# Bismuthyl (ion)

Bismuthyl is an inorganic oxygen-containing singly charged ion with the chemical formula BiO$^+$, and is an oxycation of bismuth in the +3 oxidation state. Most often it is formed during the hydrolysis of trivalent bismuth salts, primarily nitrate, chloride and other halides. In chemical compounds, bismuthyl plays the role of a monovalent cation.
In inorganic chemistry bismuthyl has been used to describe compounds such as BiOCl which were assumed to contain the diatomic bismuthyl, BiO$^+$, cation, that was also presumed to exist in aqueous solution.
This diatomic ion is not now believed to exist. Unlike other inorganic radicals such as hydroxyl, carbonyl, chromyl, uranyl or vanadyl, according to the current IUPAC rules, the name bismuthyl for BiO$^+$ is not recommended, since individual molecules of these groups are not identifiable but atomic layers of Bi and O. Their presence in compounds preferably should be referred to as oxides.$^{:16}$ However, the latter position remains controversial. For example, to this day the Russian school of inorganic chemistry still operates with bismuthyl and stibil (antimonyl) cations as actually existing radicals.

## In the history of chemistry

Until the last quarter of the 20th century, the real existence of the bismuthyl ion was not in doubt; it was fully present in all reference books and manuals on inorganic chemistry, including German and English ones. The most famous compound of this class was considered bismuthyl chloride, the chemical properties of which were studied in detail and were considered titular for all other bismuth compounds.$^{:144}$ In addition, the compound with the calculation formula BiOCl exists in nature in the form of bismoclitea, one of the secondary metamorphosed minerals from the class of halides.
In the fundamental three-volume book “Modern Inorganic Chemistry” by Nobel laureate Frank Cotton and Geoffrey Wilkinson, summarizing the latest achievements of science in the first half of the 20th century, the real existence of the bismuthyl cation is not only not questioned, but is not even discussed in any detail. This inorganic radical is mentioned without further explanation and is by default considered a legacy of the fundamental corpus of inorganic chemistry of the 19th century. First of all, the authors note that of the entire group of pnictogens, only bismuth has a truly extensive and detailed cation chemistry. According to the authors, aqueous solutions of bismuth salts contain well-defined hydrated cations. Moreover, bismuthyl in the newest version at that time also acquires quasi-polymeric properties, connecting into chains or hexagons. For example, in neutral perchlorate solutions the main ions are [Bi$_6$O$_6$]$^{6+}$ or its hydrated form [Bi$_6$(OH)$_{12}$]$^{6+}$, and at higher pH values [Bi$_6$O$_6$(OH)$_3$]$^{3+}$ are formed.$^{:II:364}$

## In mineralogy and geochemistry

Previously, it was believed that bismuthyl plays almost the main role in the geochemistry of bismuth and metamorphic processes taking place in a liquid medium. Already in ore waters, bismuth and its main compounds are oxidized, forming a sparingly soluble oxychloride — bismoclite, which, when mixed with bicarbonate background waters, is replaced by an even more sparingly soluble — bismuthite. As a result, small amounts of bismuth circulate in both ore and background waters precisely in the form of bismuthyl ion.$^{:291}$
The migration of bismuth in neutral and slightly alkaline groundwater in the form of a simple bismuth ion is hindered as a result of the low threshold pH for the precipitation of its hydroxide from solution. According to thermodynamic calculations carried out in the late 1960s for the stability fields of native bismuth, bismuthinite, bismuth oxides and bismuthyl chloride, in the pH–Eh coordinates the main ion form of bismuth migration was the bismuthyl ion BiO$^+$. According to calculations, it occupied a leading place in the metabolic and oxidative processes that constantly take place in the erosion zones of bismuth minerals.
Bismuthyl chloride, along with BiO(NO)$_3$ nitrate, which was originally considered the title compound of this cation, actually exists in nature in the form of bismoclite, one of the secondary metamorphosed minerals from the class of halides. According to the chemical formula conventionally recognized back in the 19th century, bismoclite consisted precisely of bismuthyl cations (BiO$^+$) and chlorine anions (Cl$^–$). Thus, previously the chemical composition of this mineral was traditionally called bismuthyl chloride. However, by the end of the 20th century, based on the results of targeted chemical analyses, the reality of the existence of the diatomic bismuthyl ion was called into question. Since then, bismoclite has been characterized as bismuth oxide-chloride (oxychloride). In the same way, it was proposed to rename all similar bismuthyl compounds, primarily the remaining halides (from fluoride to iodide) and nitrate.

