Tetrakis(triphenylphosphine)palladium(0)
Overview
 
Tetrakispalladium(0) is the chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 Pd[P(C6H5)3]4, often abbreviated Pd
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...

(PPh3
Triphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...

)4, or even PdP4. It is a bright yellow crystalline solid that becomes brown upon decomposition in air.
The four P
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

 atoms are at the corners of a tetrahedron surrounding the palladium(0) center. This structure is typical for four-coordinate 18e
18-Electron rule
The 18-electron rule is a rule of thumb used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal consists of nine valence orbitals, which collectively can accommodate 18 electrons either as nonbinding electron pairs or...

 complexes.
Encyclopedia
Tetrakispalladium(0) is the chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 Pd[P(C6H5)3]4, often abbreviated Pd
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...

(PPh3
Triphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...

)4, or even PdP4. It is a bright yellow crystalline solid that becomes brown upon decomposition in air.

Structure and properties

The four P
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

 atoms are at the corners of a tetrahedron surrounding the palladium(0) center. This structure is typical for four-coordinate 18e
18-Electron rule
The 18-electron rule is a rule of thumb used primarily for predicting formulas for stable metal complexes. The rule rests on the fact that valence shells of a transition metal consists of nine valence orbitals, which collectively can accommodate 18 electrons either as nonbinding electron pairs or...

 complexes. The corresponding complexes Ni(PPh3)4 and Pt(PPh3)4 are also well known. Such complexes reversibly dissociate PPh3 ligands in solution, releasing the 16e M(PPh3)3. Thus, reactions attributed to Pd(PPh3)4 in fact arise from Pd(PPh3)3 or even Pd(PPh3)2.

Preparation

Tetrakis(triphenylphosphine)palladium(0) was first prepared by Lamberto Malatesta and his group in Milan in the 1960s by reduction of sodium chloropalladate with hydrazine in the presence of the phosphine. It is commercially available, but can be prepared in two steps from Pd(II) precursors:
PdCl2 + 2 PPh3cis-PdCl2(PPh3)2
cis-PdCl2(PPh3)2
Bis(triphenylphosphine)palladium(II) dichloride
Bispalladium dichloride is a coordination compound of palladium containing two triphenylphosphine and two chloride ligands. This yellow complex is often used for palladium-catalyzed coupling reactions, e.g. the Sonogashira–Hagihara reaction. The complex is square planar...

  + 2 PPh3 + 2.5 N2H4
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

  → Pd(PPh3)4 + 0.5 N2 + 2 N2H5+Cl-

Both steps may be carried out in a one-pot reaction, without isolating and purifying the cis-PdCl2(PPh3)2 intermediate. Reductants other than hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

 can be employed. The compound is sensitive to air, but can be purified by washing with methanol to give the desired yellow powder. It is usually stored cold under nitrogen.

Applications

Pd(PPh3)4 is widely used as a catalyst for palladium-catalyzed coupling reactions. Prominent applications include the Heck reaction
Heck reaction
The Heck reaction is the chemical reaction of an unsaturated halide with an alkene and a base and palladium catalyst to form a substituted alkene. Together with the other palladium-catalyzed cross-coupling reactions, this reaction is of great importance, as it allows one to do substitution...

, Suzuki coupling, Stille coupling, Sonogashira coupling
Sonogashira coupling
In organic chemistry, a Sonogashira coupling is a coupling reaction of terminal alkynes with aryl or vinyl halides. This reaction was first reported by Kenkichi Sonogashira and Nobue Hagihara in 1975.-Catalyst:...

, and Negishi coupling
Negishi coupling
The Negishi coupling is a cross coupling reaction in organic chemistry involving an organozinc compound, an organic halide and a nickel or palladium catalyst creating a new carbon-carbon covalent bond:* The halide X can be chloride, bromine or iodine but also a triflate or acetyloxy group with as...

. These processes begin with the oxidative addition
Oxidative addition
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre...

of an aryl halide to the Pd(0) center:
Pd(PPh3)4 + ArBr → PdBr(Ar)(PPh3)2 + 2 PPh3
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