A metal-phosphine complex is a In

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

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 phosphines ...

of the type R₃P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis. Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh₃)₃Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0). Wilkinson's-catalyst-2D

Preparation

Many metal phosphine complexes are prepared by reactions of metal halides with preformed phosphines. For example, treatment of a suspension of

palladium 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 ethanol with triphenylphosphine yields monomeric

bis(triphenylphosphine)palladium(II) chloride Bis(triphenylphosphine)palladium chloride is a coordination compound of palladium containing two triphenylphosphine and two chloride ligands. It is a yellow solid that is soluble in some organic solvents. It is used for palladium-catalyzed coupli ...

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 widely used in the synthesis of organic and organometallic compounds. PPh3 exists a ...

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

M-PR₃ bonding

Phosphines are L-type ligands. Unlike most metal ammine complexes, metal phosphine complexes tend to be lipophilic, displaying good

solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubil ...

in organic solvents. Phosphine ligands are also π-acceptors. Their π-acidity arises from overlap of P-C σ*

anti-bonding orbital In chemical bonding theory, an antibonding orbital is a type of molecular orbital that weakens the chemical bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms. Such an orbital has one or more no ...

s with filled metal orbitals. Aryl- and fluorophosphines are stronger π-acceptors than alkylphosphines.

Trifluorophosphine 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 indeed i ...

(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 orbital In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any spe ...

s 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 A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as ''side ...

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 indeed ...

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 amines, 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 a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...

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

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 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 (CHO) and a hydrogen atom to a carbon-carbon d ...

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 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 simple ...

, and

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

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) is widely used to catalyse C-C coupling reactions 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 ...

, 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 (or palladium nanomaterial-based catalyst) to form a s ...

. * Wilkinson's catalyst, RhCl(PPh₃)₃ is a square planar Rh(I) complex of historical significance used to catalyze 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 reactions. This early work provided the insights that led to the flowering of the area of homogeneous catalysis. * NiCl₂(PPh₃)₂ is a tetrahedral (

spin triplet In quantum mechanics, a triplet is a quantum state of a system with a spin of quantum number =1, such that there are three allowed values of the spin component, = −1, 0, and +1. Spin, in the context of quantum mechanics, is not a mechanical r ...

) 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 (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: :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 In organic chemistry, hydrocyanation is a process for conversion of alkenes to nitriles. The reaction involves the addition of hydrogen cyanide and requires a catalyst. This conversion is conducted on an industrial scale for the production of pr ...

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 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 ligands are ...

, 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 P2H4. This colourless liquid is one of several binary phosphorus hydrides. It is the impurity that typically causes samples of phosphine to ignite in air. Propert ...

s makes them especially useful in asymmetric catalysis, e.g. Noyori asymmetric hydrogenation. Several diphosphines have been developed, prominent examples include

1,2-bis(diphenylphosphino)ethane 1,2-Bis(diphenylphosphino)ethane (dppe) is an organophosphorus compound with the formula (PhPCH) (Ph = phenyl). It is a commonly used bidentate ligand in coordination chemistry. It is a white solid that is soluble in organic solvents. Preparation ...

(dppe) and

1,1'-Bis(diphenylphosphino)ferrocene 1,1-Bis(diphenylphosphino)ferrocene, commonly abbreviated dppf, is an organophosphorus compound commonly used as a ligand in homogeneous catalysis. It contains a ferrocene moiety in its backbone, and is related to other bridged diphosphines such ...

, the trans spanning

xantphos Xantphos is an organophosphorus compound derived from the heterocycle xanthene. It is used as a bidentate diphosphine ligand and is noteworthy for having a particularly wide bite angle (108°). Such ligands are useful in the hydroformylation of ...

and spanphos. The complex dichloro(1,3-bis(diphenylphosphino)propane)nickel is useful in Kumada coupling.

References

{{Coordination complexes Coordination complexes Catalysis *