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Platinum(II) chloride

Platinum(II) chloride describes the inorganic compounds with the formula PtCl<sub>2</sub>. They are precursor used in the preparation of other platinum compounds. Platinum(II) chloride exists in two crystalline forms (polymorphs), but the main properties are somewhat similar: dark brown, insoluble in water, diamagnetic, and odorless.

Structure

The structures of PtCl<sub>2</sub> and PdCl<sub>2</sub> 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&nbsp;°C. In the β form, the Pt-Pt distances are 3.32–3.40&nbsp;Å, indicative of some bonding between the pairs of metals. In both forms of PtCl<sub>2</sub>, 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. The structure of α-PtCl<sub>2</sub> is reported to be disordered and contain edge- and/or corner-sharing square-planar PtCl<sub>4</sub> units.

Preparation

β-PtCl<sub>2</sub> is prepared by heating chloroplatinic acid to 350&nbsp;°C in air.

H<sub>2</sub>PtCl<sub>6</sub> → PtCl<sub>2</sub> + Cl<sub>2</sub> + 2 HCl

This method is convenient since the chloroplatinic acid is generated readily from Pt metal. Aqueous solutions of H<sub>2</sub>PtCl<sub>6</sub> can also be reduced with hydrazinium salts, but this method is more laborious than the thermal route of Kerr and Schweizer.

Although PtCl<sub>2</sub> can form when platinum metal contacts hot chlorine gas, this process suffers from over-chlorination to give PtCl<sub>4</sub>. Berzelius and later Wöhler and Streicher showed that upon heating to 450&nbsp;°C, this Pt(IV) compound decomposes to the Pt(II) derivative:

PtCl<sub>4</sub> → PtCl<sub>2</sub> + Cl<sub>2</sub>

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, PtCl<sub>2</sub> releases more chlorine to give metallic Pt. This conversion is the basis of the gravimetric assay of the purity of the PtCl<sub>2</sub> product.

Reactions

Most reactions of PtCl<sub>2</sub> proceed via treatment with ligands (L) to give molecular derivatives. These transformations entail depolymerization via cleavage of Pt-Cl-Pt linkages:

PtCl<sub>2</sub> + 2 L → PtCl<sub>2</sub>L<sub>2</sub>

Addition of ammonia gives initially "PtCl<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub>", "Magnus's green salt", also described as [Pt(NH<sub>3</sub>)<sub>4</sub>][PtCl<sub>4</sub>].

Many complexes have been described, the following are illustrative:

  • pink K<sub>2</sub>PtCl<sub>4</sub>, a widely employed water-soluble derivative.
  • colorless cis-PtCl<sub>2</sub>(NH<sub>3</sub>)<sub>2</sub>, better known as cisplatin.
  • colorless cis-PtCl<sub>2</sub>(P(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>)<sub>2</sub>, a common precursor to other complexes of the type PtX(Cl)(P(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>)<sub>2</sub> (X = H, CH<sub>3</sub>, etc.).
  • yellow trans-PtCl<sub>2</sub>(P(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>)<sub>2</sub>, a metastable relative of the cis- isomer.
  • colorless dichloro(cycloocta-1,5-diene)platinum(II) (Pt(cod)Cl<sub>2</sub>), 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

References