1. Nickel Complexes and Colors

1. Ni(H₂O)₆²⁺ complex (Figure 1a)
   1. Prepare a 1 M solution of Ni(H₂O)₆²⁺ by dissolving NiSO₄ in the appropriate volume of water.
   2. Further dilute the Ni(H₂O)₆²⁺solution by adding 70 mL of the 1 M solution to 1,000 mL of deionized water.
   3. Divide the Ni(H₂O)₆²⁺ among seven 400-mL beakers.
   4. The aqueous nickel solution takes on a light green color since water is a weak-field ligand.
   5. The absorbance spectrum indicates red wavelengths are absorbed, justifying the opposite, green, which is observed.
2. Ni(NH₃)₆²⁺ complex (Figure 1b)
   1. Add a 5 M aqueous ammonia solution to a beaker and stir.
   2. The solution takes on a deep blue color, indicating the solution is absorbing orange light which is higher in energy than red light.
   3. The absorbance spectrum indicates yellow wavelengths are absorbed, justifying the opposite, blue, which is observed.
      1. Ammonia is a stronger field ligand than water, which increases the splitting between the t_2g and e_g orbitals.
3. Ni(en)₃²⁺ complex (Figure 1c)
   1. Add a 30% ethylenediamine (en) solution to the aqueous Ni(H₂O)₆²⁺ complex and stir.
   2. The solution progressively turns from light blue to blue to purple as ethylenediamine molecules progressively coordinate around the metal center to eventually form Ni(en)³⁺.
   3. Ethylenediamine is a stronger ligand than water or ammonia and it is bidentate. The purple color indicates the solution is absorbing yellow light which is higher in energy than orange or red light.
   4. The absorbance spectrum indicates yellow wavelengths are absorbed, justifying the opposite, purple, which is observed.
4. Ni(dmg)₂²⁺ complex (Figure 1d)
   1. Dimethylgloxine (dmg) is a bidentate ligand that chelates a large number of metals. Only two dmg molecules are required per metal center because Ni(dmg)₂²⁺ has a square-planar geometry.
   2. Add 1% dmg to the aqueous complex.
   3. A solid pink/red precipitate forms, the insoluble Ni(dmg)₂²⁺ complex.
   4. A visible transmission spectrum of the complex is not possible, but the red color indicates green light is being absorbed. Green is higher energy than yellow, orange, and red.
5. Ni(CN)₄^2- complex (Figure 1e)
   1. The cyanide ion (CN^-) is monodentate, but a very strong field ligand, which also forms square-planar complexes with nickel (II).
   2. Add a 1 M KCN solution.
   3. A yellow Ni(CN)₄^2- complex forms almost immediately.
      1. Note: Working with cyanide salts must be done with great care. Addition of acid may result in the formation of cyanide gas.
   4. Cyanide is a stronger ligand than any of the other ligands because there is σ-bonding from the ligand to the metal and π-back bonding from the metal to the ligand. The yellow color indicates the solution is absorbing blue light, which is higher in energy than green, yellow, orange, and red.
   5. The absorbance spectrum indicates yellow wavelengths are absorbed, justifying the opposite, purple, which is observed.

2. Ligand Strength

1. According to the spectrochemical series, some ligands are stronger-field than others, which correspond to the size of the splitting of the d-orbitals of the central metal ion.
2. Stronger field ligands replace weaker field ligands in solution.
3. An aqueous solution of nickel sulfate appears light green because the Ni(H₂O)₆²⁺ complex forms.
4. Sequentially add solutions of ammonia, ethylenediamine, dimethylglyoxime, and cyanide to the nickel-containing solution while stirring.
5. After each addition, the previous color disappears and the new color appears.
6. The color change indicates the formation of a new coordination complex driven by the strength of the ligand. These can be quantified by the equilibrium constant for each reaction:
   Ni(H₂O)₆²⁺(aq) + 6 NH₃ (aq) → Ni(NH₃)₆²⁺ (aq) + 6 H₂OK_eq = 1.2 x 10⁹
   Ni(NH₃)₆²⁺ (aq) + 3 en(aq) → Ni(en)₃²⁺ (aq) + 6 NH₃ (aq)K_eq = 1.1 x 10⁹
   Ni(en)₃²⁺ (aq) + 2 Hdmg(aq) → Ni(dmg)₂ (s) + 3 en(aq) + 2 H⁺ (aq)K_eq = 1.35 x 10⁵
   Ni(dmg)₂ (s) + 4 CN^- (aq) - → Ni(CN)₄^-2 (aq) + 2 dmg^- (aq)K_eq = 6.3 x 10⁷
7. The equilibrium constant in each reaction is very large (>1), indicating that the reactions are all product driven.

Figure 1. Structures of nickel (II) coordination complexes a-e.