Metal halides

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Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, such as palladium chloride. [1] [2]

Contents

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.

Sample of silver chloride AgCl-neerslag.jpg
Sample of silver chloride

In practice, this type of reaction may be very exothermic, hence impractical as a preparative technique. Additionally, many transition metals 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 can be prepared thus, but ferrous chloride cannot. Heating the higher halides may produce the lower halides; this occurs by thermal decomposition or by disproportionation. For example, gold(III) chloride to gold(I) chloride: [1]

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: [1]

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: [1] [3]

MCln·xH2O + x SOCl2 → MCln + x SO2 + 2x 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):

TiO2 + 2Cl2 + C → TiCl4(l) + CO2(g)

Structure and reactivity

Antimony pentafluoride is the prototypical Lewis acid for the Gutmann scale Antimony-pentafluoride-2D.png
Antimony pentafluoride is the prototypical Lewis acid for the Gutmann scale

"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. [1]

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. [1]

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: [4]

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: [5]

[PdCl2]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: [6] [7]

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. [8]

Halide ligands

Complexcolourelectron config.geometry
[TiCl4]colourless(t2g)0tetrahedral
[Ti2Cl10]2−colourless(t2g)3bioctahedral
[TiCl6]2−yellow(t2g)0octahedral
[CrCl6]3− ??(t2g)3octahedal
[MnCl4]2−pale pink(eg)2(t2g)3tetrahedral
[FeCl4]2−colourless(eg)3(t2g)3tetrahedral
[CoCl4]2−blue(eg)4(t2g)3tetrahedral
[NiCl4]2−blue(eg)4(t2g)4tetrahedral
[CuCl4]2−green(eg)4(t2g)5tetrahedral
[PdCl4]2−brownd8square planar
[PtCl4]2−pinkd8square planar
Aluminium trichloride dimer Aluminium-trichloride-dimer-3D-balls.png
Aluminium trichloride dimer

Halides are X-type ligands in coordination chemistry. 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 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 ligands. 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 [CrCl6]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, whereas the tetrahalides are usually tetrahedral. Square planar tetrahalides are known as are examples with 2- and 3-coordination.

Alfred Werner studied hexamminecobalt(III) chloride, and was the first to propose the correct structures of coordination complexes. Cisplatin, cis-Pt(NH3)2Cl2, is a platinum drug bearing two chloride ligands. The two chloride ligands are easily displaced, allowing the platinum center to bind to two guanine units, thus damaging DNA.

Due to the presence of filled pπ orbitals, halide ligands on transition metals are able to reinforce π-backbonding onto a π-acid. They are also known to labilize cis-ligands. [9]

Applications

The volatility of the tetrachloride and tetraiodide complexes of Ti(IV) is exploited in the purification of titanium by the Kroll and van Arkel–de Boer processes, respectively.

Metal halides act as Lewis acids. Ferric and aluminium chlorides are catalysts for the Friedel-Crafts reaction, but due to their low cost, they are often added in stoichiometric quantities.

Chloroplatinic acid (H2PtCl6) is an important catalyst for hydrosilylation.

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 or the hexafluorophosphate. In many transition metal compounds, the empty coordination site is stabilized by a coordinating solvent like tetrahydrofuran. Halide ligands may also be displaced by the alkali salt of an X-type ligand, such as a salen-type ligand. [10] 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.

For example, sodium cyclopentadienide reacts with ferrous chloride to yield ferrocene: [11]

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 may be often be used in lieu of bis(triphenylphosphine)palladium(II) chloride for palladium-catalyzed coupling reactions.

Lamps

Some halides are used in metal-halide lamps.

See also

Related Research Articles

Iron(III) chloride describes the inorganic compounds with the formula FeCl3(H2O)x. Also called ferric chloride, these compounds are some of the most important and commonplace compounds of iron. They are available both in anhydrous and in hydrated forms which are both hygroscopic. They feature iron in its +3 oxidation state. The anhydrous derivative is a Lewis acid, while all forms are mild oxidizing agents. It is used as a water cleaner and as an etchant for metals.

Benzonitrile is the chemical compound with the formula C6H5(CN), 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.

The Stille reaction is a chemical reaction widely used in organic synthesis. The reaction involves the coupling of two organic groups, one of which is carried as an organotin compound (also known as organostannanes). A variety of organic electrophiles provide the other coupling partner. The Stille reaction is one of many palladium-catalyzed coupling reactions.

<span class="mw-page-title-main">Manganese(II) chloride</span> Chemical compound

Manganese(II) chloride is the dichloride salt of manganese, MnCl2. This inorganic chemical exists in the anhydrous form, as well as the dihydrate (MnCl2·2H2O) and tetrahydrate (MnCl2·4H2O), with the tetrahydrate being the most common form. Like many Mn(II) species, these salts are pink, with the paleness of the color being characteristic of transition metal complexes with high spin d5 configurations.

<span class="mw-page-title-main">Copper(II) chloride</span> Chemical compound

Copper(II) chloride, also known as cupric chloride, is an inorganic compound with the chemical formula CuCl2. The monoclinic yellowish-brown anhydrous form slowly absorbs moisture to form the orthorhombic blue-green dihydrate CuCl2·2H2O, with two water molecules of hydration. It is industrially produced for use as a co-catalyst in the Wacker process.

<span class="mw-page-title-main">Chromium(III) chloride</span> Chemical compound

Chromium(III) chloride (also called chromic chloride) is an inorganic chemical compound with the chemical formula CrCl3. It forms several hydrates with the formula CrCl3·nH2O, among which are hydrates where n can be 5 (chromium(III) chloride pentahydrate CrCl3·5H2O) or 6 (chromium(III) chloride hexahydrate CrCl3·6H2O). The anhydrous compound with the formula CrCl3 are violet crystals, while the most common form of the chromium(III) chloride are the dark green crystals of hexahydrate, CrCl3·6H2O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.

<span class="mw-page-title-main">Nickel(II) chloride</span> Chemical compound

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 chemical synthesis. The nickel chlorides are deliquescent, absorbing moisture from the air to form a solution. Nickel salts have been shown to be carcinogenic to the lungs and nasal passages in cases of long-term inhalation exposure.

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 the greenish tetrahydrate, which is the form that is most commonly encountered in commerce and the laboratory. There is also a dihydrate. The compound is highly soluble in water, giving pale green solutions.

<span class="mw-page-title-main">Rhodium(III) chloride</span> Chemical compound

Rhodium(III) chloride refers to inorganic compounds with the formula RhCl3(H2O)n, where n varies from 0 to 3. These are diamagnetic red-brown solids. The soluble trihydrated (n = 3) salt is the usual compound of commerce. It is widely used to prepare compounds used in homogeneous catalysis.

<span class="mw-page-title-main">Tin(II) chloride</span> Chemical compound

Tin(II) chloride, also known as stannous chloride, is a white crystalline solid with the formula SnCl2. It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl2 is widely used as a reducing agent (in acid solution), and in electrolytic baths for tin-plating. Tin(II) chloride should not be confused with the other chloride of tin; tin(IV) chloride or stannic chloride (SnCl4).

<span class="mw-page-title-main">Palladium(II) chloride</span> Chemical compound

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 in organic synthesis. It is prepared by the reaction of chlorine with palladium metal at high temperatures.

<span class="mw-page-title-main">Hafnium tetrachloride</span> Chemical compound

Hafnium(IV) chloride is the inorganic compound with the formula HfCl4. This colourless solid is the precursor to most hafnium organometallic compounds. It has a variety of highly specialized applications, mainly in materials science and as a catalyst.

<span class="mw-page-title-main">Palladium(II) oxide</span> Chemical compound

Palladium(II) oxide is the inorganic compound of formula PdO. It is the only well characterised oxide of palladium. It is prepared by treating the metal with oxygen. Above about 900 °C, the oxide reverts to palladium metal and oxygen gas. It is not attacked by acids.

<span class="mw-page-title-main">Organozinc chemistry</span>

Organozinc chemistry is the study of the physical properties, synthesis, and reactions of organozinc compounds, which are organometallic compounds that contain carbon (C) to zinc (Zn) chemical bonds.

<span class="mw-page-title-main">Sodium tetraphenylborate</span> Chemical compound

Sodium tetraphenylborate is the organic compound with the formula NaB(C6H5)4. It is a salt, wherein the anion consists of four phenyl rings bonded to boron. This white crystalline solid is used to prepare other tetraphenylborate salts, which are often highly soluble in organic solvents. The compound is used in inorganic and organometallic chemistry as a precipitating agent for potassium, ammonium, rubidium, and caesium ions, and some organic nitrogen compounds.

<span class="mw-page-title-main">Bis(triphenylphosphine)palladium chloride</span> Chemical compound

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 coupling reactions, e.g. the Sonogashira–Hagihara reaction. The complex is square planar. Many analogous complexes are known with different phosphine ligands.

