Thallium(I) oxalate is an inorganic compound of thallium in the +1 oxidation state and the oxalate ion, with the formula Tl2C2O4. It is a colorless to white crystalline solid and is only slightly soluble in water.
Preparation Thallium(I) oxalate can be prepared by reaction of thallium(I) carbonate with oxalic acid in aqueous solution:
Tl2CO3 + H2C2O4 → Tl2C2O4 + CO2↑ + H2O
This reaction has also been used in crystallographic studies of thallium(I) oxalate derivatives.
Thallium(I) oxalate has also been obtained as a product of redox chemistry in the Tl2O3–HCN/CN−–H2O system. In one study, prolonged reaction of thallium(III) oxide with hydrocyanic acid at about 70 °C produced transparent crystals of Tl2C2O4.
Properties Structure The crystal structure of Tl2C2O4 has been determined by single-crystal X-ray diffraction.
The compound contains planar oxalate anions and irregularly coordinated Tl+ ions. The coordination geometry around thallium is strongly influenced by the stereochemically active 6s2 lone pair characteristic of Tl(I).
A later crystallographic study of the related adduct Tl2C2O4·H2C2O4 likewise found highly irregular oxygen coordination around Tl(I), with nine oxygen atoms within about 3.25 Å.
Solubility Thallium(I) oxalate is only slightly soluble in water. A solubility of about 1.8 g per 100 g of water at room temperature has been tabulated.
Thermal decomposition On heating, thallium(I) oxalate decomposes rather than boiling. Older thermal studies place decomposition in the approximate range 315–320 °C.
The decomposition can be represented overall as formation of thallium(I) oxide and gaseous carbon oxides. Thermal studies of thallium oxalates show Tl2C2O4 as a stable intermediate below about 290 °C in decomposition sequences of thallium(III) oxalato compounds.
Oxalic acid adduct Reaction of thallium(I) carbonate with a twofold molar excess of oxalic acid does not necessarily give simple thallium hydrogen oxalate. Instead, the crystalline adduct Tl2C2O4·H2C2O4 can form.
This compound crystallizes in the monoclinic space group P21/a with a = 6.2891 Å, b = 6.3974 Å, c = 10.3618 Å, β = 94.526°, and Z = 2. The oxalate ion and oxalic acid molecule are both planar and linked by strong O–H···O hydrogen bonds into polymeric chains.
Safety Thallium(I) oxalate is highly toxic because of the toxicity of soluble and bioavailable thallium(I) compounds. Contact and ingestion should be avoided.
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