Phosphine ligand selection is usually taught as a list of names – XPhos for this, BINAP for that – which is why so many optimisation campaigns end up screening eight ligands and learning nothing. The ligands are not interchangeable brands. They differ along two axes that can be read off the structure before you run a single reaction: how much steric bulk sits around the phosphorus, and how electron-rich the phosphorus is.
This guide sets out those two axes, sorts the 43 phosphine ligands we hold into three structural families, and gives the full CAS list with molecular weights so that a specification or a quotation can be checked against the structure rather than against a catalogue name.
The two axes, and why they are the only two that matter first
A phosphine ligand does two jobs in a cross-coupling cycle: it keeps the metal soluble and electron-rich enough to oxidatively add, and it creates a crowded environment that accelerates reductive elimination. Those two jobs pull in opposite directions, and every ligand design is a compromise between them.
| Axis | What it controls | How to read it from a structure |
|---|---|---|
| Electron donation | Rate of oxidative addition, especially for aryl chlorides and sulfonates | Alkyl substituents on phosphorus (cyclohexyl, tert-butyl) donate strongly; aryl and alkoxy-substituted aryl groups donate less; fluorinated aryl groups are electron-poor |
| Steric bulk | Reductive elimination rate, and whether the complex stays mono- or bis-phosphine | Ortho substituents on a biaryl backbone, and the size of the groups directly on phosphorus |
The practical consequence is that the electron-rich, bulky dialkylbiaryl phosphines – the Buchwald family – operate mainly as monophosphine complexes, because two of them will not fit around one metal. That single structural fact explains why they are so effective with hindered substrates: a mono-ligated, electron-rich palladium centre oxidatively adds readily and then eliminates quickly.
Family 1 – the dialkylbiaryl phosphines (Buchwald type)
These are built on the same skeleton: a biphenyl with a dialkylphosphino group at the 2-position. What changes is the substitution on the second ring, and that is where the steric tuning happens.
| Ligand | CAS | Backbone substitution | Phosphorus substituents | MW | Where it earns its price |
|---|---|---|---|---|---|
| JohnPhos | 224311-51-7 | None on the second ring | Di-tert-butyl | 298.4 | The starting point of the family – very electron-rich, moderate bulk, good for simple aryl bromides and chlorides |
| SPhos | 657408-07-6 | 2,6-dimethoxy | Dicyclohexyl | 410.5 | Least hindered of the three shown here; the methoxy groups are electron-donating, which helps electron-poor aryl electrophiles |
| XPhos | 564483-18-7 | 2,4,6-triisopropyl | Dicyclohexyl | 476.7 | Most hindered of the three; the workhorse for hindered aryl halides, aminations with hindered amines and C-C couplings that stall with simpler ligands |
Note what changed between SPhos and XPhos: the groups directly on phosphorus are identical (dicyclohexyl). The entire difference in reactivity comes from the second ring – two methoxy groups against three isopropyl groups. That is the cleanest demonstration in the whole ligand field that sterics and electronics are separable variables.
Family 2 – the ferrocene bisphosphines
Two cyclopentadienyl rings bridged by iron, bearing phosphino groups. The sandwich structure holds the two phosphorus atoms at a fixed, wide separation, and the flat, rigid backbone tolerates a large bite angle without strain. Planar chirality becomes available when the two rings carry different substitution, which is how the Josiphos class is built.
