Similarly,
Cr^3 3NH 3 3H 2 O88nCr(OH) 3 (s)3NH 4
In general terms we can represent this reaction as
MnnNH 3 nH 2 O88nM(OH)n(s)nNH 4
where Mnrepresents all of the common metal ions exceptthe cations of the strong bases
(Group IA cations and the heavier members of Group IIA: Ca^2 , Sr^2 , and Ba^2 ).
The hydroxides of some metals and some metalloids are amphoteric (Section 10-6).
Aqueous NH 3 is a weak base (Kb1.8 10 ^5 ), so the [OH] is too low to dissolve
amphoteric hydroxides to form hydroxo complexes.
However, several metal hydroxides do dissolve in an excess of aqueous NH 3 to form
ammine complexes. For example, the hydroxides of copper and cobalt are readily soluble
in an excess of aqueousammonia solution.
Co(OH) 2 (a blue compound that
turns gray quickly) dissolves in
excess aqueous NH 3 to form
[Co(NH 3 ) 6 ]^2 ions (yellow-orange).
976 CHAPTER 25: Coordination Compounds
TABLE 25-3 Common Metal Ions That Form Soluble Complexes with an
Excess of Aqueous Ammoniaa
Insoluble Hydroxide Formed Complex Ion Formed
Metal Ion by Limited Aq. NH 3 by Excess Aq. NH 3
Co^2 Co(OH) 2 [Co(NH 3 ) 6 ]^2
Co^3 Co(OH) 3 [Co(NH 3 ) 6 ]^3
Ni^2 Ni(OH) 2 [Ni(NH 3 ) 6 ]^2
Cu CuOH88n^12 Cu 2 Ob [Cu(NH 3 ) 2 ]
Cu^2 Cu(OH) 2 [Cu(NH 3 ) 4 ]^2
Ag AgOH88n^12 Ag 2 Ob [Ag(NH 3 ) 2 ]
Zn^2 Zn(OH) 2 [Zn(NH 3 ) 4 ]^2
Cd^2 Cd(OH) 2 [Cd(NH 3 ) 4 ]^2
Hg^2 Hg(OH) 2 [Hg(NH 3 ) 4 ]^2
aThe ions of Rh, Ir, Pd, Pt, and Au show similar behavior.
bCuOH and AgOH are unstable and decompose to the corresponding oxides.
Cu(OH) 2 (light blue) dissolves in excess aqueous NH 3 to form [Cu(NH 3 ) 4 ]^2 ions (deep blue).