Experiment 1: Coordination Chemistry - Nickel Complexes

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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.

Answers:

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
    125 mL Erlenmeyer flask
    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.