Metal halides

Source: Wikipedia, the free encyclopedia.
"Metal halide" redirects here. For the type of lighting, see
metal halide lamp
.

Metal halides are compounds between

palladium chloride.[1][2]

  • Sodium chloride crystal structure
    Sodium chloride crystal structure
  • Discrete UF6 molecules
    Discrete UF6 molecules
  • Infinite chains of one form of palladium chloride
    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.

Sample of silver chloride


In practice, this type of reaction may be very exothermic, hence impractical as a preparative technique. Additionally, many

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)
See also:
solubility constant

Structure and reactivity

See also:
Structural chemistry of the metal fluorides
Antimony pentafluoride is the prototypical Lewis acid for the Gutmann scale

"Ionic" metal halides (predominantly of the

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,

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)

Gutmann donor number.[8]

See also: § Precursor to inorganic compounds

Halide ligands

Main article: transition metal chloride complex
Complex colour electron config. geometry
[TiCl4] colourless (t2g)0 tetrahedral
[Ti2Cl10]2− colourless (t2g)3 bioctahedral
[TiCl6]2− yellow (t2g)0 octahedral
[CrCl6]3− ?? (t2g)3 octahedal
[MnCl4]2− pale pink (eg)2(t2g)3 tetrahedral
[FeCl4]2− colourless (eg)3(t2g)3 tetrahedral
[CoCl4]2− blue (eg)4(t2g)3 tetrahedral
[NiCl4]2− blue (eg)4(t2g)4 tetrahedral
[CuCl4]2− green (eg)4(t2g)5 tetrahedral
[PdCl4]2− brown d8 square planar
[PtCl4]2− pink d8 square planar
Aluminium trichloride dimer

Halides are X-type

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.

See also: crystal field theory, ligand field theory, and spectrochemical series

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.

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

"Metal halide" redirects here. For the type of lighting, see
metal halide lamp
.

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.

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

lattice energies
.

For example,

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

palladium-catalyzed coupling reactions
.

Lamps

Some halides are used in metal-halide lamps.

See also

  • Hard and soft acids and bases
  • Alkali halides
  • Silver halides

References

  1. ^
    ISBN 978-0-08-037941-8
    .
  2. ISBN 9781119951438
    .
  3. ISBN 978-0-470-13259-3
    .
  4. ISBN 0-471-31506-0
    .
  5. ISBN 9780470132593. {{cite book}}: |journal= ignored (help
    )
  6. ISBN 9780470132401
    .
  7. doi:10.1107/S0365110X67004268
    .
  8. doi:10.1016/S0010-8545(00)82045-7
    .
  9. ISBN 978-1-891389-53-5
    .
  10. PMID 15354222
    .
  11. ^ Geoffrey Wilkinson (1963). "Ferrocene". Organic Syntheses; Collected Volumes, vol. 4, p. 473.
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