
triphenylphosphine Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists a ...
may be often be used in lieu of
bis(triphenylphosphine)palladium(II) chloride Bis(triphenylphosphine)palladium chloride is a coordination compound of palladium containing two triphenylphosphine and two chloride ligands. It is a yellow solid that is soluble in some organic solvents. It is used for palladium-catalyzed coupli ...
for
palladium-catalyzed coupling reactions In organic chemistry, a cross-coupling reaction is a reaction where two fragments are joined together with the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M (R = organic fragment, M = ...
.


See also

* Hard and soft acids and bases *
Alkali halides In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
* Silver halides


References

{{reflist Inorganic compounds