A metal-phosphine complex is a

coordination complex A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

containing one or more phosphine ligands. Almost always, the phosphine is an

organophosphine Organophosphines are organophosphorus compounds with the formula PR''n''H3−''n'', where R is an organic substituent. These compounds can be classified according to the value of ''n'': primary phosphines (''n'' = 1), secondary phosphin ...

of the type R₃P (R = alkyl, aryl). Metal phosphine complexes are useful in

homogeneous catalysis In chemistry, homogeneous catalysis is catalysis where the catalyst is in same phase as reactants, principally by a soluble catalyst in a solution. In contrast, heterogeneous catalysis describes processes where the catalysts and substrate are in d ...

. Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh₃)₃Cl),

Grubbs' catalyst Grubbs catalysts are a series of transition metal carbene complexes used as catalysts for olefin metathesis. They are named after Robert H. Grubbs, the chemist who supervised their synthesis. Several generations of the catalyst have also been develo ...

, and

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 d ...

Preparation

Many metal phosphine complexes are prepared by reactions of metal halides with preformed phosphines. For example, treatment of a suspension of palladium chloride in ethanol with triphenylphosphine yields monomeric bis(triphenylphosphine)palladium(II) chloride units. : dCl₂sub>n + 2PPh₃ → PdCl₂(PPh₃)₂ The first reported phosphine complexes were ''cis''- and ''trans''-PtCl₂(PEt₃)₂ reported by Cahours and Gal in 1870. Often the phosphine serves both as a ligand and as a reductant. This property is illustrated by the synthesis of many platinum-metal complexes of

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 versatile compound that is widely used as a reagent in organic synthesis and as a l ...

: :RhCl₃(H₂O)₃ + 4PPh₃ → RhCl(PPh₃)₃ + OPPh₃ + 2HCl + 2H₂O

M-PR₃ bonding

Phosphines are L-type ligands. Unlike most

metal ammine complex In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia () ligand. "Ammine" is spelled this way for historical reasons; in contrast, alkyl or aryl bearing ligands are spelt with a single "m". Almost al ...

es, metal phosphine complexes tend to be

lipophilic Lipophilicity (from Greek language, Greek λίπος "fat" and :wikt:φίλος, φίλος "friendly") is the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such compounds are c ...

, displaying good

solubility In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...

organic solvent A solvent (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...

s. Phosphine ligands are also π-acceptors. Their π-acidity arises from overlap of P-C σ* anti-bonding orbitals with filled metal orbitals. Aryl- and fluorophosphines are stronger π-acceptors than alkylphosphines. Trifluorophosphine (PF₃) is a strong π-acid with bonding properties akin to those of the carbonyl ligand. In early work, phosphine ligands were thought to utilize 3 d orbitals to form M-P pi-bonding, but it is now accepted that d-orbitals on phosphorus are not involved in bonding. The energy of the σ* orbitals is lower for phosphines with

electronegative Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...

substituent In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...

s, and for this reason

phosphorus trifluoride Phosphorus trifluoride (formula P F3), is a colorless and odorless gas. It is highly toxic and reacts slowly with water. Its main use is as a ligand in metal complexes. As a ligand, it parallels carbon monoxide in metal carbonyls, and indee ...

is a particularly good π-acceptor. Image:Connelly-Orpen-R3P-M-sigma-bonding.png, R₃P–M σ bonding File:Connelly-Orpen-R3P-M-pi-backbonding.png, R₃P–M π backbonding

Steric properties

220px, Cone angle is a common and useful parameter for evaluating the steric properties of phosphine ligands. In contrast to tertiary phosphines,

tertiary amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

s, especially arylamine derivatives, are reluctant to bind to metals. The difference between the coordinating power of PR₃ and NR₃ reflects the greater steric crowding around the nitrogen atom, which is smaller. By changes in one or more of the three organic substituents, the

steric Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivi ...

and electronic properties of phosphine ligands can be manipulated. The steric properties of phosphine ligands can be ranked by their Tolman cone angle or percent buried volume.

Spectroscopy

An important technique for the characterization of metal-PR₃ complexes is ³¹P NMR spectroscopy. Substantial shifts occur upon complexation. ³¹P-³¹P spin-spin coupling can provide insight into the structure of complexes containing multiple phosphine ligands.

Reactivity

Phosphine ligands are usually "spectator" rather than "actor" ligands. They generally do not participate in reactions, except to dissociate from the metal center. In certain high temperature

hydroformylation In organic chemistry, hydroformylation, also known as oxo synthesis or oxo process, is an industrial process for the production of aldehydes () from alkenes (). This chemical reaction entails the net addition of a formyl group () and a hydrogen ...

reactions, the scission of P-C bonds is observed however. The thermal stability of phosphines ligands is enhanced when they are incorporated into pincer complexes.

