Platinum(II) chloride

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.

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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]

Preparation

β-PtCl₂ is prepared by heating chloroplatinic acid to 350 °C in air.^[2]

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 must 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:^[3]

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.

Uses

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 → PtCl₂L₂

Sometimes, such reactions can be deceptive. Addition of ammonia gives initially "PtCl₂(NH₃)₂", but this material is in fact Magnus' green salt, [PtCl₄][Pt(NH₃)₄].

Of the many such complexes that have been described, the following are illustrative:^[4]

• 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 commonly employed, easily crystallized species that is widely employed as a precursor 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 PtCl₂(cyclooctadiene), 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.

References