Palladium(II) chloride

Palladium(II) chloride

Names
Other names Palladium dichloride, Palladous chloride
Identifiers
CAS Number	7647-10-1 N
EC Number	231-596-2
Jmol interactive 3D	Image
PubChem	24290
RTECS number	RT3500000
InChI InChI=1S/2ClH.Pd/h2*1H;/q;;+2/p-2 Key: PIBWKRNGBLPSSY-UHFFFAOYSA-L
SMILES Cl[Pd]Cl
Properties
Chemical formula	PdCl2
Molar mass	177.326 g/mol (anhydrous) 213.357 g/mol (dihydrate)
Appearance	dark red solid hygroscopic (anhydrous) dark brown crystals (dihydrate)
Density	4.0 g/cm3
Melting point	679 °C (1,254 °F; 952 K) (decomposes)
Solubility in water	soluble in trace amounts, better solubility in cold water
Solubility	soluble in organic solvents dissolves rapidly in HCl
Structure
Crystal structure	rhombohedral
Coordination geometry	square planar
Hazards
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (Median dose)	2704 mg/kg (rat, oral)
Related compounds
Other anions	Palladium(II) fluoride Palladium(II) bromide Palladium(II) iodide
Other cations	Nickel(II) chloride Platinum(II) chloride Platinum(II,IV) chloride Platinum(IV) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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 chlorination of palladium.

Structure

Two forms of PdCl₂ are known. In both forms, the palladium centres adopt the square-planar coordination geometry that is characteristic of Pd(II). Furthermore, in both forms, the Pd(II) centres are linked by μ₂-chloride bridges. The α-form of PdCl₂ is a polymer, consisting of "infinite" slabs or chains. The β-form of PdCl₂ is molecular, consisting of an octahedral cluster of six Pd atoms. Each of the twelve edges of this octahedron is spanned by Cl⁻. PtCl₂ adopts similar structures, whereas NiCl₂ adopts the CdCl₂ motif, featuring hexacoordinated Ni(II).^[1]

Evolution of β-PdCl₂ structure: Start with cubic lattice, remove corner and centered lattice points, inscribe octahedron (red lines), label corners as X (twelve Cl⁻ centers) and face-centered atoms as M (six Pd(II) centers).

ball-and-stick model of the crystal structure of α-PdCl2	thermal ellipsoid model of the Pd6Cl12 molecule found in the crystal structure of β-PdCl2

Preparation

Palladium(II) chloride is prepared by dissolving palladium metal in aqua regia or hydrochloric acid in the presence of chlorine. Alternatively, it may be prepared by heating palladium sponge metal with chlorine gas at 500 °C.

Reactions

Palladium(II) chloride is a common starting point in the synthesis of other palladium compounds. It is not particularly soluble in water or non-coordinating solvents, so the first step in its utilization is often the preparation of labile but soluble Lewis base adducts, such as those derived from acetonitrile or benzonitrile.^[2] The acetonitrile complex is prepared by treating PdCl₂ in refluxing acetonitrile:

PdCl₂ + 2 MeCN → PdCl₂(MeCN)₂

Although occasionally recommended, inert-gas techniques are not necessary if the complex is to be used in situ. As an example, bis(triphenylphosphine)palladium(II) dichloride may be prepared from palladium(II) chloride by reacting it with triphenylphosphine in benzonitrile:^[3]

PdCl₂ + 2 PPh₃ → PdCl₂(PPh₃)₂

Further reduction in the presence of more triphenylphosphine gives tetrakis(triphenylphosphine)palladium(0); the second reaction may be carried out without purifying the intermediate dichloride:^[4]

PdCl₂(PPh₃)₂ + 2 PPh₃ + 2.5 N₂H₄ → Pd(PPh₃)₄ + 0.5 N₂ + 2 N₂H₅⁺Cl⁻

Alternatively, palladium(II) chloride may be solubilized in the form of the tetrachloropalladate anion, e.g. sodium tetrachloropalladate, by reacting with the appropriate alkali metal chloride in water:^[5] Palladium(II) chloride is insoluble in water, whereas the product dissolves:

PdCl₂ + 2 MCl → M₂PdCl₄

This compound may also further react with phosphines to give phosphine complexes of palladium.^[5]

Palladium chloride may also be used to give heterogeneous palladium catalysts: palladium on barium sulfate, palladium on carbon, and palladium chloride on carbon.^[6]

A solution of [PdCl₄]²⁻(aq)

Uses

Even when dry, palladium(II) chloride is able to rapidly stain stainless steel. Thus, palladium(II) chloride solutions are sometimes used to test for the corrosion-resistance of stainless steel.^[7]

Palladium(II) chloride is sometimes used in carbon monoxide detectors. Carbon monoxide reduces palladium(II) chloride to palladium:

PdCl₂ + CO + H₂O → Pd + CO₂ + 2HCl

Residual PdCl₂ is converted to red PdI₂, the concentration of which may be determined colorimetrically:^[8]

PdCl₂ + 2 KI → PdI₂ + 2 KCl

Palladium(II) chloride can also be used for cosmetic tatooing of leukomas in the cornea.

References