# Bismuth oxynitrate

Bismuth oxynitrate is the name applied to a number of compounds that contain Bi$^{3+}$, nitrate ions and oxide ions and which can be considered as compounds formed from Bi$_2$O$_3$, N$_2$O$_5$ and H$_2$O. Other names for bismuth oxynitrate include bismuth subnitrate and bismuthyl nitrate. In older texts bismuth oxynitrate is often simply described as BiONO$_3$ 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 Bi$_5$O(OH)$_9$(NO$_3$)$_4$ (CAS number: 1304-85-4 ) or as BiONO$_3$·H$_2$O (CAS Number: 13595-83-0 ).
Some compounds have been fully characterised with single crystal studies and found to contain the octahedral [Bi$_6$O$_x$(OH)$_{8−x}$]$^{(10−x)+}$ cation. There is indirect evidence that either the octahedral cation Bi $_6$O $_4$(OH)$^{6+}$ $_4$ or the octahedral cation Bi $_6$(OH)$^{6+}$ $_{12}$ is present in aqueous solution following the polymerisation of Bi(H $_2$O)$^{3+}$ $_8$, the Bi$^{3+}$ ion present in acidic solutions. The ion Bi $_6$O $_4$(OH)$^{6+}$ $_4$ is found in the perchlorate compound Bi$_6$O$_4$(OH)$_4$ClO$_4$·7H$_2$O and is isoelectronic with the octahedral Sn$_6$O$_4$(OH)$_4$ cluster found in the hydrate of tin(II) oxide, 3SnO·H$_2$O. The compounds that contain this are:
    Bi$_6$O$_4$(HO)$_4$(NO$_3$)$_6$·H$_2$O (equivalent to BiONO$_3$·⁠1/2⁠H$_2$O; Bi$_2$O$_3$·N$_2$O$_5$·H$_2$O )
    Bi$_6$O$_4$(OH)$_4$(NO$_3$)$_6$·4H$_2$O (equivalent to BiONO$_3$·H$_2$O; Bi$_2$O$_3$·N$_2$O$_5$·6H$_2$O )
    [Bi$_6$O$_4$(OH)$_4$][Bi$_6$O$_5$(OH)$_3$](NO$_3$)$_{11}$, which contains two different cations, [Bi$_6$O$_4$(OH)$_4$]$^{6+}$ and [Bi$_6$O$_5$(OH)$_3$]$^{5+}$
The compound Bi$_6$O$_5$(OH)$_3$(NO$_3$)$_5$·3H$_2$O (equivalent to 6Bi$_2$O$_3$·5N$_2$O$_5$·9H$_2$O) also contains the octahedral units but this time they are joined to form {[Bi$_6$O$_5$(OH)$_3$]$^{5+}$}$_2$.
Additionally some oxynitrates have layer structures (a common motif also found in bismuth(III) oxyhalides):
    Bi$_2$O$_2$(OH)NO$_3$ (equivalent to BiONO$_3$·⁠1/2⁠H$_2$O) contains "[Bi$_2$O$_2$]$^{2+}$" layers
    Bi$_5$O$_7$NO$_3$, which is isostructural with β-Bi$_5$O$_7$I

## Cluster cation structure

The octahedral ion has 6 Bi$^{3+}$ 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 Bi$^{3+}$ and the structure concludes that the lone pairs on the Bi$^{3+}$ 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, BiNO$_3$·5H$_2$O with KOH, or the controlled thermal decomposition of the pentahydrate.
The thermal decomposition of bismuth nitrate pentahydrate proceeds through the following stages:
    At pH below 1.0, Bi$_6$O$_4$(OH)$_4$(NO$_3$)$_6$·4H$_2$O (equivalent to BiNO$_3$·H$_2$O) is the first solid product, which when heated produces Bi$_6$H$_2$O(NO$_3$)O$_4$(OH)$_4$ (equivalent to BiNO$_3$.⁠1/2⁠H$_2$O).
    Between pH 1.2 and 1.8, further hydrolysis occurs and Bi$_6$O$_5$(OH)$_3$(NO$_3$)$_5$·3H$_2$O is formed.
The final oxynitrate product of thermal dehydration is believed to be Bi$_5$O$_7$NO$_3$, which is isostructural with β–Bi$_5$O$_7$I and has a layer structure. The ultimate stage of thermal decomposition of oxynitrates is bismuth(III) oxide, Bi$_2$O$_3$.