| Ligand | CAS | MW | Character |
|---|---|---|---|
| dppf | 12150-46-8 | 554.4 | The reference ferrocene bisphosphine: two diphenylphosphino groups, wide bite angle, the standard for palladium-catalysed coupling and the ligand inside the common PdCl2(dppf) precatalyst |
| dtbpf | 84680-95-5 | 474.4 | Electron-rich version – di-tert-butyl groups, much stronger donor, more sensitive to air |
| dcpf | 146960-90-9 | not indexed | Dicyclohexyl analogue, intermediate electron density |
| Ferrocenyl oxazoline | 163169-29-7 | 481.3 | P,N ligand – one phosphorus donor and one nitrogen donor, used where a different electronic asymmetry is wanted |
| Planar-chiral ferrocenyl bisphosphines | 158923-11-6, 155941-31-4, 155806-35-2, 155830-69-6, 849924-76-1, 849924-77-2, 184095-69-0, 162291-02-3, 277306-29-3 | see table below | Josiphos-type ligands: planar chirality plus a stereogenic carbon, used in asymmetric hydrogenation |
Family 3 – rigid bidentate and wide-bite-angle ligands
| Ligand | CAS | MW | Bite angle | Use |
|---|---|---|---|---|
| Xantphos | 161265-03-8 | 578.6 | Very wide | Amination, carbonylation and hydroxycarbonylation, where a wide P-Pd-P angle favours the desired pathway; also common in copper photoredox work |
| dppb | 1179-06-2 | 446.5 | Rigid, para-phenylene | Bridges two metals – coordination polymers, MOFs and dinuclear complexes rather than simple mononuclear catalysis |
| BINAP (racemic) | 98327-87-8 | 622.7 | Axially chiral, wide | Ligand screening baseline, racemic catalysis, and the starting point for resolution |
| (R)-BINAP | 76189-55-4 | 622.7 | as above | Asymmetric hydrogenation and isomerisation, one enantiomer |
| (S)-BINAP | 76189-56-5 | 622.7 | as above | The other enantiomer – same physical data, opposite selectivity |
Bite angles are quoted as a qualitative class rather than a number because the observed angle depends on the metal, the oxidation state and the coordination mode. A ligand that is described as “wide bite angle” in one paper may be crystallographically narrower in another complex of the same metal.
The full list: monodentate aryl and diaryl phosphines
These are the classic triarylphosphines and the secondary phosphines. They are cheaper than the Buchwald family and they are the right answer in a large number of ordinary applications – and the wrong answer when a hindered substrate needs a mono-ligated electron-rich centre.
| Compound | CAS | MW | Type | Note |
|---|---|---|---|---|
| Tri-tert-butylphosphine | 13716-12-6 | 202.32 | Trialkyl, very electron-rich | Pyrophoric as the neat liquid – supplied as a solution, never as the pure material |
| Triphenylphosphine-type, tri-o-tolyl | 6163-58-2 | 304.4 | Hindered triaryl | Ortho methyls give bulk without changing the donor strength much |
| Tri-p-tolylphosphine | 1038-95-5 | 304.4 | Electron-rich triaryl | Para methyls donate electronically with almost no steric change – the clean electronic control against tri-o-tolyl |
| Tris(4-chlorophenyl)phosphine | 1159-54-2 | 365.6 | Electron-poor triaryl | Para chloro withdrawing groups |
| Tris(4-fluorophenyl)phosphine | 18437-78-0 | 316.3 | Electron-poor triaryl | Milder withdrawal than chloro |
| Tris(4-trifluoromethylphenyl)phosphine | 13406-29-6 | 466.3 | Electron-poor triaryl | Strongly withdrawing |
| Tris(4-bromophenyl)phosphine | 29949-81-3 | 499.0 | Functional triaryl | Bromine handles for further functionalisation or for heavier analogues |
| Tris(pentafluorophenyl)phosphine | 1259-35-4 | 532.1 | Strongly electron-poor | A classic Lewis acid partner in frustrated Lewis pair chemistry |
| Diphenylphosphine | 829-85-6 | 186.19 | Secondary phosphine | A building block rather than a finished ligand – oxidises readily, needs inert storage |
| Bis(2-methoxyphenyl)phosphine | 10177-79-4 | 246.24 | Secondary phosphine | Ortho-methoxy donor for chelation after deprotonation; precursor to ether-functionalised ligands |
| Bis(3,5-dimethylphenyl)phosphine | 71360-06-0 | 242.29 | Secondary phosphine | Bulky diaryl secondary phosphine |
The phosphine oxides: a different product, not a degraded ligand
Roughly a third of the list is diaryl and dialkyl phosphine oxides. These are frequently bought in the mistaken belief that they are ligands. They are not: the phosphorus lone pair is already bonded to oxygen, so there is nothing to coordinate through. Phosphine oxides are used as extractants (notably for actinides and rare earths), as Lewis base synergists, as flame-retardant or polymer additives, and as precursors that are reduced back to the phosphine when a ligand is being made.
