Many d block compounds give colours in aqueous state. Also d block metal ions forms precipitates with anions such as CO32- , OH-. We can identify these metals or compounds using these precipitates and colours. Some d block metal ions give precipitates with NaOH, ammonia and more solutions. These d block metal hydroxides, sulfates, chlorides and other precipitates and solutions take different colours. d metals have different oxidation numbers such as +2 and +3 for ions. Therefore, Fe(OH)2(s) and Fe(OH)3(s) are green and brown precipitates.
In this tutorial,
we discuss about transition metal compounds and its colours, oxidation states, stable compounds, what are the precipitates and solutions. Then we list all the transition metal compounds according to the colours and solubility. Finally we discuss some questions about transition metal compounds.
Transition metal ions are often coloured. When a single transition metal atom is considered, it's five d orbitals are in same energy level(degenerate).In a complex ion, energy of d orbitals are changed slightly due to overlapping differently with the ligands they are non-degenerate. When d metal ion absorb energy, it's electrons of d orbitals can jump to another d orbit. The frequency absorbed in this energy transitions is in the visible region of the spectrum and the ion appears coloured. The colour of the ion is complementary to the colours absorbed.
The common electron configuration is [Ar] 4s2 3dn. Once the 4s electrons have been removed, the difference in energy between the 3d and the 4s electrons is much smaller than the difference between the 3s and the 3p electrons.
Transition elements exits in several oxidation states. Also oxidation states change in units of one.
Fe2+ and Fe3+ , Cu+ and Cu2+
An oxidization state is stable,
Rare oxidation states are given inside paranthesis.
Electronic structure : d1 s2
Oxidation states : +2 , +3
Stable compounds: Sc2O3 , ScF3 , ScCl3 , ScBr3 , ScI3
Electronic structure: d2 s2
Oxidation states: +2 , +3 , +4
Stable compounds: TiO , TiCl2 , TiBr2 , TiI2 , Ti2O3 , TiO2
Electronic structure: d3 s2
Oxidation states: +2 , +3 , +4, +5
Stable compounds: VO , VF2 , VCl2 , VBr2 , VI2 , V2O3 , VO2 , V2O5
Electronic structure: d5 s1
Oxidation states: (+1) , +2 , +3 , +4 , +5 , +6
Stable compounds: CrO , CrF2 , CrCl2 , CrBr2 , CrI2 , Cr2O3 , Cr3 , CrO2 , CrF5 , CrO3
Electronic structure: d5 s2
Oxidation states: +2 , +3 , +4 , +5 , +6 , +7
Stable compounds: MnO , MnF2 , MnCl2 , MnBr2 , MnI2 , Mn2O3 , MnO2 , Mn2O7 , KMnO4
Electronic structure: d6 s2
Oxidation states: +2 , +3
Stable compounds: FeO , FeF2 , FeCl2 , FeBr2 , FeI2 , Fe2O3 , FeF3
Electronic structure: d7 s2
Oxidation states: +2 , +3 , (+4) , (+5)
Stable compounds: CoO , CoF2 , CoCl2 , CoBr2 , CoI2 , Co2O3
Electronic structure: d8 s2
Oxidation states: +2 , (+3) , +4
Stable compounds: NiO , NiF2 , NiCl2 , NiBr2 , NiI2 , Ni2O3 , NiO2
Electronic structure: d10 s1
Oxidation states: +1 , +2
Stable compounds: Cu2O , CuCl , CuBr , CuI , CuO , CuF2 , CuCl2 , CuBr2
Electronic structure: d10 s2
Oxidation states: +2
Stable compounds: ZnO , ZnF2 , ZnCl2 , ZnBr2 , ZnI2
d block elements can show different oxidation states. As an example Mn has major oxidation numbers +2 , +4 , +6 , +7. When oxidation number changes, the colour of d metal ion in aqueous state changes.
[Sc(H2O)6]3+ - Colourless
[Ti(H2O)6]3+ - Purple
[V(H2O)6]3+ - Green
[Cr(H2O)6]3+ - Violet
[Mn(H2O)6]2+ - Pink
[Mn(H2O)6]3+ - purple
[Fe(H2O)6]2+ - Liht green
[Fe(H2O)6]3+ - Yellow - Brown
[Co(H2O)6]2+ - Pink
[Ni(H2O)6]3+ - Green
[Cu(H2O)6]2+ - Blue
[Cu(H2O)6]+ - Colourless
[Zn(H2O)6]2+ - Colourless
Sc3+ ions don't have d electrones. Therefore it is colourless. Also Cu+ and Zn2+ are with 3d10 configuration. Therefore there is no d to d electrons transition in Cu+ and Zn2+. So Cu+ and Zn2+ ions are colourless.
All of d block metal hydroxides are precipitates and they have different colours. All the d block metals are gelatinous owing to hydration and all are basic. When we add dilute NaOH to d metal ion, a precipitate forms. Some of these hydroxides are amphoteric and some of hydroxides forms soluble complexions with ammonia. Following compounds are some of d metal hydroxides which exist as precipitates and colours of each precipitates are noted.
When concentrated hydrochloric acid is added to some d block(transition) metal ions(existing as complex compounds in water), they give coloured solutions. Fe2+, Co2+, Ni2+, Cu2+ ions give colours with concentrated HCl.
In this lesson, we are going to identify some cations in d block. We can identify d7, d8,d9 and d10 configurations' cations by solving their precipitates in excess ammonia solutions.
When we add excess ammonia solution to precipitates of hydroxides of d block cations, precipitates dissolve and give different colours related to different cations.
We can identify Co2+, Ni2+, Cu2+, Zn2+ and Ag+ from this method.
All of d block metal sulfides are precipitates. Some of those sulfides are formed in acidic medium and some are in acidic medium. Hydrogen sulfide (H2S) gas is sent to the 3d block metal ion solutions to form their respective sulfides. Basic medium is made by adding aqueous ammonia solution.
Hydrogen sulfide (H2S) gas is sent to the below acidified d block metal ion solutions to form their respective sulfides
Hydrogen sulfide (H2S) gas is sent to the below basic d block metal ion solutions to form their respective sulfides
Add potassium cyanide (KCN) solution to d metal ions and check their colours.
Now, we list all precipitates and solutions according to the colours. It's very useful to remember compounds with colours. This can be used as a short note / summary.
Now we summarize all of 3d metal precipitates according to their colours. Exact colour of compound is noted with every precipitate.
Cr2O3 : green colour precipitate
Cr(OH)3 : a green precipitate
ZnO : white precipitate
Fe2O3 : brown colour precipitate
Cu2O : brick red precipitate
CuCl : a white precipitate
CuBr : a white precipitate
CuI : a white precipitate