Thiocarbonic acid

It is often referred to as trithiocarbonic acid so as to differentiate it from other carbonic acids containing sulfur, such as monothiocarbonic O,O-acid S=C(OH)₂, monothiocarbonic O,S-acid O=C(OH)(SH), dithiocarbonic O,S-acid S=C(OH)(SH) and dithiocarbonic S,S-acid O=C(SH)₂ (see thiocarbonates).

Discovery and synthesis

It was first reported in brief by Zeise in 1824 and later in more detail by Berzelius in 1826, in both cases it was produced by the action of carbon disulfide on a hydrosulfide salt (e.g. potassium hydrosulfide).

CS₂ + 2 KSH → K₂CS₃ + H₂S

Treatment with acids liberates the thiocarbonic acid as a red oil:

K₂CS₃ + 2 HX → H₂CS₃ + 2 KX

Both the acid and many of its salts are unstable and decompose via the release of carbon disulfide, particularly upon heating:

H₂CS₃ → CS₂ + H₂S

An improved synthesis involves addition of barium trithiocarbonate to hydrochloric acid at 0 °C. This method provided samples with which many measurement have been made.

BaCS₃ + 2 HCl → H₂CS₃ + BaCl₂

Despite its lability, crystals of thiocarbonic acid have been examined by X-ray crystallography, which confirms the anticipated molecular structure of a trigonal planar molecular geometry at the central carbon atom. The C-S bond lengths range from 1.69 to 1.77 Å.