Palladium(II) chloride

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.

Palladium(II) chloride

Names
Other names Palladium dichloride, Palladous chloride
Identifiers
CAS Number	7647-10-1 Y
3D model (JSmol)	monomer: Interactive image hexamer: Interactive image
ChemSpider	22710
ECHA InfoCard	100.028.724
EC Number	231-596-2
PubChem CID	24290
RTECS number	RT3500000
UNII	N9214IR8N7 N
CompTox Dashboard (EPA)	DTXSID4025824
InChI InChI=1S/2ClH.Pd/h2*1H;/q;;+2/p-2 Key: PIBWKRNGBLPSSY-UHFFFAOYSA-L
SMILES monomer: Cl[Pd]Cl hexamer: [Cl+]0[Pd-2]12[Cl+][Pd-2]34[Cl+][Pd-2]05[Cl+][Pd-2]6([Cl+]1)[Cl+][Pd-2]([Cl+]2)([Cl+]3)[Cl+][Pd-2]([Cl+]4)([Cl+]5)[Cl+]6
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
Magnetic susceptibility (χ)	−38.0×10−6 cm3/mol
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). N verify (what is YN ?) Infobox references

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, 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

Two further polymorphs, γ-PdCl₂ and δ-PdCl₂, have been reported and show negative thermal expansion. 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.^[2]

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.^[3][4][5][6]

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 bis(benzonitrile)palladium dichloride and bis(acetonitrile)palladium dichloride.^[7] 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 in benzonitrile:^[8]

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:^[9]

PdCl₂(PPh₃)₂ + 2 PPh₃ + ⁠5/2⁠ N₂H₄ → Pd(PPh₃)₄ + ⁠1/2⁠ N₂ + 2 N
₂H⁺
₅Cl⁻

Alternatively, palladium(II) chloride may be solubilized in the form of the tetrachloropalladate(II) anion, such as in sodium tetrachloropalladate, by reacting with the appropriate alkali metal chloride in water:^[10] 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.^[10]

Palladium chloride may also be used to give heterogeneous palladium catalysts: palladium on barium sulfate, palladium on carbon, and palladium chloride on carbon.^[11]

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.^[12]

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:^[13]

PdCl₂ + 2 KI → PdI₂ + 2 KCl

Palladium(II) chloride is used in the Wacker process for production of aldehydes and ketones from alkenes.

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

References

1. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
2. J. Evers, W. Beck, M. Göbel, S. Jakob, P. Mayer, G. Oehlinger, M. Rotter, T. M. Klapötke (2010). "The Structures of δ-PdCl₂ and γ-PdCl₂: Phases with Negative Thermal Expansion in One Direction". Angew. Chem. Int. Ed. 49 (33): 5677–5682. doi:10.1002/anie.201000680. PMID 20602377.{{cite journal}}: CS1 maint: multiple names: authors list (link)
3. Patnaik, Pradyot (2003). "Palladium". Handbook of inorganic chemicals (1 ed.). New York: McGraw-Hill. p. 687. ISBN 978-0-07-049439-8.
4. Patnaik, Pradyot (2003). "Palladium Dichloride". Handbook of inorganic chemicals (1 ed.). New York: McGraw-Hill. pp. 688–689. ISBN 978-0-07-049439-8.
5. Armarego, W. L. F. (2017). "4. Purification of Inorganic and Metal-Organic Chemicals". Purification of laboratory chemicals (Eighth ed.). Amsterdam: Elsevier. p. 687. ISBN 978-0-12-805457-4.
6. Kharasch, Morris S.; Seyler, Ralph C.; Mayo, Frank R. (1938). "Coördination Compounds of Palladous Chloride". J. Am. Chem. Soc. 60 (4). American Chemical Society: 882–884. doi:10.1021/ja01271a035.
7. Gordon K. Anderson, Minren Lin (2007). "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)
8. Norio Miyaura; Akira Suzuki (1993). "Palladium-catalyzed reaction of 1-alkenylboronates with vinylic halides: (1Z,3E)-1-Phenyl-1,3-octadiene". Organic Syntheses; Collected Volumes, vol. 8, p. 532.
9. D. R. Coulson; Satek, L. C.; Grim, S. O. (1972). "Tetrakis(triphenylphosphine)palladium(0)". Inorganic Syntheses. Vol. 13. pp. 121–124. doi:10.1002/9780470132449.ch23. ISBN 9780470132449. {{cite book}}: |journal= ignored (help)
10. Daniele Choueiry and 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.
11. Ralph Mozingo (1955). "Palladium Catalysts". Organic Syntheses; Collected Volumes, vol. 3, p. 685.
12. For example, http://www.marinecare.nl/assets/Uploads/Downloads/Leaflet-Passivation-Test-Kit.pdf%5B%5D
13. 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. doi:10.1016/S0021-9258(19)52307-9. PMID 13252030.