Dissolution of a Precipitate by Complex Ion Formation
A number of slightly soluble salts do not dissolve readily in acids.  eg. AgCl will not dissolve in nitric acid.
In order to dissolve AgCl, the equilibrium of  AgCl(s)  <=====>  Ag+(aq) + Cl-(aq)
must be shifted to the right.
Two things can do this:
NH3 or CN- form co-ordinate bonds with the Ag+ ions and remove them from the solution. They create complex ions that are even more insoluble then the original AgCl.
The ammonia solution has Ag(NH3)2+ in it, and the cyanide solution has the Ag(CN)2- ion in it.
These two polyatomic ions are weakly dissociated.

The part of the ion which attaches by a co-ordinate bond is called a ligand.

eg. AgCl(s) + 2 NH3(aq) <======> Ag(NH3)2+(aq) + Cl-(aq)

AgCl(s) + 2 CN-(aq) <=======>  Ag(CN)2- + Cl-(aq)

A special nomenclature has been developed for complex ions.
The name of the Ag(NH3)2+ ion is the diamminesilver(I) ion.
The name of the Ag(CN)2- ion is the dicyanoargenate(I) ion.
The rules for naming complex ions are as follows:
1. Use the mono, di, tri system for naming the ligand attachments.
The other common ligands are:
H2O
 aquo 
 Cl-
chloro
NO3-
nitrato 
 S2O32-
thiosulphato
NH3
ammine 
OH
hydroxo
C2O4-
oxalato
CN-
cyano
SO42-
sulfato 
NO2
nitro 

 
Ag(NH3)2+ has two ammine ligands therefore its diammine
 
Ag(CN)2- has two cyano ligands therefore its dicyano
 
2. The charge on the overall molecule is the sum of the metal's charge and the ligands charge.
 
Ag(NH3)2+ The silver ion has a +1 charge and the ammine has no charge therefore +1 +0 +0 = +1 which is the overall charge on the complex.
 
Ag(CN)2- The silver ion still has a +1 charge but now two CN- ions also come into play. +1 -1 -1 = -1 as an overall charge on the complex.
 
3. The overall charge of the metallic atom help determine the name of the complex. Complexes with a positive or neutral charge get the normal metallic name and the charge in brackets.
 
Ag(NH3)2+ The overall charge is +1 therefore the metallic portion is named silver(I) because the silver has a charge of +1.
 
A negative complex has slightly different rules. If the metal is named after an old element use the older name, such as argentum for silver, hydroargentum for mercury, ferrum for iron, etc. Drop the suffix and add the suffix 'ate' which is the signifier of a negative charge. The value of the charge is given in brackets. If the metals name does not stem from an older classical name then use its modern name along with its 'ate' suffix.
 
Ag(CN)2- The overall charge is -1 and the metal is silver whose symbol comes from the old name of argentum, therefore argentate(I) is its name. The 'ate' signifies that the complex is -ve in charge but the (I) is the value of the positive charge on the silver atom.
 
Tl(NO3)63- This would be hexanitrothallate(III)
 
Six nitro groups would be hexanitro
 
Thallium has no old classical name therefore it must remain thallium. Drop the suffix and adding 'ate' gives thallate.
 
What is the charge? 6 nitro's donate -6 to the complex. The overall charge is -3, therefore the thallium's charge is +3. This is indicated by the thallate(III) portion of the name.
 
Examples of complex ions:
  Ligand Formula Name
Ag+ NH3 Ag(NH3)2+ diamminesilver(I)
Ag+ CN-   Ag(CN)2  dicyanoargenate(I)
Ag+  Cl-   Ag(Cl)2 dichloroargenate(I)
Cu2+   NH3 Cu(NH3)42+  tetramminecopper(II)
Cu+2 CN-  Cu(CN)42- tetramminecuprate(II)
Cd2+  NH3 Cd(NH3)42+  tetramminecadmium(II)
Cd2+   CN- Cd(CN)42-  tetracyanocadimate(II)
Co3+ NH3   Co(NH3)63+ hexamminecobalt(III)
Al3+ OH- Al(OH)4-  tetrahydroxoaluminate(III)
Zn2+ NH3 Zn(NH3)42+ tetramminezinc(II)
Zn2+  OH-  Zn(OH)42- tetrahydroxozincate(II)
Follow-Up Problems
Write net ionic equation using complex ions, for the reactions below. In each case name the complex ion formed.

a) copper(II) hydroxide with excess aqueous ammonia.

b) nickel(II) hydroxide with excess sodium cyanide.

c) silver bromide with excess sodium thiosulphate. The complex formed is Ag(S2O3)23-

d) zinc hydroxide with excess sodium hydroxide