Palladium(II) chloride

Palladium(II) chloride
Identifiers
CAS number	7647-10-1 Y
EC number	231-596-2
RTECS number	RT3500000
Properties
Molecular formula	PdCl2
Molar mass	177.33 g/mol
Appearance	dark red solid hygroscopic
Density	4.0 g/cm3
Melting point	679 °C (decomp.)
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
EU Index	Not listed
Flash point	Non-flammable
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
Y chloride (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Palladium(II) chloride, also known as palladium dichloride, 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 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]

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

References

1. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. ^ Gordon K. Anderson, Minren Lin (1990). "Bis(Benzonitrile)Dichloro Complexes of Palladium and Platinum". Inorg. Synth. 28: 60–63. doi:10.1002/9780470132593.ch13. 
3. ^ Norio Miyaura and Akira Suzuki (1993), "Palladium-catalyzed reaction of 1-alkenylboronates with vinylic halides: (1Z,3E)-1-Phenyl-1,3-octadiene", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv8p0532 ; Coll. Vol. 8: 532 
4. ^ D. R. Coulson; Satek, L. C.; Grim, S. O. (1972). "23. Tetrakis(triphenylphosphine)palladium(0)". Inorg. Synth. 13: 121. doi:10.1002/9780470132449.ch23. 
5. ^ ^{a b} Daniele Choueiry and Ei-ichi Negishi (2002). "II.2.3 Pd(0) and Pd(II) Complexes Containing Phosphorus and Other Group 15 Atom Ligands". In Ei-ichi Negishi (Google Books excerpt). Handbook of Organopalladium Chemistry for Organic Synthesis. John Wiley & Sons, Inc.. ISBN 0-471-31506-0. http://books.google.com/?id=mTMA2hExAaIC&pg=PA47. 
6. ^ Ralph Mozingo (1955), "Palladium Catalysts", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv3p0685 ; Coll. Vol. 3: 685 
7. ^ For example, http://www.marinecare.nl/assets/Uploads/Downloads/Leaflet-Passivation-Test-Kit.pdf
8. ^ T. H. Allen, W. S. Root (1955). "Colorimetric Determination of Carbon Monoxide in Air by an improved Palladium Chloride Method". J. Biol. Chem. 216 (1): 309–317. PMID 13252030. http://www.jbc.org/content/216/1/309.