Zeise salt: Structure, CAS Number, Properties and Uses

Zeise's salt, potassium trichloro(ethylene)platinate(II) hydrate, K[PtCl3(C2H4)]·H2O (CAS 123334-22-5, PubChem CID 71311337), was the first transition-metal alkene complex ever isolated, prepared in 1830. Its platinum(II) center carries 16 valence electrons rather than 18, and platinum makes up 50.5% of the compound by weight. Its structure, electron count, PGM content, properties, preparation, uses, commercial form and hazard classification follow below, section by section.

What Is Zeise's Salt?#

Zeise's salt is a platinum(II) coordination compound in which one ethylene molecule binds side-on to a PtCl3 unit, giving the anion [PtCl3(C2H4)]-, isolated as its potassium salt monohydrate. PubChem catalogs the compound under CID 71311337, with the systematic name "potassium;ethene;trichloroplatinum(1-);hydrate," and assigns it CAS number 123334-22-5, the identifier this page uses throughout. A second registry number, CAS 16405-35-9 (PubChem CID 193510), also identifies Zeise's salt hydrate and still appears on some secondary chemistry references and older supplier listings; PubChem confirms both numbers describe the same substance and formula, a dual-CAS pattern this site also documents for Karstedt's and Grubbs' catalysts, so the older number is a parallel registration rather than an error, and this page uses the CAS 123334-22-5 / CID 71311337 pair throughout. The "hydrate" in its name matters: the anion and potassium counter-ion crystallize with one water molecule per formula unit, which is why the 386.6 g/mol molecular weight printed on this page is higher than an anhydrous [PtCl3(C2H4)]- salt would carry on its own. Zeise's salt is one of the platinum group metal compounds and catalysts cataloged with a dedicated CAS registry entry on this site.

Zeise's Salt Structure and Bonding#

Platinum in Zeise's salt is four-coordinate and square planar, the geometry every platinum(II), d8 complex adopts, a pattern the site's chemistry registry documents across the whole platinum(II) family, from cisplatin to Magnus's green salt to the K2PtCl4 starting material itself. Three chloride ions and one ethylene molecule fill the four coordination sites around platinum, with the ethylene bound side-on through its carbon-carbon double bond rather than through a single atom, the defining feature of a pi complex. That side-on bonding lengthens the platinum-chlorine bond trans to the ethylene to 234 picometers, against roughly 230 picometers for the other two platinum-chlorine bonds, according to Wikipedia's Zeise's salt entry, a direct structural measurement of ethylene's trans influence on the metal center. A single-crystal X-ray redetermination published in Acta Crystallographica Section B (Jarvis, Kilbourn and Owston, 1971) refines these figures further: the trans platinum-chlorine bond measures 232.7 picometers (2.327(5) angstrom) against a 230.5-picometer average (2.305(7) angstrom) for the two cis bonds, and the coordinated ethylene's own carbon-carbon bond stretches to 137 picometers (1.37(3) angstrom), longer than the 134-picometer carbon-carbon bond of free ethylene gas, direct crystallographic evidence that platinum donates electron density back into the alkene's antibonding orbital rather than only accepting it.

This side-on, pi-bound arrangement is what the electron-counting exercise below tests directly, since a two-electron donor ligand behaves differently in the count than a simple single-atom donor would. Wikipedia's Zeise's salt entry describes this side-on binding as the structural feature that later founded pi-complex bonding theory: ethylene donates electron density from its carbon-carbon pi bond toward platinum rather than bonding through a single carbon atom, the bonding pattern that distinguishes a pi complex from an ordinary sigma-bonded ligand.

Electron Count: Is Zeise's Salt an 18-Electron Complex?#

Zeise's salt's platinum center carries 16 valence electrons, not 18, computed with the standard ionic electron-counting method. Pt2+ is a d8 ion contributing 8 electrons, each of the three chloride ligands contributes 2 electrons as an anionic 2-electron donor for 6 electrons total, and the neutral ethylene ligand contributes 2 electrons as a pi donor, for a sum of 16.

Square-planar d8 platinum(II) complexes are a well-known exception to the 18-electron rule; cisplatin and Magnus's green salt are also 16-electron, per the site's chemistry registry's d-count and geometry table. That 16-electron, coordinatively unsaturated count is consistent with square-planar d8 chemistry generally, though unlike a catalytic 16-electron complex, Zeise's salt does not turn over a reaction cycle, its significance is structural and historical rather than catalytic.

