science

Metal-phosphine complex

Metal-phosphine complex is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Metal-phosphine complex rather than just read about it. In short: A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl).

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Reference excerpt

A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine 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).

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.

[PdCl2]n + 2n PPh3 → n 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 + 4 PPh3 → RhCl(PPh3)3 + OPPh3 + 2 HCl + 2 H2O

M-PR3 bonding

Phosphines are L-type ligands. 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 3d 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 substituents, and for this reason phosphorus trifluoride is a particularly good π-acceptor.

Steric properties

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 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 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 triphenylphosphine in "Reppe" chemistry (1948), which included reactions of alkynes, carbon monoxide, and alcohols. 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.

Illustrative PPh3 complexes Tetrakis(triphenylphosphine)palladium(0) is widely used to catalyse C-C coupling reactions in organic synthesis, 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) complex of Ni(II). In contrast PdCl2(PPh3)2 is square planar. Stryker's reagent, [(PPh3)CuH]6, PPh3-stabilized transition metal hydride cluster that used as a reagent 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 arsines 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 LnM(PR2H)Cl → [LnM(μ-PR2)]2 + 2 HCl

Complexes of PRx(OR')3−x Nickel(0) complexes of phosphites, e.g., Ni[P(OEt)3]4 are useful catalysts for hydrocyanation of alkenes. Related complexes are known for phosphinites (R2P(OR')) and phosphonites (RP(OR')2).

Diphosphine complexes

Due to the chelate effect, ligands with two phosphine groups bind more tightly to metal centers than do two monodentate phosphines. The conformational properties of diphosphines 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 (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.

See also Transition metal trifluorophosphine complexes

References

Illustrations

Worked examples

Example 1 — a first encounter with Metal-phosphine complex

Start with the simplest possible case. Write down what Metal-phosphine complex claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Metal-phosphine complex before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Metal-phosphine complex ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Metal-phosphine complex

In research: Metal-phosphine complex appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Metal-phosphine complex in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Metal-phosphine complex is common in secondary-school and first-year university syllabi. It links to neighbouring topics Catalysis, Coordination complexes, Phosphanes, so understanding it makes those chapters shorter.
In everyday life: Look for Metal-phosphine complex outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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Frequently asked questions

What is Metal-phosphine complex in simple terms?

A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl).

Why does Metal-phosphine complex matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Metal-phosphine complex?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Metal-phosphine complex.