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Palladium(II) chloride, also known as palladium dichloride and palladous chloride, are the
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
s with the
formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a '' chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betw ...
PdCl2. PdCl2 is a common starting material in
palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself ...
chemistry – palladium-based catalysts are of particular value in
organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
. It is prepared by the reaction of
chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine ...
with palladium metal at high temperatures.


Structure

Two forms of PdCl2 are known, denoted α and β. In both forms, the palladium centres adopt a square-planar coordination geometry that is characteristic of Pd(II). Furthermore, in both forms, the Pd(II) centers are linked by μ2-chloride bridges. The α-form of PdCl2 is a
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
, consisting of "infinite" slabs or chains. The β-form of PdCl2 is
molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
, 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). Two further polymorphs, γ-PdCl2 and δ-PdCl2, have been reported and show
negative thermal expansion Negative thermal expansion (NTE) is an unusual physicochemical process in which some materials contract upon heating, rather than expand as most other materials do. The most well-known material with NTE is water at 0~4 °C. Water's NTE is the ...
. The high-temperature δ form contains planar ribbons of edge-connected PdCl4 squares, like α-PdCl2. The low-temperature γ form has corrugated layers of corner-connected PdCl4 squares.


Preparation

Palladium(II) chloride is prepared by dissolving palladium metal in aqua regia or
hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the digesti ...
in the presence of
chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. 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
adduct An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all co ...
s, such as bis(benzonitrile)palladium dichloride and bis(acetonitrile)palladium dichloride. These complexes are prepared by treating PdCl2 with hot solutions of the nitriles: :PdCl2 + 2 RCN → PdCl2(RCN)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 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 ...
in benzonitrile: :PdCl2 + 2 PPh3 → PdCl2(PPh3)2 Further reduction in the presence of more triphenylphosphine gives
tetrakis(triphenylphosphine)palladium(0) Tetrakis(triphenylphosphine)palladium(0) (sometimes called quatrotriphenylphosphine palladium) is the chemical compound d(P(C6H5)3)4 often abbreviated Pd( PPh3)4, or rarely PdP4. It is a bright yellow crystalline solid that becomes brown upon de ...
; the second reaction may be carried out without purifying the intermediate dichloride: :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: 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. Palladium chloride may also be used to give heterogeneous palladium catalysts: palladium on
barium sulfate Barium sulfate (or sulphate) is the inorganic compound with the chemical formula Ba SO4. It is a white crystalline solid that is odorless and insoluble in water. It occurs as the mineral barite, which is the main commercial source of barium and ...
, palladium on carbon, and palladium chloride on carbon.


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 Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engin ...
-resistance of stainless steel. Palladium(II) chloride is sometimes used in
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simp ...
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: : PdCl2 + 2 KI → PdI2 + 2 KCl Palladium(II) chloride is used in the
Wacker process The Wacker process or the Hoechst-Wacker process (named after the chemical companies of the same name) refers to the oxidation of ethylene to acetaldehyde in the presence of palladium(II) chloride as the catalyst. This chemical reaction was one ...
for production of
aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s and
ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double bon ...
s from
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s. Palladium(II) chloride can also be used for the cosmetic tattooing of leukomas in the
cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical po ...
.


References

{{Chlorides Palladium compounds Chlorides Platinum group halides