Complex ions usually refer to cations in which surrounding water molecules have been replaced by some other electron pair donor. We write:
Cu2+ --> Cu(NH3)42+
In this reaction, we neglect water molecules surrounding the copper(II) ion. Complex ions are studied for many reasons, not the least of which is that many such ions are very colorful:
The well at the left contains copper chloride and water. Beginning from the right, the wells contain increasing amounds of aqueous ammonia solution. The deep blue color in the 2nd well from the left is due to Cu(NH3)42+.
The pale bule color at the left is due to Cu(H2O)42+ and some copper chloride complexes (greenish).
Questions that deal with the interplay of precipitation reactions and complex ion formation frequently are found on the AP exam. Here is a sequence of reactions of Ag+:
These reactions are written as:
Ag+ + CO32- --> Ag2CO3
Ag2CO3 + PO43- --> Ag3PO4 + CO32-
Ag3PO4 + OH- --> Ag2O + PO43- + H2O
Ag2O + Cl- + H+ --> AgCl + H2O
AgCl + NH3 --> Ag(NH3)2+ + Cl-
Ag(NH3)2+ + Br- --> AgBr + NH3
AgBr + S2O32- --> Ag(S2O3)23- + Br-
Ag(S2O3)23- + I- --> AgI + S2O32-
AgI + HS- --> Ag2S + I- + H+
The silver ammine complex, Ag(NH3)2+, is a relatively frequent visitor on AP tests. Many cations (Ag+, Cu2+, Cd2+, Zn2+, and many others) form complexes with ammonia. These sometimes are called ammine complexes.
Another reaction type frequent seen is the formation of a hydroxide followed by the formation of a soluble hydroxide complex ion. Fe(OH)3 and Mg(OH)2 do not form soluble complexes, but aluminum and zinc do:
Zn(OH)2 + OH- --> Zn(OH)42-
Al(OH)3 + OH- --> Al(OH)4-
Finally, a favorite reaction is that of ferric ion with thiocyanante ion (SCN-):
Fe3+ + SCN- FeSCN2+