<span class="mw-page-title-main">Sodium tetrachloropalladate</span> Chemical compound

Sodium tetrachloropalladate is an inorganic compound with the chemical formula Na2PdCl4. This salt, and the analogous alkali metal salts of the form M2PdCl4, may be prepared simply by reacting palladium(II) chloride with the appropriate alkali metal chloride in aqueous solution. Palladium(II) chloride is insoluble in water, whereas the product dissolves:

<span class="mw-page-title-main">Metal amides</span>

Metal amides (systematic name metal azanides) are a class of coordination compounds composed of a metal center with amide ligands of the form NR2−. Amido complexes of the parent amido ligand NH2− are rare compared to complexes with diorganylamido ligand, such as dimethylamido. Amide ligands have two electron pairs available for bonding.

<span class="mw-page-title-main">Bis(benzonitrile)palladium dichloride</span> Chemical compound

Bis(benzonitrile)palladium dichloride is the coordination complex with the formula PdCl2(NCC6H5)2. It is the adduct of two benzonitrile (PhCN) ligands with palladium(II) chloride. It is a yellow-brown solid that is soluble in organic solvents. The compound is a reagent and a precatalyst for reactions that require soluble Pd(II). A closely related compound is bis(acetonitrile)palladium dichloride.

Palladium forms a variety of ionic, coordination, and organopalladium compounds, typically with oxidation state Pd0 or Pd2+. Palladium(III) compounds have also been reported. Palladium compounds are frequently used as catalysts in cross-coupling reactions such as the Sonogashira coupling and Suzuki reaction.

References

  1. 1 2 3 4 5 6 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 819–824. ISBN   978-0-08-037941-8.
  2. ↑ Köhler, J. (2014). "Halides: Solid-State Chemistry". Encyclopedia of Inorganic and Bioinorganic Chemistry. pp. 1–22. doi:10.1002/9781119951438.eibc0078.pub2. ISBN   9781119951438.
  3. ↑ Alfred R. Pray; Richard F. Heitmiller; Stanley Strycker (1990). "Anhydrous Metal Chlorides". Inorganic Syntheses. Vol. 28. pp. 321–323. doi:10.1002/9780470132593.ch80. ISBN   978-0-470-13259-3.
  4. ↑ Daniele Choueiry & Ei-ichi Negishi (2002). "II.2.3 Pd(0) and Pd(II) Complexes Containing Phosphorus and Other Group 15 Atom Ligands" (Google Books excerpt). In Ei-ichi Negishi (ed.). Handbook of Organopalladium Chemistry for Organic Synthesis. John Wiley & Sons, Inc. ISBN   0-471-31506-0.
  5. ↑ Gordon K. Anderson; Minren Lin (1990). "Bis(Benzonitrile)Dichloro Complexes of Palladium and Platinum". Inorganic Syntheses. Vol. 28. pp. 60–63. doi:10.1002/9780470132593.ch13. ISBN   9780470132593.{{cite book}}: |journal= ignored (help)
  6. ↑ Gill, N. S. & Taylor, F. B. (1967). "Tetrahalo Complexes of Dipositive Metals in the First Transition Series". Inorganic Syntheses. Inorganic Syntheses. Vol. 9. pp. 136–142. doi:10.1002/9780470132401.ch37. ISBN   9780470132401.
  7. ↑ G. D. Stucky; J. B. Folkers; T. J. Kistenmacher (1967). "The Crystal and Molecular Structure of Tetraethylammonium Tetrachloronickelate(II)". Acta Crystallographica . 23 (6): 1064–1070. doi:10.1107/S0365110X67004268.
  8. ↑ V. Gutmann (1976). "Solvent effects on the reactivities of organometallic compounds". Coord. Chem. Rev. 18 (2): 225–255. doi:10.1016/S0010-8545(00)82045-7.
  9. ↑ J. F. Hartwig (2009). "4: Covalent (X-Type) Ligands Bound Through Metal-Heteroatom Bonds". Organotransition Metal Chemistry. ISBN   978-1-891389-53-5.
  10. ↑ Cozzi, Pier Giorgio (2004). "Metal-Salen Schiff base complexes in catalysis: Practical aspects". Chem. Soc. Rev. 33 (7): 410–21. doi:10.1039/B307853C. PMID   15354222.
  11. ↑ Geoffrey Wilkinson (1963). "Ferrocene". Organic Syntheses ; Collected Volumes, vol. 4, p. 473.
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