A metal-phosphine complex is a In coordination complex 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 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: :RhCl₃(H₂O)₃ + 4PPh₃ → RhCl(PPh₃)₃ + OPPh₃ + 2HCl + 2H₂O

M-PR₃ bonding

Phosphines are

L-type ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electro ...

s. Unlike most metal ammine complexes, metal phosphine complexes tend to be lipophilic, displaying good solubility in organic solvents. 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

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

pincer complex In chemistry, a transition metal pincer complex is a type of coordination complex with a pincer ligand. Pincer ligands are chelating agents that binds tightly to three adjacent coplanar sites in a meridional configuration. The inflexibility of ...

es.

Applications to homogeneous catalysis

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

s, carbon monoxide, 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. * 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 (Triphenylphosphine)iron tetracarbonyl is a coordination complex with the formula Fe(CO)4(PPh3) (Ph = C6H5). An off-white solid, this complex is derived from iron pentacarbonyl by replacement of one carbonyl ligand by triphenylphosphine (PPh3). ...

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

References

{{Coordination complexes Coordination complexes Catalysis *