One specification detail worth knowing: PubChem indexes several of these oxides in their protonated form, so the formula shown for a phosphine oxide may carry a positive charge (for example C16H18OP+ for a compound whose neutral formula is C16H17OP). Quote molecular weight against the neutral molecule when you are checking a certificate.
| Compound | CAS | Note |
|---|---|---|
| Bis(3,5-dimethylphenyl)phosphine oxide | 187344-92-9 | Diaryl oxide, metal extractant |
| Bis(2-methoxyphenyl)phosphine oxide | 71360-04-8 | Ortho-methoxy chelating oxide |
| Bis(p-tolyl)phosphine oxide | 2409-61-2 | Electron-rich diaryl oxide |
| Bis(4-methoxyphenyl)phosphine oxide | 15754-51-5 | Electron-rich diaryl oxide |
| Bis(4-fluorophenyl)phosphine oxide | 94940-35-9 | Fluorinated diaryl oxide |
| Bis(4-chlorophenyl)phosphine oxide | 15948-60-4 | Chlorinated diaryl oxide |
| Bis(4-bromophenyl)phosphine oxide | 15754-53-7 | Brominated diaryl oxide |
| Di(naphthalen-1-yl)phosphine oxide | 13440-07-8 | Bulky naphthyl oxide |
| Dibutylphosphine oxide | 15754-54-8 | Dialkyl oxide, liquid, extractant |
| Dicyclohexylphosphine oxide | 14717-29-4 | Dialkyl oxide |
The chiral ligands, and the trap in the certificate
Nine of the ligands in this list are planar-chiral or axially chiral ferrocene derivatives, and three are BINAP. For these, chemical purity is not the specification that decides whether the experiment works. A 98% pure (R)-BINAP that is 20% racemised will give a worse enantiomeric excess than a 95% pure sample that is enantiopure.
| Ligand | CAS | Chemical purity on our sheet | What else to ask for |
|---|---|---|---|
| (+-)-BINAP | 98327-87-8 | 99.0% | Nothing – it is racemic by definition, which is what makes it the right screening baseline |
| (R)-BINAP | 76189-55-4 | 98.0% | Optical purity (enantiomeric excess or specific rotation) – without it the sheet does not describe the product |
| (S)-BINAP | 76189-56-5 | 98.0% | Same as above |
| Josiphos-type ligands | 158923-11-6, 155806-35-2, 155830-69-6, 849924-76-1, 849924-77-2, 184095-69-0, 162291-02-3, 277306-29-3 | 97.0% | Optical purity, and confirmation of which diastereomer the CAS refers to |
| (1S)-1-(Diphenylphosphino)-2-[(1R)-…]ferrocene | 155941-31-4 | 97.0% | Optical purity; this is a single diastereomer of a planar-chiral ligand |
| (S)-Ferrocenyl oxazoline | 163169-29-7 | 97.0% | Optical purity; P,N ligand |
The CID trap
A chemical database will happily give you one identifier for all three BINAP grades: racemic, (R) and (S) BINAP all resolve to the same PubChem CID (634876), because that record describes the constitution of the molecule and not its stereochemistry. The same happens with molecular formula and molecular weight – all three are C44H32P2, 622.7.
So a specification cannot distinguish (R) from (S) from racemic. Only the CAS number and an optical purity measurement can. If a supplier’s certificate quotes formula, molecular weight and assay but not rotation or enantiomeric excess, it has not told you which of the three products you are buying.
Storage and weighing: the part that ruins experiments
Almost every ligand in this list is flagged air-sensitive, and that flag is not decoration. Electron-rich alkylphosphines oxidise to the phosphine oxide in air, and the oxide is a spectator at best and an inhibitor at worst – the reaction simply gets slower and the operator blames the catalyst.
- Store under inert gas at low temperature, and let containers reach room temperature before opening to avoid condensation onto the solid.
- Weigh in a glovebox or under a blanket, and return the container promptly. A ligand left open on a balance for an afternoon is a different compound.
- Watch the colour. Most of these ligands are white to off-white powders; yellowing or a greasy appearance means oxide formation, not a batch variation.
