«The Structure of Complex Ions.»—The ability of ammonia to form complex ions with simple metal ions is commonly ascribed to the unsaturated condition of ammonia (see p. 65). It is interesting to recall the fact, that the marked power of the cyanide-ion to form complexes of extraordinary stability, is also associated with a similarly unsaturated condition of the cyanide-ion (p. 66). According to the results of Nef's researches,[476] we have, in hydrocyanic acid and the cyanides, unsaturated or bivalent carbon; potassium cyanide, for instance, has the structure K—N=C<. The two free valences of the carbon atom may, according to the electrical theory of valence, be again considered to consist of one negative and one positive charge. The possibility of the formation of complexes is then self-evident, and we can readily see, how silver cyanide, Ag—N=C±, should absorb cyanide-ion,[477] ∓C=N^{−}, and form a complex (Ag—N=C=C=N^{−}) or Ag(CN)_{2}^{−}, whose potassium salt would be potassium argenticyanide, K[Ag(CN)_{2}]. For the potassium salt K_{2}Ag(CN)_{3}, of the second complex ion[478] [Ag(CN)_{3}^{2−}] of silver and cyanogen, the most likely structure is Ag—N=C—C=N—K. \ / C=N—K The other complex cyanide ions, ferrocyanide, ferricyanide, cobalticyanide, etc., are considered to have structures entirely similar to those given to the argenticyanide ions.[479] «Complex Halide, Sulphide, Oxide and Oxonium Ions.»—In conclusion, there are other elements besides nitrogen (in ammonia and its derivatives) and carbon (in cyanides), which form complex ions with simple ions; notably the halogens form such complexes. Chloroplatinic acid, H_{2}PtCl_{6}, and its salts (Lab. Manual, ‹q. v.›), fluorosilicic acid, H_{2}SiF_{6}, potassium-mercuric iodide, [p238] KHgI_{3} and K_{2} HgI_{4},[480] which forms a most sensitive reagent for the detection of traces of ammonia,[481] are instances of complexes of the halogens that are of importance in analysis. It is worthy of note that the most likely structure[482] for these compounds, ‹e.g.› Cl_{2}=Pt=(Cl=Cl—H)_{2}, bears a very striking analogy to the structures frequently assigned to the cyanide and ammonia complex ions. Oxygen and sulphur form complex ions, of great stability, with many elements, and we shall presently have occasion to discuss in detail some of the complex ions formed by sulphur. All of the oxygen acids may be treated as containing complex ions of oxygen and some other element, and their instability constants[483] are factors in determining their chemical behavior. For instance, such is the case, most likely, for their behavior as oxidizing and reducing agents (see Chapter XVI). The unsaturated condition of oxygen in water, (H_{2}O±), makes possible, also, the formation of complex ions, [(H_{2}O)_{‹x›}Me]^{+}, etc., called oxonium ions and comparable with metal-ammonium ions. They form a most inviting field for rigorous investigation.