## Chemical properties

The classic method for obtaining bismuthyl salts was the treatment of bismuth oxide (Bi $_2$O $_3$) with nitric acid. This reaction produces bismuthyl salts such as BiO(NO$_3$ and Bi$_2$O$_2$(OH)(NO$_3$) as end products. The same bismuthyl salts precipitate when strongly acidic solutions of various bismuth compounds are diluted.$^{:II:364}$
The formation of bismuthyl was also considered to be a process that constantly occurs as a result of hydrolysis. Thus, bismuth nitrate, Bi(NO$_3$)$_3$ • 5H$_2$0, crystallizes from a solution resulting from the reaction of bismuth with nitric acid. It dissolves in a small amount of water acidified with nitric acid. However, when the solution is diluted with larger quantities of water, hydrolysis occurs and basic salts precipitate, the composition of which depends on the conditions. A salt of the composition BiONO$_3$ is often formed.$^{:416}$
Bismuthyl chloride (BiOCl) is readily soluble in hydrochloric acid. Moreover, this process, like nitrate, proceeds through a reversible reaction; a shift of the reaction to the left or right also occurs along the line of hydrolysis, depending on the relative amount of water and the (residual) hydrochloric acid present. Adding water to a slightly acidic solution of ВіСl$_3$ immediately causes the appearance of a white precipitate of basic bismuth chloride, BiOCl. When hydrochloric acid is added, the precipitate dissolves again, but it immediately falls out when more water is added. All other bismuth compounds behave in aqueous solutions similarly to chloride.$^{:144}$
In more detail, the ongoing hydrolysis reactions using bismuth chloride as an example are usually represented by the following reversible equations:
                BiCl$_3$ + H$_2$O ↔ BiOHCl$_2$ + HCl
                BiOHCl$_2$ + H$_2$O ↔ Bi(OH)$_2$Cl + HCl
The resulting dihydroxobismuth chloride is unstable and easily splits off a water molecule:
                Bi(OH)$_2$Cl = BiOCl + H$_2$O
The output is a basic salt containing a bismuthyl cation ВiO$^+$, i.e. ″bismuthyl″ chloride.
Bismuth nitrate is hydrolyzed in the same way, forming the main salt of the composition BiONO$_3$. However, the reaction with it in an aqueous environment is much less successful and does not have such a clear result, since the resulting bismuthyl nitrate is much more soluble in water than its chloride.
The hydrolysis reaction of bismuth salts is reversible, and therefore when heated and hydrochloric acid is added to the precipitate, it dissolves again:
                BiOCl + 2HCl = BiCl$_3$ + H$_2$O
When the solution is diluted again with water, a precipitate of the basic salt precipitates again.$^{:104}$
The main mechanism in such reactions is the pronounced amphotericity of X(ОН)$_3$ hydroxides for arsenic and antimony and the basic properties for bismuth, as a result of which the salts are susceptible to hydrolysis, especially in the case of antimony and bismuth, which are characterized by the formation of antimonyl cations SbO$^+$ and bismuthyl BiO$^+$. According to this principle, Bi(OH)$_3$, losing water when heated, turns into yellow bismuthyl hydroxide with the formula BiO(OH), sparingly soluble in water, which upon further dehydration forms Bi$_2$O$_3$ oxide.$^{:129}$
At elevated temperatures, the vapors of the metal combine rapidly with oxygen, forming the yellow trioxide, Bi $_2$O $_3$. When molten, at temperatures above 710 °C, this oxide corrodes any metal oxide and even platinum. On reaction with a base, it forms two series of oxyanions: BiO$^−$ $_2$, which is polymeric and forms linear chains, and BiO$^{3−}$ $_3$. The anion in Li $_3$BiO $_3$ is a cubic octameric anion, Bi $_8$O$^{24−}$ $_{24}$, whereas the anion in Na $_3$BiO $_3$ is tetrameric.
In addition to bismuthyl itself, thiocompounds corresponding to bismuthyl salts are also considered indicative for the chemistry of bismuth, for example, gray thiobismuthyl chloride with the formula BiSCl and others similar to it. These substances, unlike bismuthyl salts, are very stable with respect to water, and can be easily prepared by the action of hydrogen sulfide gas on the corresponding bismuth trihalide.$^{:278}$

## Practical significance

* The mineral bismoclite (bismuthyl chloride) has a traditional use as one of the secondary bismuth ores that are constantly formed in oxidation zones. When mixed with other associated ores, it will become the raw material for the production of pure bismuth and its compounds.
* In medical diagnostics, bismoclite (in the form of purified bismuth oxychloride) is used as a local radiocontrast agent.
* In addition, in the production of cosmetics, bismoclite is used as an enhancing additive; it gives a pearlescent shine to lipstick, nail polish and eye shadow.
* In the chemical industry, in the process of cracking hydrocarbons, bismuthyl chloride is used as a catalyst.
* The bismuthyl cation is also widely involved in the synthesis of bismuth-organic compounds, including those with pharmaceutical applications.