- Use 31P NMR as the acceptance test rather than appearance. The phosphine and its oxide sit at clearly different chemical shifts, so a single spectrum tells you the oxidation state of the whole sample.
Pairing a ligand with a precatalyst
Most users buy the ligand and the palladium source separately and prepare the catalyst in situ. That is fine, but it means two purchases, two shipments and two chances of an air-exposure problem. We can also supply the common palladium precatalysts – palladium acetate, Pd2(dba)3, PdCl2(dppf) and the Buchwald-type precatalysts – alongside the ligand, sourced from precious-metal manufacturers we work with, so that a single order delivers a matched pair with the metal content stated on the certificate.
For the ligand-only purchase, the questions that decide whether a quote is usable are set out below.
Buying checklist
- Confirm the CAS against the structure, not the name. “Bis(diphenylphosphino)ferrocene” and “1,1-bis(diphenylphosphino)ferrocene” are the same compound; “dppb” refers to the benzene-bridged ligand, not the ferrocene one.
- Ask for 31P NMR and elemental analysis for air-sensitive ligands. Appearance is not a specification.
- For chiral ligands, ask for optical purity as well as chemical purity.
- Ask how it will be packaged – sealed under argon, ampouled, or glovebox-packed. This matters more than the difference between 98% and 99% chemical purity.
- Check the molecular weight basis for phosphine oxides – PubChem indexes some of them protonated.
- Decide whether you want the ligand or the precatalyst. For a first screen, the in-situ route is cheaper; for a reproducible process, a defined precatalyst removes a variable.
Product pages
The ligands with dedicated pages are listed here; the rest of the list is available on request with the same certificate package.
- XPhos – CAS 564483-18-7
- SPhos – CAS 657408-07-6
- JohnPhos – CAS 224311-51-7
- Xantphos – CAS 161265-03-8
- dppf – CAS 12150-46-8
- dtbpf – CAS 84680-95-5
- dppb – CAS 1179-06-2
- (+/-)-BINAP – CAS 98327-87-8
- (R)-BINAP – CAS 76189-55-4
- (S)-BINAP – CAS 76189-56-5
- Tri-tert-butylphosphine – CAS 13716-12-6
FAQ
Which Buchwald ligand should I start with?
For an unhindered aryl bromide or chloride with a primary amine, JohnPhos or SPhos will usually work and costs less. Move to XPhos when the substrate is hindered – ortho-substituted aryl halides, secondary amines, or a coupling that stalls at partial conversion.
Are SPhos and XPhos the same ligand with a different name?
No. Both carry dicyclohexylphosphino groups, but SPhos has 2,6-dimethoxy substitution on the second ring and XPhos has 2,4,6-triisopropyl. The steric difference is large enough to change which substrates react at all.
Why is tri-tert-butylphosphine supplied as a solution?
The neat liquid is pyrophoric in air. It is handled and shipped as a solution in toluene or similar, which is the only sensible way to deliver it.
Can a phosphine oxide be used as a ligand?
Not as a conventional phosphine ligand – the lone pair is bonded to oxygen. Phosphine oxides are extractants, synergists, additives and synthetic precursors.
How do I know an (R)-BINAP is really (R)?
By the CAS number and an optical purity measurement. Formula, molecular weight and even the PubChem CID are identical for the racemic, (R) and (S) grades.
References and notes
- CAS numbers, molecular formulas and molecular weights verified against PubChem compound records; nine ferrocene ligands in this list have no PubChem record and are listed without formula or molecular weight rather than estimated.
- PubChem CID 634876 covers racemic, (R) and (S) BINAP – the record describes constitution, not stereochemistry.
- Purity figures are the values on our current specification sheets and refer to chemical purity, not optical purity.
- Bite angles are described qualitatively because observed values depend on metal, oxidation state and coordination mode.
Disclaimer. This guide describes ligand structure and commercial specification. It is not a reaction-development recommendation: the right ligand for a given substrate is established by screening, and reported conditions rarely transfer unchanged between substrates. Purity figures are chemical purity and do not describe optical purity.
For a specific ligand, a certificate with 31P NMR, or a quotation that includes a matched palladium precatalyst, contact PolyBlueChem.
Related reading.