Oxides and chalcogenides Silver and gold have rather low
chemical affinities for oxygen, lower than copper, and it is therefore expected that silver oxides are thermally quite unstable. Soluble silver(I) salts precipitate dark-brown
silver(I) oxide, Ag2O, upon the addition of alkali. (The hydroxide AgOH exists only in solution; otherwise it spontaneously decomposes to the oxide.) Silver(I) oxide is very easily reduced to metallic silver, and decomposes to silver and oxygen above 160 °C. This and other silver(I) compounds may be oxidised by the strong oxidising agent
peroxodisulfate to black AgO, a mixed
silver(I,III) oxide of formula AgIAgIIIO2. Some other mixed oxides with silver in non-integral oxidation states, namely Ag2O3 and Ag3O4, are also known, as is Ag3O which behaves as a metallic conductor. In stark contrast to this, all four silver(I) halides are known. The
fluoride,
chloride, and
bromide have the sodium chloride structure, but the
iodide has three known stable forms at different temperatures; that at room temperature is the cubic
zinc blende structure. They can all be obtained by the direct reaction of their respective elements. :X− +
hν → X + e− (excitation of the halide ion, which gives up its extra electron into the conduction band) :Ag+ + e− → Ag (liberation of a silver ion, which gains an electron to become a silver atom) The process is not reversible because the silver atom liberated is typically found at a
crystal defect or an impurity site, so that the electron's energy is lowered enough that it is "trapped". It is often used for gravimetric analysis, exploiting the insolubility of the heavier silver halides which it is a common precursor to. Yellow
silver carbonate, Ag2CO3 can be easily prepared by reacting aqueous solutions of
sodium carbonate with a deficiency of silver nitrate. Its principal use is for the production of silver powder for use in microelectronics. It is reduced with
formaldehyde, producing silver free of alkali metals: :Ag2CO3 + CH2O → 2 Ag + 2 CO2 + H2 Silver carbonate is also used as a
reagent in organic synthesis such as the
Koenigs–Knorr reaction. In the
Fétizon oxidation, silver carbonate on
celite acts as an
oxidising agent to form
lactones from
diols. It is also employed to convert
alkyl bromides into
alcohols. and
silver acetylide, Ag2C2, formed when silver reacts with
acetylene gas in ammonia solution. By far the most important oxidation state for silver in complexes is +1. The Ag+ cation is diamagnetic, like its homologues Cu+ and Au+, as all three have closed-shell electron configurations with no unpaired electrons: its complexes are colourless provided the ligands are not too easily polarised such as I−. Ag+ forms salts with most anions, but it is reluctant to coordinate to oxygen and thus most of these salts are insoluble in water: the exceptions are the nitrate, perchlorate, and fluoride. The tetracoordinate tetrahedral aqueous ion [Ag(H2O)4]+ is known, but the characteristic geometry for the Ag+ cation is 2-coordinate linear. For example, silver chloride dissolves readily in excess aqueous ammonia to form [Ag(NH3)2]+; silver salts are dissolved in photography due to the formation of the thiosulfate complex [Ag(S2O3)2]3−; and
cyanide extraction for silver (and gold) works by the formation of the complex [Ag(CN)2]−. Silver cyanide forms the linear polymer {Ag–C≡N→Ag–C≡N→}; silver
thiocyanate has a similar structure, but forms a zigzag instead because of the sp3-
hybridized sulfur atom.
Chelating ligands are unable to form linear complexes and thus silver(I) complexes with them tend to form polymers; a few exceptions exist, such as the near-tetrahedral
diphosphine and
diarsine complexes [Ag(L–L)2]+.
Organometallic Under standard conditions, silver does not form simple carbonyls, due to the weakness of the Ag–C bond. A few are known at very low temperatures around 6–15 K, such as the green, planar paramagnetic Ag(CO)3, which dimerises at 25–30 K, probably by forming Ag–Ag bonds. Additionally, the silver carbonyl [Ag(CO)][B(OTeF5)4] is known. Polymeric AgLX complexes with
alkenes and
alkynes are known, but their bonds are thermodynamically weaker than even those of the
platinum complexes (though they are formed more readily than those of the analogous gold complexes): they are also quite unsymmetrical, showing the weak
π bonding in group 11. Ag–C
σ bonds may also be formed by silver(I), like copper(I) and gold(I), but the simple alkyls and aryls of silver(I) are even less stable than those of copper(I) (which tend to explode under ambient conditions). For example, poor thermal stability is reflected in the relative decomposition temperatures of AgMe (−50 °C) and CuMe (−15 °C) as well as those of PhAg (74 °C) and PhCu (100 °C). The C–Ag bond is stabilised by
perfluoroalkyl ligands, for example in AgCF(CF3)2. Alkenylsilver compounds are also more stable than their alkylsilver counterparts. Silver-
NHC complexes are easily prepared, and are commonly used to prepare other NHC complexes by displacing labile ligands. For example, the reaction of the bis(NHC)silver(I) complex with
bis(acetonitrile)palladium dichloride or
chlorido(dimethyl sulfide)gold(I): :
Intermetallic Silver forms
alloys with most other elements on the periodic table. The elements from groups 1–3, except for
hydrogen,
lithium, and
beryllium, are very miscible with silver in the condensed phase and form intermetallic compounds; those from groups 4–9 are only poorly miscible; the elements in groups 10–14 (except
boron and
carbon) have very complex Ag–M phase diagrams and form the most commercially important alloys; and the remaining elements on the periodic table have no consistency in their Ag–M phase diagrams. By far the most important such alloys are those with copper: most silver used for coinage and jewellery is in reality a silver–copper alloy, and the
eutectic mixture is used in vacuum
brazing. The two metals are completely miscible as liquids but not as solids; their importance in industry comes from the fact that their properties tend to be suitable over a wide range of variation in silver and copper concentration, although most useful alloys tend to be richer in silver than the eutectic mixture (71.9% silver and 28.1% copper by weight, and 60.1% silver and 28.1% copper by atom). Most other binary alloys are of little use: for example, silver–gold alloys are too soft and silver–
cadmium alloys too toxic. Ternary alloys have much greater importance: dental
amalgams are usually silver–tin–mercury alloys, silver–copper–gold alloys are very important in jewellery (usually on the gold-rich side) and have a vast range of hardnesses and colours, silver–copper–zinc alloys are useful as low-melting brazing alloys, and silver–cadmium–
indium (involving three adjacent elements on the periodic table) is useful in
nuclear reactors because of its high thermal neutron capture
cross-section, good conduction of heat, mechanical stability, and resistance to corrosion in hot water. ==Etymology==