Inorganic carbonates usually contain a planar trigonal [CO₃]^2– unit. There are also carbonates with a tetrahedrally coordinated carbon atom with four surrounding oxygen atoms, i.e., carbonates with [CO₄]^4– units that may polymerize by corner-sharing. These can be obtained, for example, at very high pressures and temperatures. Pyrocarbonates, which contain [C₂O₅]^2– units, can be found as pyrocarbonate esters in organic chemistry, but inorganic pyrocarbonates with isolated [C₂O₅]^2– complex anions had not been reported so far.

Dominik Spahr, Goethe University Frankfurt, Frankfurt am Main, Germany, and colleagues have synthesized the inorganic pyrocarbonate salt Sr[C₂O₅], which contains isolated [C₂O₅]^2– groups (structure pictured). The team reacted SrCO₃ and CO₂ at pressures of ca. 30 GPa and temperatures of ca. 2000 K in a laser-heated diamond anvil cell. The product was characterized using synchrotron-based single-crystal X-ray diffraction, which confirmed the presence of a novel pyrocarbonate with the composition Sr[C₂O₅].

The product crystallizes in the monoclinic space group P2₁/c. The [C₂O₅]^2– groups can be considered to consist of two [CO₃]^2– units that share one oxygen atom. The team proposes that other inorganic pyrocarbonates, such as Mg[C₂O₅], Ca[C₂O₅], Ba[C₂O₅], and Pb[C₂O₅], might also be experimentally accessible.

• Sr[C₂O₅] is an Inorganic Pyrocarbonate Salt with [C₂O₅]^2– Complex Anions,
  Dominik Spahr, Jannes König, Lkhamsuren Bayarjargal, Victor Milman, Alexander Perlov, Hanns-Peter Liermann, Björn Winkler,
  J. Am. Chem. Soc. 2022.
  https://doi.org/10.1021/jacs.2c00351

Correction (February 10, 2022)
The article originally misidentified pyrocarbonates as pyroborates in the last sentence.