Ligand Exchange Reaction Of Transition Metals Assignment Sample

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Ligand Exchange Reaction Of Transition Metals Assignment

Task AC 2.1


Ligand exchange reaction is associated with the transitional metals due to presence of incompletely filled d orbitals in the antepenultimate shell (Wang et al. 2020). This is why it reacts with ligands and forms a colored complex. Ligand exchange reaction is the substitution of ligands by other ligands (Davankov, 2020). In this case different aqua metal complexes have been replaced by the amine group or NH3 ligands reacting with aqueous ammonia solution. In this case, NH3 forms stronger bonds than H2O ligands due to higher nucleophilic character of NH3. The lone pair in NH3 gets stable with the metal ions and donating electronic clouds in the d orbital. Polarization capacity of NH3 is quite greater than that of H2O and forms a stronger color than before.

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Aqueous solution of Cu (II), Ni (II), Fe (III), Zn (II), and Sc (III) are treated with the ammonia to allow the ligand exchange reactions and forms aqua-amine or amine complex of the metals replacing water molecules from them.

Copper sulfate, nickel chloride, ferric chloride, zinc sulphate, and scandium sulphate are taken and also ammonia solution is prepared to start the experiments. Strength of the metal complex solutions are 0.1 mol/dm3 and the ammonia solution is 2 mol/dm3.

Here, ammonia reacts with the metal ions in two ways and these two ways also can be determined by the experimental observations like color changing, precipitation formation, etc. Ammonia acts both as base and ligands in this case. When a small quantity of the ammonia solution is added in the aqueous solution of the metal complex then it forms metal hydroxide complex which mainly forms precipitation and excess adding of ammonia would help to dissolve the ppt and forms metal-amine complexes (Zhang et al. 2021). In these cases, oxidation number as well as coordination number of the metal ion can be changed and therefore it would show excellent features of color changing, ppt formation, etc. But generally, when the H2O ligand is replaced by the NH3 ligand, there is no change of oxidation state or coordination number experienced because both are neutral in nature and favorable in octahedral coordinates.

When the reactions are proceeded, we must follow the safety measures as the solutions of the transitional metals may be irritating sometimes and also ammonia vapor can harm our eyes. Therefore, eye protection glasses, hand gloves, etc to avoid accidents.

Copper, nickel, and iron solutions are taken separately in the test tubes and then an aqueous ammonia solution is added in the test tubes by dropper. The tests are done separately and the observations are also recorded for individual reaction.

Reaction of ammonia with Cu (II)

At first, aqueous solution of Cu (II) sulphate is taken in the test tube and then ammonia is added dropwise. At first, some ppt of is formed which is actually the metal hydroxide complex which is blue in color. And then, some more ammonia is added in the test tubes and then stirred. The ppt dissolves in the water and then forms a deep blue solution. The reactions are given below.

Reaction of copper with ammonia

(Source: Clark, 2020)

[Cu (H2O)6]2+ (aq) + NH3. H2O → [Cu (OH)2 (H2O)4] + NH3 (excess) → [Cu (NH3)4 (H2O)2]2+

In the above equation, it is very clear that the ammonia acts as a base and forms hydroxide ion and then when more ammonia is added then the ppt has dissolved in the solution and the color changes to dark blue which is tetraamine diaqua copper complex.

Final Observations and comments: The color of the solution becomes dark blue from the light blue or pale blue of the copper sulphate solution. The oxidation number and coordination number remain the same as octahedral. But there present a Jahn Teller effect and therefore the altercation of the axial and equatorial position happens frequently.

Reaction with Ni (II)

When ammonia is added in the nickel hexa aqua solution, the green color changes to the ppt first and then deep violet. At first, the nickel forms tetraaqua nickel hydroxide complex which is due to the basic character of ammonium hydroxide. When excess ammonia is added then the ppt dissolves in the solution and forms a deep violet color which is the color of hexammine nickel complex (Botelho et al. 2019). The H2O ligands are replaced by the NH3 ligands.

Figure: Reaction of nickel with ammonia

(Source: Clark, 2020)

[Ni (H2O)6]2+ (aq) + NH4OH → [Ni (OH)2 (H2O)4] + NH3 (excess) → [Ni (NH3)6]2+

The ammonia works as a base, forming hydroxide ions, and when more ammonia is added, the ppt has dissolved in the solution, changing the hue to dark violet, indicating the presence of hexammine nickel complex.

Final Observations and comments:

The color of the green changes into deep violet but the oxidation state and coordination number remains the same as before.

