Chem 15: TM complex ion colours

What does adding NaOH or NH3 do?

?? OH in excess sol only with Cr (amphoteric)

[Cu(H2O)6]2+ + ?NH3

Dropwise) 2NH3 => Cu(OH)2(H2O)4

Excess) 4NH3 => [Cu(NH3)4(H2O)2]2+

Pale blue solution => Blue precipitate => Deep blue solution

[Cu(H2O)6]2+ + ?NaOH

Dropwise) 2OH− => Cu(OH)2(H2O)4

Excess) No further change

Pale blue solution => Blue precipitate => No further change in excess

[Cr(H2O)6]3+ + ?NaOH

Dropwise) 3OH− => Cr(OH)3(H2O)3

Excess) Cr(OH)3(H2O)3 + OH− => [Cr(OH)6]3−

Green (Violet) solution => Grey-green precipitate => Dark green solution

[Cr(H2O)6]3+ + ?NH3

Dropwise) 3NH3 => Cr(OH)3(H2O)3

Excess) Cr(OH)3(H2O)3 + 6NH3 => [Cr(NH3)6]3+

Green (Violet) solution => Grey-green precipitate => Purple solution

[Fe(H2O)6]2+ + ?NaOH

Dropwise) 2OH− => Fe(OH)2(H2O)4

Excess) No further change (turns brown in air)

Pale green solution => Green precipitate => No further change (turns brown in air due to oxidation to Fe(OH)₃)

[Fe(H2O)6]2+ + ?NH3

Dropwise) 2NH3 => Fe(OH)2(H2O)4

Excess) No further change (turns brown in air)

Pale green solution => Green precipitate => No further change (oxidises to brown Fe(OH)₃)

[Fe(H2O)6]3+ + ?NaOH

Dropwise) 3OH− => Fe(OH)3(H2O)3

Excess) No further change

Yellow solution => Brown precipitate => No further change in excess

[Fe(H2O)6]3+ + ?NH3

Dropwise) 3NH3 => Fe(OH)3(H2O)3

Excess) No further change

Yellow solution => Brown precipitate => No further change in excess

[Co(H2O)6]2+ + ?NaOH

Dropwise) 2OH− => Co(OH)2(H2O)4

Excess) Co(OH)2 oxidises to Co(OH)3 in air

Pink solution => Blue precipitate => Slowly turns brown in air

[Co(H2O)6]2+ + ?NH3

Dropwise) 2NH3 => Co(OH)2(H2O)4

Excess) 6NH3 => [Co(NH3)6]2+

Pink solution => Blue precipitate => Yellow-brown solution