Bismuth oxynitrate

Bismuth oxynitrate is the name applied to a number of compounds that contain Bi³⁺, nitrate ions and oxide ions and which can be considered as compounds formed from Bi₂O₃, N₂O₅ and H₂O. Other names for bismuth oxynitrate include bismuth subnitrate and bismuthyl nitrate. In older texts bismuth oxynitrate is often simply described as BiONO₃ or basic bismuth nitrate. Bismuth oxynitrate was once called magisterium bismuti or bismutum subnitricum, and was used as a white pigment, in beauty care, and as a gentle disinfectant for internal and external use. It is also used to form Dragendorff's reagent, which is used as a TLC stain.

Hydrates

Bismuth oxynitrate is commercially available as Bi₅O(OH)₉(NO₃)₄ (CAS number: ) or as BiONO₃·H₂O (CAS Number: ).

Some compounds have been fully characterised with single crystal studies and found to contain the octahedral [Bi₆O_x(OH)_{8−x}]^{(10−x)+} cation. There is indirect evidence that either the octahedral cation or the octahedral cation is present in aqueous solution following the polymerisation of , the Bi³⁺ ion present in acidic solutions. The ion is found in the perchlorate compound Bi₆O₄(OH)₄ClO₄·7H₂O and is isoelectronic with the octahedral Sn₆O₄(OH)₄ cluster found in the hydrate of tin(II) oxide, 3SnO·H₂O. The compounds that contain this are:

The compound Bi₆O₅(OH)₃(NO₃)₅·3H₂O (equivalent to 6Bi₂O₃·5N₂O₅·9H₂O) also contains the octahedral units but this time they are joined to form {[Bi₆O₅(OH)₃]⁵⁺}₂.

Additionally some oxynitrates have layer structures (a common motif also found in bismuth(III) oxyhalides):

Cluster cation structure

The octahedral ion has 6 Bi³⁺ ions at the corners of an octahedron. There is no covalent bond between the Bi atoms, they are held in position by bridging O^{2−} and OH^− anions, one at the centre of each of the eight triangular faces, bridging three Bi ions. The Bi ions are essentially four coordinate and are at the apex of a flat square pyramid. An ab initio theoretical study of the hydration mechanism of Bi³⁺ and the structure concludes that the lone pairs on the Bi³⁺ ions are stereochemically active.

Preparation

Bismuth oxynitrates can be prepared from bismuth(III) nitrate. For example, hydrolysis of a solution of bismuth nitrate through the addition of alkali or the reaction of the pentahydrate, BiNO₃·5H₂O with KOH, or the controlled thermal decomposition of the pentahydrate.

The thermal decomposition of bismuth nitrate pentahydrate proceeds through the following stages:

The final oxynitrate product of thermal dehydration is believed to be Bi₅O₇NO₃, which is isostructural with β–Bi₅O₇I and has a layer structure. The ultimate stage of thermal decomposition of oxynitrates is bismuth(III) oxide, Bi₂O₃.

References