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Werner’s Coordination Complexes - Evidence and Arguments (Worksheet)

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    126985
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    Work in groups on these problems. You should try to answer the questions without referring to your textbook. If you get stuck, try asking another group for help.

    Learning Objectives

    • Be able to visualize the predicted isomers for hexagonal planar, trigonal prismatic and octahedral coordination geometries.
    • Be able to state the arguments Werner used to establish the concepts of coordination number and coordination geometry, based on conductance of solutions or chloride ions quickly precipitated and on the predicted isomers.
    • Ability to visualize the 3-dimensional structure of a complex and predict isomers.
    • Accurate deduction of coordination geometries from evidence.

    Alfred Werner made the break-through in formulating the structure of coordination complexes and eventually received the Nobel Prize for his work. His hypothesis that complexes had coordination numbers, including corresponding sites on a geometrical framework on which the ligands could be placed, formed the basis for modern coordination chemistry. In this activity we examine the lines of evidence that Werner pursued in his research that led to the acceptance of his hypothesis.

    Resources

    • Lecture.
    • Plan presented in this activity.
    • Table 2.2 in Rodgers as a Resource and Model.
    • Chapter 2 in Rodgers (Glen E. Rodgers, “Descriptive Inorganic, coordination and Solid-State Chemistry,” Brooks/Cole, Thompson Learning, USA, 2002, ISBN 0-12-592060-1).

    Prerequisites

    Understand the following: coordination number, coordination geometry, difference between behavior of chloro ligands and chloride counterions in a compound, prediction of possible isomers from the formula of a complex.

    Pre-activity assignment

    Problem 2.10 in Rodgers.

    Plan

    1. Form groups of two to collaborate in the completion of this plan.
    2. Evidence is given in the table below.

    Compound Label

    Formula

    No. of precipitated Cl- ions with AgNO3

    Number of observed isomers

    A CrCl3 • 6H2O 3 1
    B CrCl3 • 5H2O 2 1
    C CrCl3 • 4H2O 1 2
    1. Write a balanced chemical equation for the reaction of compound A with an aqueous solution of silver nitrate (AgNO­3­). Note that only the counter ion chlorides react with silver nitrate. Chlorides that are bound to chromium are not precipitated but remain in solution bound to chromium. The water molecules that are listed as waters of hydration are bound to chromium in the complex ion.
    2. Write the formulas of the complex ions in compounds A, B and C that are consistent with the observed number of precipitated chloride ions in each case.
    3. Write structural formulas for compound C in its two isomeric forms as Werner would have envisioned them based on a coordination number of 6 and octahedral coordination geometry.
    4. If the coordination geometry were hexagonal planar, write the structural formulas for the possible isomers of compound C.
    5. If the coordination geometry were trigonal prismatic, write the structural formulas for the possible isomers of compound C.
    6. Write a statement to convince a colleague that the evidence points to coordination number six with octahedral coordination geometry. Use both the precipitation and the isomer observations.
    Post-activity assignment

    Problem 2.12.

    Reference

    • Susan Jackels, Seattle University
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