Platinum(II) chloride explained

Platinum(II) chloride is the chemical compound PtCl₂. It is an important precursor used in the preparation of other platinum compounds. It exists in two crystalline forms, but the main properties are somewhat similar: dark brown, insoluble in water, diamagnetic, and odorless.

Structure

The structures of PtCl₂ and PdCl₂ are similar. These dichlorides exist in both polymeric, or "α", and hexameric, or "β" structures. The β form converts to the α form at 500 °C. In the β form, the Pt-Pt distances are 3.32–3.40 Å, indicative of some bonding between the pairs of metals. In both forms of PtCl₂, each Pt center is four-coordinate, being surrounded by four chloride ligands. Complementarily, each Cl center is two-coordinate, being connected to two platinum atoms.^[1] The structure of α-PtCl₂ is reported to be disordered and contain edge- and/or corner-sharing square-planar PtCl₄ units.^[2]

Preparation

β-PtCl₂ is prepared by heating chloroplatinic acid to 350 °C in air.^[3]

H₂PtCl₆ → PtCl₂ + Cl₂ + 2 HCl

This method is convenient since the chloroplatinic acid is generated readily from Pt metal. Aqueous solutions of H₂PtCl₆ can also be reduced with hydrazinium salts, but this method is more laborious than the thermal route of Kerr and Schweizer.

Although PtCl₂ can form when platinum metal contacts hot chlorine gas, this process suffers from over-chlorination to give PtCl₄. Berzelius and later Wöhler and Streicher showed that upon heating to 450 °C, this Pt(IV) compound decomposes to the Pt(II) derivative:^{[4] [5]}

PtCl₄ → PtCl₂ + Cl₂

Transformations such as this are "driven" by entropy, the free energy gained upon the release of a gaseous product from a solid. Upon heating to still higher temperatures, PtCl₂ releases more chlorine to give metallic Pt. This conversion is the basis of the gravimetric assay of the purity of the PtCl₂ product.

Reactions

Most reactions of PtCl₂ proceed via treatment with ligands (L) to give molecular derivatives. These transformations entail depolymerization via cleavage of Pt-Cl-Pt linkages:

PtCl₂ + 2 L → PtCl₂L₂Addition of ammonia gives initially "PtCl₂(NH₃)₂", "Magnus's green salt", also described as [Pt(NH₃)₄][PtCl₄].

Many complexes have been described, the following are illustrative:^[6]

• pink K₂PtCl₄, a widely employed water-soluble derivative.
• colorless cis-PtCl₂(NH₃)₂, better known as cisplatin.
• colorless cis-PtCl₂(P(C₆H₅)₃)₂, a common precursor to other complexes of the type PtX(Cl)(P(C₆H₅)₃)₂ (X = H, CH₃, etc.).
• yellow trans-PtCl₂(P(C₆H₅)₃)₂, a metastable relative of the cis- isomer.
• colorless dichloro(cycloocta-1,5-diene)platinum(II) (Pt(cod)Cl₂), an "organic-soluble" compound containing a labile organic ligand.

Several of these compounds are of interest in homogeneous catalysis in the service of organic synthesis or as anti-cancer drugs.

See also

• Platinum(IV) chloride

Notes and References

1. Holleman, A. F.; Wiberg, E. Inorganic Chemistry Academic Press: San Diego, 2001. .
2. Bernt . Krebs . Claus . Brendel . Harald . Schäfer . Neue Untersuchungen an α-Platindichlorid Darstellung und Struktur . . 1988 . 561 . 1 . 119–131 . 10.1002/zaac.19885610113 .
3. Book: Kerr, G. T. . Schweizer, A. E. . Inorganic Syntheses . β-Platinum(II) Chloride . . 20 . 48–49 . 10.1002/9780470132517.ch14 . 2007 . 978-0-470-13251-7 . https://archive.org/details/inorganicsynthes0000unse/page/48 .
4. Wöhler, L. . Streicher, S. . . Über das Beständigkeitsgebiet von vier wasserfreien Platinchloriden, über die Flüchtigkeit des Metalls im Chlorgas und die Darstellung sauerstoff-freien Chlors . 1913 . 46 . 1591–1597 . 10.1002/cber.19130460252 . 2.
5. A. E. Schweizer . G. T. Kerr . Thermal decomposition of hexachloroplatinic acid . Inorganic Chemistry . 1978 . 17 . 8 . 2326–2327 . 10.1021/ic50186a067 . en.
6. Cotton, S. A. Chemistry of Precious Metals, Chapman and Hall (London): 1997.