section_path: Compounds/Coordination compounds
section_name: Coordination compounds
paragraph_text: Coordination compounds
  
  Silver complexes tend to be similar to those of its lighter homologue copper. Silver(III) complexes tend to be rare and very easily reduced to the more stable lower oxidation states, though they are slightly more stable than those of copper(III). For instance, the square planar periodate and tellurate complexes may be prepared by oxidising silver(I) with alkaline peroxodisulfate. The yellow diamagnetic is much less stable, fuming in moist air and reacting with glass.
  
  Silver(II) complexes are more common. Like the valence isoelectronic copper(II) complexes, they are usually square planar and paramagnetic, which is increased by the greater field splitting for 4d electrons than for 3d electrons. Aqueous Ag, produced by oxidation of Ag by ozone, is a very strong oxidising agent, even in acidic solutions: it is stabilized in phosphoric acid due to complex formation. Peroxodisulfate oxidation is generally necessary to give the more stable complexes with heterocyclic amines, such as and: these are stable provided the counterion cannot reduce the silver back to the +1 oxidation state. is also known in its violet barium salt, as are some silver(II) complexes with N - or O -donor ligands such as pyridine carboxylates.
  
  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 polarized 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 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; silver salts are dissolved in photography due to the formation of the thiosulfate complex; and cyanide extraction for silver (and gold) works by the formation of the complex. 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 sp - 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.