# Experiment 1: Coordination Chemistry - Nickel Complexes


Background

Ni$$^{2+}$$ in aqueous solution exists as the octahedral complex ion hexaaquanickel (II), [Ni(H$$_2$$O)$$_6$$]$$^{2+}$$. When the bidentate ethylenediamine ligand (en) is added to the solution in a 1:1 concentration ratio, it replaces two water ligands of the [Ni(H$$_2$$O)$$_6$$]$$^{2+}$$ to form [Ni(H$$_2$$O)$$_4$$(en)]$$^{2+}$$:

Further additions of ethylenediamine form the di- and tri- ethylenediamine complex, which have geometric and/or optical isomers.

Bidentate isomers:

Tridentate isomer:

Materials:

2 mL of 1.0 M Nickel sulfate (NiSO$$_4$$) solution
3 mL of 2.0 M Ethylenediamine (H$$_2$$NCH$$_2$$CH$$_2$$NH$$_2$$) solution
Four small test tubes (and a small beaker to hold them

Safety

Ethylenediamine is a caustic, flammable liquid with an irritating vapor similar to ammonia. Nickel sulfate is a suspected carcinogen.

Procedure:

• Add 2 mL of nickel sulfate solution to the 125 mL Erlenmeyer flask. Bring up to 30 mL with water. Calculate the final concentration of nickel (II) ion. Transfer about 1 mL into a test tube.

• Add to the Erlenmeyer flask 1.0 mL of ethylenediamine solution. Record your observations. Transfer about 1 mL into a clean test tube.

• Add another 1.0 mL aliquot* of ethylenediamine solution to the Erlenmeyer flask. Record your observations. Transfer about 1 mL into a clean test tube. (aliquot = a volume of a solution)

• Add another 1.0 mL of ethylenediamine into the Erlenmeyer flask. Record your observations. Transfer about 1 mL into a clean test tube.

• Group test tubes together to contrast the four colors.

• Optional: Work in teams to obtain the spectrum of each solution by UV-VIS spectroscopy.

Clean-Up: Collect all waste in the designated container.

Discussion:

• Draw the missing isomers in the Background section and describe in each case the type of isomerism.

• At approximately what wavelengths of the electromagnetic spectrum do these complexes, [Ni(H$$_2$$O)$$_6$$]$$^{2+}$$, [Ni(H$$_2$$O)$$_4$$(en)]$$^{2+}$$, [Ni(H$$_2$$O)$$_2$$(en)$$_2$$]$$^{2+}$$, [Ni (en)$$_3$$]$$^{2+}$$, absorb light?
• Use your understanding of the splitting of d orbitals and the spectrochemical series to explain your results.

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