Reaction with Fe (III)

The color of the Fe (III) is dark brown or rusty brown. The color of ferric chloride is the mix of yellow and rusty brown and sometimes looks like pale yellow. When ammonia is added, the color changes to orange from pale yellow. In this case, ammonia does not act as a ligand and form bonds with the lone pair of NH3. Fe (III) forms a comparatively stable complex with the aqueous solution and does not really agree to replace it by NH3 (Boukar et al. 2019). But, three H2O molecules are replaced by three OH ions and in this case ammonia acts as a base.

reaction of ferric ion with ammonia

(Source: Clark, 2020)

[Fe (H2O)6]3+ (aq) + NH3 → [Fe (OH)3 (H2O)3]

Final observations and comments: Color of pale yellow of the solution turns into orange coloration but coordination number remains the same as before. Only, the oxidation number changes from +3 to zero due to accepting three electrons from the three OH ions.


If ZnSO4 and Sc2(SO4)3 are taken instead of CuSO4 and FeCl3 then reactions occur differently. The color changing of the complexes would be not interesting in both cases. Both solutions are colorless and in the middle they form white ppt and after that those again become colorless. These are the borderline elements of d-block and therefore character of representative elements show like the formation of colorless or white colored compounds.

In the reaction of ammonia with Sc (III), it acts as a base rather than a ligand.

[Sc (H2O)6]3+ (aq) +NH4OH → [Sc (OH)3 (H2O)3]

But, when ammonia reacts with Zn the coordination number also changes from octahedral to tetrahedral (Clark et al. 2022). At first ammonia acts as a base because two hydroxide ions replace two water molecules forming a white color compound and when excess ammonia is added the ppt dissolves and the solution becomes colorless again. It indicates the formation of tetrammine zinc sulphate.

Ammonia reacts with zinc

(Source: Clark, 2020)

Reactions and stability

According to the “Le Chatelier” principle, the stability of an equilibrium reaction shifts to the right end if new products form as well as removing free ion concentration. In the case of ligand exchange reaction by ammonia, the ammonia first reacts as a base and then as a ligand with the increase of concentration (Clark, 2020). Therefore, aqueous ammonia solution also produces OH ion which is also present in the solution too. The water ligands are replaced by the OH ion and neutralize the charge of the metal ion. The hydronium ion (H+) combines with ammonia and removes H+ from the equilibrium. Therefore, the equilibrium shifts towards the right side.

Explanation of stability

(Source: Clark, 2020)

[M (H2O)6]2+ (aq) ? [M (H2O)5 (OH)]+ (aq) + H+ (aq)

[M (H2O)6]2+ (aq) + NH3 (aq) ? [M (H2O)5 (OH)]+ (aq) + NH4+ (aq)

[M (H2O)6]2+ (aq) + 2NH3 (aq) ? [M (H2O)4 (OH)2] (s) + 2NH4+ (aq)

[M (H2O)6]3+ (aq) + 3NH3 (aq) ? [M (H2O)3 (OH)3] (s) + 3NH4+ (aq)

Question 1

Transition metals are hard, ductile, and shiny, possess magnetic properties, form colorful complex compounds, etc. They have high melting and boiling points too.

Question 2

Transition metals react with water in different conditions and differently. The metal ions can attract water ligands and form octahedral aqueous complexes which are generally light colored in occurrence. 

Question 3

Not all transition metals form colored compounds and not all react equally. For example zinc, cadmium, mercury, scandanium, others form white colored compounds. Only the common thing among all transition metals is that they possess different oxidation states.

Reference list


Botelho Junior, A.B., Dreisinger, D.B. and Espinosa, D.C., 2019. A review of nickel, copper, and cobalt recovery by chelating ion exchange resins from mining processes and mining tailings. Mining, Metallurgy & Exploration36(1), pp.199-213.

Boukar, O., Fifen, J.J., Malloum, A., Dhaouadi, Z., Ghalila, H. and Conradie, J., 2019. Structures of solvated ferrous ion clusters in ammonia and spin-crossover at various temperatures. New Journal of Chemistry43(25), pp.9902-9915.

Clark, B., Gilles, G. and Tarpeh, W.A., 2022. Resin-Mediated pH Control of Metal-Loaded Ligand Exchangers for Selective Nitrogen Recovery from Wastewaters. ACS Applied Materials & Interfaces.

Davankov, V.A., 2020. Ligand-Exchange Chromatography of Chiral Compounds. In Complexation Chromatography (pp. 197-246). CRC Press.

Wang, L., Song, B., Khalife, S., Li, Y., Ming, L.J., Bai, S., Xu, Y., Yu, H., Wang, M., Wang, H. and Li, X., 2020. Introducing seven transition metal ions into terpyridine-based supramolecules: self-assembly and dynamic ligand exchange study. Journal of the American Chemical Society142(4), pp.1811-1821.

Zhang, Y., Duan, X., Hu, C., Du, G. and Wang, Y., Separation of Cadmium from Base Metals in Chloride Medium with Halide-Loaded Anion Exchanger.


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