Applications to homogeneous catalysis

One of the first applications of phosphine ligands in catalysis was the use of

in " Reppe" chemistry (1948), which included reactions of

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

, and

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

s. In his studies, Reppe discovered that this reaction more efficiently produced acrylic esters using NiBr₂( PPh₃)₂ as a catalyst instead of NiBr₂. Shell developed cobalt-based catalysts modified with trialkylphosphine ligands for hydroformylation (now a rhodium catalyst is more commonly used for this process). The success achieved by Reppe and his contemporaries led to many industrial applications.P. W.N.M. van Leeuwen "Homogeneous Catalysis: Understanding the Art, 2004 Kluwer, Dordrecht.

Illustrative PPh₃ complexes

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 d ...

is widely used to catalyse C-C coupling reactions in

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

, see

Heck reaction The Heck reaction (also called the Mizoroki–Heck reaction) is the chemical reaction of an unsaturated halide (or triflate) with an alkene in the presence of a base and a palladium catalyst to form a substituted alkene. It is named after T ...

. * Wilkinson's catalyst, RhCl(PPh₃)₃ is a square planar Rh(I) complex of historical significance used to

catalyze Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

the hydrogenation of alkenes. * Vaska's complex, ''trans''-IrCl(CO)(PPh₃)₂, is also historically significant; it was used to establish the scope of

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. Oxidat ...

reactions. This early work provided the insights that led to the flowering of the area of

. * NiCl₂(PPh₃)₂ is a tetrahedral (

spin triplet In quantum mechanics, a triplet state, or spin triplet, is the quantum state of an object such as an electron, atom, or molecule, having a quantum spin ''S'' = 1. It has three allowed values of the spin's projection along a given axis ''m''S = � ...

) complex of Ni(II). In contrast PdCl₂(PPh₃)₂ is square planar. * Stryker's reagent, PPh₃)CuHsub>6, PPh₃-stabilized transition metal hydride cluster that used as a

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

for "conjugate reductions". * (Triphenylphosphine)iron tetracarbonyl (Fe(CO)₄(PPh₃)) and

bis(triphenylphosphine)iron tricarbonyl Tricarbonylbis(triphenylphosphine)iron(0) is a coordination complex with the formula Fe(CO)3(PPh3)2 (Ph = C6H5). A yellow solid, this complex is derived from iron pentacarbonyl by replacement of two carbonyl ligands by triphenylphosphine (PPh3). ...

(Fe(CO)₃(PPh₃)₂).

Complexes of other organophosphorus ligands

The popularity and usefulness of phosphine complexes has led to the popularization of complexes of many related organophosphorus ligands. Complexes of

arsine Arsine (IUPAC name: arsane) is an inorganic compound with the formula As H3. This flammable, pyrophoric, and highly toxic pnictogen hydride gas is one of the simplest compounds of arsenic. Despite its lethality, it finds some applications in th ...

s have also been widely investigated, but are avoided in practical applications because of concerns about toxicity.

Complexes of primary and secondary phosphines

Most work focuses on complexes of triorganophosphines, but primary and secondary phosphines, respectively RPH₂ and R₂PH, also function as ligands. Such ligands are less basic and have small cone angles. These complexes are susceptible to deprotonation leading to phosphido-bridged dimers and

oligomers In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...

: :2 L_nM(PR₂H)Cl → _nM(μ-PR₂)sub>2 + 2 HCl

Complexes of PR_x(OR')_3−x

Nickel(0) complexes of phosphites, e.g., Ni (OEt)₃sub>4 are useful catalysts for hydrocyanation of alkenes. Related complexes are known for

phosphinite In organic chemistry, phosphinites are organophosphorus compounds with the formula . They are used as ligands in homogeneous catalysis and coordination chemistry. Preparation Phosphinites are prepared by alcoholysis of organophosphinous chlori ...

s (R₂P(OR')) and

phosphonite In organic chemistry, phosphonites are organophosphorus compounds with the formula P(OR)2R. They are found in some pesticides and are used as ligands. Preparation Although they are derivatives of phosphonous acid (RP(OH)2), they are not prepar ...

s (RP(OR')₂).

Diphosphine complexes

Due to the

chelate effect Chelation () is a type of bonding of ions and their molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These l ...

, ligands with two phosphine groups bind more tightly to metal centers than do two monodentate phosphines. The conformational properties of

diphosphine Diphosphane, or diphosphine, is an inorganic compound with the chemical formula . This colourless liquid is one of several binary phosphorus hydrides. It is the impurity that typically causes samples of phosphine to ignite in air. Properties, ...

s makes them especially useful in

asymmetric catalysis Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IUPAC as "a chemical reaction (or reaction sequence) in which one or more new elements of Chirality (chemistry), chirality are formed ...

, e.g. Noyori asymmetric hydrogenation. Several diphosphines have been developed, prominent examples include 1,2-bis(diphenylphosphino)ethane (dppe) and 1,1'-Bis(diphenylphosphino)ferrocene, the trans spanning xantphos and spanphos. The complex dichloro(1,3-bis(diphenylphosphino)propane)nickel is useful in

Kumada coupling In organic chemistry, the Kumada coupling is a type of cross coupling reaction, useful for generating carbon–carbon bonds by the reaction of a Grignard reagent and an organic halide. The procedure uses transition metal catalysts, typically ...

References

{{Coordination complexes Coordination complexes Catalysis *