Colourful Nickel Complexes

Solutions of different colour are obtained when initial green solution is added to colourless solutions.

Scientific name: Nickel ions combine easily with different ligands to form coordination complexes. Depending on the ligand type and coordination number complexes have different colors.

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Nickel complexes

by ChemToddler

Nickel complexes (1)

by Adam Bridgeman

[Ni(H2O)6]2+ + en → [Ni(en)(H2O)4]2+ + en → [Ni(en)2(H2O)2]2+

by Terpsichore13th

Safety

Wear goggles and disposable gloves. Nickel sulphate, ethilenediamine and potassium cyanide are very dangerous substances, be careful when working with it. This experiment is intended for carrying out by qualified specialists

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

Beaker 1: Ni(H2O)62+ + en → [Ni(H2O)4(en)]2+ + 2H2O (blue)

Beaker 2: Ni(H2O)62+ + 2en → [Ni(H2O)2(en)2]2+ + 4H2O (dark blue)

Beaker 3: Ni(H2O)62+ + 3en → [Ni(en)3]2+ + 6H2O (violet)

Beaker 4: Ni(H2O)62+ + 2DMG-OH + 2OH- → [Ni(DMG)2]2+ + 8H2O (red)

Beaker 5: Ni(H2O)62+ + 6NH3 → [Ni(NH3)6]2+ + 6H2O (light blue)

Beaker 6: Ni(H2O)62+ + 4CN- → [Ni(CN)4]2+ + 6H2O (orange)

Step-by-step instruction

1.In order to perform this experiment we need:

  [Ni(H<sub>2</sub>O)6<sub>6</sub>]SO<sub>4</sub> solution in water

  ethilenediamine solution in water

  dimethylglyoxime solution in ethanol

  NH<sub>3</sub>  water solution

  KCN water solution

  All these solutions except [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub> are colorless. The [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub>  solution is light-green.

2.The solution of [Ni(H2O)6]SO4 should be added to all other solutions in the right proportions:

  Beaker 1) [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub> + ethilenediamine, molar ratio 1:1

  For example: 50ml of 0.2M [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub>   + 50ml of 0.2M ethilenediamine

  Beaker 2) [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub> + ethilenediamine, molar ratio 1:2

  For example: 50ml of 0.2M [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub>  + 100ml of 0.2M ethilenediamine or 50ml of 0.2M [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub>   + 10 ml of 2 M ethilenediamine

  Beaker 3) [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub> + ethilenediamine, molar ratio 1:3

  For example: 50ml of 0.2M [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub> + 150ml of 0.2M ethilenediamine or 50ml of 0.2M [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub>  + 5 ml of 6 M ethilenediamine

  Beaker 4) [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub> + dimethylglyoxime, molar ratio 1:2

  Beaker 5) [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub> + NH<sub>3</sub>*H2O, molar ratio 1:6

  Beaker 6) [Ni(H<sub>2</sub>O)<sub>6</sub>]SO<sub>4</sub> + KCN, molar ratio 1:4

3.When the [Ni(H2O)6]SO4 solution is added to the second component the formation of a colored complex compound is observed.

Scientific background

This experiment is based on a very important phenomena: the ability of metals to form chemical structures called coordination compounds. In such structure the central metal atom is bounded with one or more ligands. A ligand is an atom or a molecule which contains one or more electron pairs that can be shared with a metal ion. Nickel ions are well known to combine easily with different ligands to form coordination complexes. These compounds have very symmetrical, octahedral geometry. Depending on the ligand type and coordination number (the number of neighbors near a metal) complexes have different colors. The water solutions in the beakers in the considered experiment contain various ligand ions which form well defined colored complexes.

The component added to all solutions is the [Ni(H2O)6]SO4 (light-green colored solution). It dissociates in the water solution and gives hydrated Ni2+ ions - [Ni(H2O)6]2+ and SO42- ions:

[Ni(H2O)6]SO4 = [Ni(H2O)6]2+ + SO42-

When it comes into contact with solutions in the beakers, ligand ions (molecules) replace water molecules from the Ni coordination shell.

The first three beakers are filled with different amounts of ethilenediamine (en) solution.

This ligand is polydentate. That means that it may replace H2O molecules while there are points of attachment in the ligand. The en has two points and thus is called bidentate ligand. It can attach to the central Ni atom with its two nitrogen atoms which has two unshared electron pairs. This process could be divided into three steps:

[Ni(H2O)6]2+ + en =[Ni(H2O)4(en)]2+ + 2H2O (blue)

[Ni(H2O)6]2+ + en = [Ni(H2O)2(en)2]2+ + 2H2O (dark blue)

[Ni(H2O)6]2+ + en = [Ni(en)3]2+ + 2H2O (violet)

In the experiment in the first three beakers we have different amounts of substance, so the molar ratio of the components are different too. The reaction proceeds to different extent and we can observe the formation of complexes with various numbers of ligands:

Beaker 1: [Ni(H2O)6]2+ + en = [Ni(H2O)4(en)]2+ + 2H2O (blue)

Beaker 2: [Ni(H2O)6]2+ + 2en = [Ni(H2O)2(en)2]2+ + 4H2O (dark blue)

Beaker 3: [Ni(H2O)6]2+ + 3en =[Ni(en)3]2+ + 6H2O (violet)

The forth beaker contains an ethanol solution of dimethylglyoxime (DMG). Ethanol helps to turn dimethylglyoxime to the form of its conjugate base, that interacts with [Ni(H2O)6]2+. These reagent is often used to detect nickel in qualitative analysis. One DMG molecule replaces 3 water molecules.

Beaker 4: [Ni(H2O)6]2++ 2DMG-OH + 2OH- =[Ni(DMG)2]2+ + 8H2O (red)

The five beaker contains a NH3*H2O solution. A NH3molecule replaces 1 water molecule (monodentate ligand) in a stepwise fashion:

[Ni(H2O)6]2+ + NH3 = [Ni(H2O)5(NH3)]2+ + H2O

[Ni(H2O)5(NH3)]2+ + NH3 = [Ni(H2O)4(NH3)2]2+ + H2O

and so until all water molecules are replaced by ammonia. In the experiment the concentration of NH3 is enough to complete the reaction. The overall equation looks as follows:

Beaker 5: [Ni(H2O)6]2+ + 6NH3 = [Ni(NH3)6]2+ + 6H2O (light blue)

The six beaker contains sodium cyanid:

Beaker 6: [Ni(H2O)6]2+ + 4CN- = [Ni(CN)4]2- + 6H2O (orange)

Danger:
Coolness:
Difficulty:

Published on 22 January 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst