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Experiment 1: Coordination Chemistry - Nickel Complexes

  • Page ID
    211947
  • 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)]\(^{2+}\):

    clipboard_e1d6b1972957583fce866171b1d7283a2.png

     

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

    clipboard_e33d706cc834eb6b945d55f6f6f9d9b39.png
    clipboard_ea0033b79e0371428db7db89a66e1615d.png

     

    Answers:

    Bidentate isomers:

    clipboard_eaca7c630693ae6c3cff8b22a0956e444.png

    Tridentate isomer:

    clipboard_e3ec57b6e6c94c23c5ce48f0fe01defdc.png

     

     

    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.
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