PGM Content of Zeise's Salt#

Platinum makes up 50.5% of Zeise's salt by weight, computed from the compound's formula mass of 386.6 g/mol and platinum's atomic mass of 195.08 g/mol (195.08 / 386.6 = 0.505). That single platinum atom, three chlorine atoms, one potassium atom, two carbon atoms and the water of hydration together account for the remaining 49.5% of the formula mass, so roughly half of every gram of Zeise's salt is the metal itself and half is the surrounding ligand and counter-ion framework. That figure places Zeise's salt in the same order of magnitude as other platinum coordination compounds sold by contained-metal content, though as the commercial section below notes, Zeise's salt itself is not priced against a metal-content benchmark the way bulk platinum precursors are. For the metal's own current market value, DailyPlatinum publishes platinum price data separately from any compound's contained-metal calculation.

Physical and Chemical Properties#

Zeise's salt is a yellow, water-soluble crystalline solid with a molecular weight of 386.6 g/mol. It decomposes at 220°C rather than showing a true melting point, and has a density of 2.88 g/mL at 25°C, according to ChemicalBook's aggregated technical data, a single-source figure with no second source located for this page.

How Is Zeise's Salt Made?#

Zeise's salt is made by passing ethylene gas into a solution of potassium tetrachloroplatinate(II) (K2PtCl4) with tin(II) chloride (SnCl2) as catalyst, per Wikipedia's Zeise's salt entry. The preparation follows three steps:

1. Dissolve potassium tetrachloroplatinate(II), K2PtCl4, in water.
2. Add a catalytic quantity of tin(II) chloride, SnCl2, which activates the platinum center toward ethylene coordination.
3. Pass ethylene gas through the solution, displacing one chloride ligand and forming the [PtCl3(C2H4)]- anion, isolated as its potassium salt monohydrate.

This route, ethylene into a K2PtCl4 solution with catalytic SnCl2, is documented as a named preparation, "Potassium Trichloro(Ethene)platinate(II) (Zeise's Salt)," in Inorganic Syntheses, Volume 28 (1990), page 349 (Chock, Halpern and Paulik), the standard literature citation research groups use to reproduce the reaction, and the same K2PtCl4 starting material feeds several other named platinum(II) products, covered in the Related PGM Compounds section below. A faster alternative reaches completion in 15 minutes at 130°C rather than the days the classical open-flask route takes: K2PtCl4 dissolved in a 1:1:1 water-ethanol-concentrated hydrochloric acid solvent mixture reacts under 50 psi of ethylene pressure in a microwave reactor, per a method published in Inorganic Chemistry Communications.

What Is Zeise's Salt Used For?#

Zeise's salt has research and teaching significance rather than an industrial-scale application: it is the structural landmark that founded pi-complex bonding theory, per Wikipedia's Zeise's salt entry. Its side-on ethylene bond was the first direct evidence that a transition metal could bind an alkene through its pi electrons rather than through a single atom, a discovery that predated the electronic theory needed to fully explain it by more than a century.

The compound's continuing role covers three related applications:

• Organometallic chemistry teaching, where Zeise's salt is the standard first example used to introduce alkene pi-complexation.
• Structural chemistry reference, cited for its measured platinum-chlorine trans influence whenever a new pi complex's bonding is compared against a known benchmark.
• Synthetic and spectroscopic research reagent, supplied in small laboratory quantities as a source of the [PtCl3(C2H4)]- fragment for further coordination chemistry.
• Electron-microscopy staining, where its heavy platinum atom acts as an electron-opaque marker in biological and materials sample preparation, per PubChem's synonym and product data for the compound (CID 193510).

Commercial Forms and Price#

Zeise's salt is supplied as a small-pack research and teaching reagent, keyed to CAS 123334-22-5 and PubChem CID 71311337 for procurement. American Elements names Sigma-Aldrich, American Elements and Santa Cruz Biotechnology as suppliers of Zeise's salt, all in small research pack sizes rather than industrial-scale quantities, and reports no evidence that the compound is priced against its contained platinum value the way bulk platinum precursors typically are, a contrast with commodity platinum salts whose catalog prices track the metal market. ChemicalBook's aggregated supplier data additionally names Riedel-de Haen AG, Alfa Aesar, Advanced Technology & Industrial Co. Ltd. (China) and ABCR GmbH & Co. KG (Germany) as suppliers, all consistent with the same small-pack, non-industrial pattern rather than a bulk commercial listing. That pricing pattern differs from other PGM research complexes on this site: Wilkinson's catalyst, for example, is 11.1% rhodium by weight and its price tracks the rhodium market closely enough that volume buyers price a batch against contained-rhodium value, while Zeise's salt, despite carrying 50.5% platinum by weight, changes hands as a flat-rate research chemical rather than a metal-content-indexed one.

