9Exhaustive Guide to Nickel(II) Complex Color Effects and Ligand Exchange

Objective: The experiment E9, titled ‘Farbeffekte beim Ligandenaustausch in Nickel(II)‐Komplexen’ (Color Effects during Ligand Exchange in Nickel(II) Complexes), is part of the Inorganic Chemistry Internship at the Institut f%C3%BCr Chemie und Biochemie.
Required Research Topics:
* Complex compounds and their specific colors.
* Properties of Nickel and Nickel compounds.
Research Resources:
* Standard textbooks for chemical backgrounds and historical/application‐related facts.
* Databases such as SciFinder, the internet, or the library to locate the original publication: H. W. Roesky, K. M%C3%B6ckel, Chemische Kabinettst%C3%BCcke, VCH Verlagsgesellschaft mbH 1994, S. 106ff.
Specific Research Tasks:
* Identify all actual Nickel(II) species appearing in the series of experiments. They must be listed using their molecular formulas and nomenclature‐accurate names (simple notations like ‘ $Ni^{2+}$ ’ are insufficient).
* Establish detailed reaction equations for every individual reaction step.

Nickel Species in the Experiment:
* Nickel(II) chloride hexahydrate: $NiCl_2 ⋅ 6H_2O$ (Green solid). Contains the hexaaquanickel(II) complex: $[Ni(H_2O)<em>6]Cl_2$ . * Anhydrous Nickel(II) chloride: $NiCl_2$ (Yellow‐orange solid). * Hexaaquanickel(II) ion: $[Ni(H_2O)_6]^{2+}$ ( $aq$ ) (Green solution). * Ammine Complexes: Various species of $[Ni(NH_3)_n(H_2O)</em>{6-n}]^{2+}$ . Specifically, the hexaamminenickel(II) complex $[Ni(NH_3)_6]^{2+}$ which is associated with violet coloration.
Chemical Principle: The experiment demonstrates ligand exchange, where water molecules ( $H_2O$ ) in the coordination sphere of the Nickel(II) ion are replaced by ammonia ( $NH_3$ ) or alcohol molecules, leading to distinct shifts in the electronic structure and visible color.

Safety Instruction: All laboratory work must be performed under an exhaust hood (Abzug). Proper handling of heavy metal solutions is mandatory.
Chemical Hazards:
    * Nickel(II)‐chlorid‐Hexahydrat: Classified as toxic (stored in the ‘Giftkiste Pr%C3%A4parate’).
    * 25% Ammonia Solution ( $NH_3$ ): Corrosive and pungent; handled from the ‘F%C3%BCnfersatz’ storage.
    * Methanol ( $CH_3OH$ ): Toxic and highly flammable. Must be handled in the O‐Abzug.
Student Responsibility: Students must research and be able to explain the specific H‐phrases (hazard statements) and P‐phrases (precautionary statements) for these chemicals in their own words rather than simply reading a list of numbers.

Initial Setup: Fill a test tube with $1.5‐g$ of green Nickel(II)‐chlorid‐Hexahydrat under a fume hood.
Dehydration Process:
* Heat the salt using a burner flame while continuously swirling the test tube until the salt is dehydrated.
* Observation: The salt appears to melt and transitions into a distinct yellow‐orange color.
Water Removal: To remove water condensing at the top of the tube, remove the tube from the flame with the clamp, adjust the clamp to a lower position, and then heat the upper part of the test tube to drive the moisture out.
Solid Preparation: Occasionally use a glass rod to carefully crush the nickel salt residue.
Completion: The process is finished when the salt is crumbly and entirely yellow‐orange. Close the test tube with a suitable rubber stopper (obtained from the assistant).

Aqueous Dissolution:
* Dissolve most (but not all) of the anhydrous yellow nickel chloride in approximately $25 cm^3$ of water within a $50‐100‐cm^3$ beaker.
* Shake until a completely clear solution is formed.
Comparison Series in Test Tubes:
    * Distribute the Nickel(II) chloride solution equally into four test tubes.
    * Tube 1 (Baseline): Remains the original green color of the starting salt ( $[Ni(H_2O)_6]^{2+}$ ).
    * Tube 2 (Partial Exchange): Add a few drops of $NH_3$ solution. Result: The color turns turquoise.
    * Tube 3 (Further Exchange): Add more $NH_3$ solution. Result: The color turns deep blue.
    * Tube 4 (Excess Ammonia): Add excess ammonia solution. Result: The color turns violet. Note that this may be accompanied by the formation of a light violet precipitate.
Alcoholic Solution Comparison:
    * Mix a portion of the anhydrous yellow nickel(II) chloride with alcohol (Methanol).
    * Result: The liquid assumes the yellow color of the dissolved anhydrous salt.
    * Humidity Sensitivity: If the alcoholic solution is exposed to water or ammonia vapors (even atmospheric moisture), it rapidly changes to a light green (‘lindgr%C3%BCne’) color.
Reversibility: The colors can be varied back and forth between green and violet by the alternating addition of Nickel(II) chloride solution or Ammonia solution.

Required Chemicals:
    * Nickel(II)‐chlorid‐Hexahydrat ( $1.5‐g$ ).
    * $25%$ $NH_3$ solution.
    * Methanol.
Required Apparatus:
    * Test tubes (Reagenzgl%C3%A4ser).
    * Matching rubber stoppers.
    * Test tube clamp.
    * Spatula.
    * Glass rod.
    * $50‐100‐cm^3$ Beaker.
    * Bunsen burner.
Disposal Instructions:
* All solutions must be disposed of in the container for aqueous heavy metal waste (‘w%C3%A4ssrige Schwermetallabfalll%C3%B6sungen’) located in the A‐Abzug.
* Any undissolved Nickel(II) chloride must first be dissolved in water before disposal in the same heavy metal container.

Laboratory Journal: Record all observations and precise experimental conditions (e.g., exact weights of substances) carefully.
Photography: It is recommended to document experimental setups and color changes photographically.
Written Elaborations: A formal written laboratory report (‘Versuchsvorschrift’) is required for selected experiments. A model template (Muster‐Versuchsvorschrift) is provided for guidance.
Presentation Requirement: Students must give a short presentation during the introductory course (‘Einf%C3%BChrungskurs’). The presentation serves as a preliminary discussion to practice explaining chemical facts clearly and to outline the practical procedure of the synthesis/experiment.