Safety and Hazard Classification#

PubChem's mirror of the ECHA Classification and Labelling (C&L) Inventory gives Zeise's salt a full European Union CLP classification for CID 71311337 (EC number 630-445-1), aggregated from 38 company reports filed under a single C&L notification: Skin Irrit. 2, Eye Irrit. 2 and STOT SE 3, carrying hazard statements H315 (causes skin irritation), H319 (causes serious eye irritation) and H335 (may cause respiratory irritation), under GHS pictogram GHS07 and signal word "Warning." ECHA's own registered-substances portal blocks automated access, so this PubChem mirror is the source this page uses for EU classification. That mirror does not report an active REACH registration status or a REACH tonnage band for Zeise's salt, unlike some other platinum compounds in the same family, so neither is stated here. Three irritant hazard codes with a "Warning" signal word is a lighter classification than several other platinum(II) chloride salts in the same family carry; potassium tetrachloroplatinate, for comparison, carries GHS signal word "Danger" with hazard statements for oral toxicity (H301), skin irritation (H315), allergic skin reaction (H317), serious eye damage (H318) and possible respiratory allergy (H334), per PubChem's GHS classification for CID 61440. Laboratories handle the Zeise's salt solid under standard precautions, gloves, eye protection and fume-hood practice, with its dust not inhaled. That absence of any acute-toxicity code is not a certification of safety; it reflects the specific classification data notified to PubChem for this compound, not an independent safety assessment.

Where Zeise's Salt Chemistry Stops: Medical and Pharmaceutical Platinum#

Zeise's salt chemistry stops at the metal-organic complex itself; it is not a pharmaceutical compound and carries no dosing or clinical use. Platinum drugs made from the same K2PtCl4 family, such as cisplatin, are a distinct pharmacological subject, named only, without dose or regimen, and covered instead on DailyPlatinum's platinum compounds page. That page, not this one, is where platinum's medical chemistry belongs.

Related PGM Compounds#

Zeise's salt and Magnus's green salt are both made from potassium tetrachloroplatinate (K2PtCl4), by different reactions. K2PtCl4 itself, CAS 10025-99-7, formula Cl4K2Pt, molecular weight 415.1 g/mol, is a reddish-orange salt made by reducing potassium hexachloroplatinate with sulfur dioxide, per the site's chemistry registry; from that one starting material, ethylene and catalytic tin(II) chloride give Zeise's salt while a tetraammineplatinum(II) salt gives Magnus's green salt instead. Potassium tetrachloroplatinate and chloroplatinic acid are the parent platinum(II) and platinum(IV) chemistries behind much of this family, per the site's chemistry registry, though neither is linked from this page.

History of Zeise's Salt#

William Christopher Zeise prepared this salt in 1830 at the University of Copenhagen. Decades passed before chemists had the electronic theory to explain why an alkene would bind a metal center at all, and Zeise's salt was only later recognized as the founding example of pi-complex, organometallic bonding, a landmark the site's chemistry registry lists alongside Adams's catalyst (1922) and Karstedt's catalyst (1973) as one of platinum chemistry's fixed historical dates. Zeise himself worked without knowing why his yellow crystals behaved as they did; the salt sat as a chemical curiosity for over a century until 20th-century bonding theory gave chemists the tools to explain the side-on platinum-ethylene bond he had isolated.

Frequently Asked Questions#

What is the CAS number and formula of Zeise's salt? Zeise's salt carries CAS number 123334-22-5 and the molecular formula C2H6Cl3KOPt, with a molecular weight of 386.6 g/mol, per PubChem CID 71311337.

What is the geometry of Zeise's salt? Zeise's salt is square planar and four-coordinate, with platinum(II) bonded to three chloride ions and one side-on ethylene ligand.

How many electrons does platinum have in Zeise's salt? Sixteen. Pt2+ contributes 8 electrons, the three chloride ligands contribute 6 electrons combined, and ethylene contributes 2 electrons as a neutral donor, for a 16-electron total, not the 18 electrons a filled-shell complex would carry.

What is Zeise's salt used for today? Zeise's salt is used as a research and teaching reagent, chiefly to introduce alkene pi-complexation and as a structural reference point for platinum-chlorine bond lengths, rather than in any industrial-scale process.

Who discovered Zeise's salt, and when? William Christopher Zeise prepared it in 1830 at the University of Copenhagen, decades before chemists could explain its side-on ethylene bond.

Is Zeise's salt dangerous to handle? PubChem's mirror of the ECHA Classification and Labelling Inventory classifies it Skin Irrit. 2, Eye Irrit. 2 and STOT SE 3 under GHS signal word "Warning," with hazard statements for skin irritation (H315), eye irritation (H319) and possible respiratory irritation (H335), moderate irritant hazards handled with standard laboratory precautions.

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