A Guided Inquiry Activity for Teaching Ligand Field Theory | Journal of

Jun 17, 2015 - The guided inquiry activity for LFT, a daily reading and implementation schedule, sample quizzes and an answer key are provided...
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A Guided Inquiry Activity for Teaching Ligand Field Theory Brian J. Johnson* and Kate J. Graham Department of Chemistry, College of Saint Benedict and Saint John’s University, Saint Joseph, Minnesota 56374, United States

J. Chem. Educ. 2015.92:1369-1372. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/08/18. For personal use only.

S Supporting Information *

ABSTRACT: This paper will describe a guided inquiry activity for teaching ligand field theory. Previous research suggests the guided inquiry approach is highly effective for student learning. This activity familiarizes students with the key concepts of molecular orbital theory applied to coordination complexes. Students will learn to identify factors that affect the ligand field splitting diagram for octahedral and other common geometries. In addition, students use this understanding of ligand field theory to explain the properties of transition metal complexes. This activity could be used in an advanced inorganic chemistry course or at a different level.

KEYWORDS: Upper Division Undergraduate, First Year Undergraduate/General, Inorganic Chemistry, Collaborative/Cooperative Learning, Inquiry-Based/Discovery Learning, Crystal Field/Ligand Field Theory, Coordination Compounds, Transition Elements

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INTRODUCTION Much of inorganic chemistry deals with the structure and properties of the transition metal complexes. One of the key approaches to understanding these properties is crystal field theory, which was originally developed by Bethe in the late 1920s to explain the electronic structure of metal ions in crystals using a purely electrostatic bonding model.1 In the 1950s, Griffith and Orgel combined these ideas with molecular orbital theory in order to better explain covalent aspects of the metal−ligand bond; the result was ligand field theory (LFT).2 Many early efforts to employ these theories in the teaching of transition metal chemistry appeared in this Journal.3−8 Ligand field theory, or the closely related crystal field theory, is a staple of many different chemistry courses. Crystal field theory and ligand field theory give similar end results, the d orbital splitting diagram and an energy difference between these levels, although the former does not rely on molecular orbital considerations. Current textbooks employed in advanced inorganic chemistry courses give significant treatment to the development of LFT and its application to explain structural, magnetic, and spectroscopic aspects of transition metals.9−12 Insights based on symmetry and group theory are also included at this level. There is similar treatment at the intermediate or “sophomore” inorganic level without the inclusion of symmetry and group theory.13 Even most general chemistry textbooks have a chapter on transition metal and/or coordination chemistry containing an introduction to crystal field theory.14−17 However, traditional textbooks are not designed for a student-centered classroom environment, and instructors who want to use alternative pedagogical strategies need other © 2015 American Chemical Society and Division of Chemical Education, Inc.

resources. There is a substantial body of evidence that students learn better in classes that incorporate active learning.18−25 Although there are student-centered activities for other inorganic topics or practicing concepts from ligand field theory,26−31 there is no publicly accessible guided inquiry approach to teach ligand field theory and the applications. The approach described herein utilizes a guided inquiry unit in which students use models and worksheets to draw conclusions about ligand field theory. This approach challenges students to develop their own understanding of the material.

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ACTIVITY DESIGN AND IMPLEMENTATION

While there have been a variety of guided inquiry approaches developed for general chemistry and organic chemistry, there are only a few available resources for inorganic guided inquiry materials. The goal of this project was to introduce students to the key ideas of ligand field theory using a guided inquiry inclass activity rather than a lecture or a multimedia presentation. This resource will fill a gap in the current pedagogical literature. Learning Objectives

This activity was designed for a first year foundational class in principles of reactivity.32 It has also been implemented in an upper division inorganic class as well. The complete activity, an instructor’s guide, and how the activity aligns with the CSB/ SJU curriculum are available in the Supporting Information. When utilized with first year students, this activity usually takes five to six 55 min class periods. The primary objectives include the following: Published: June 17, 2015 1369

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1. To understand metal−ligand bonding and field splitting diagrams. 2. To determine whether a ligand is a sigma-only ligand or also a pi-donor or pi acceptor. 3. To predict a ligand’s place in the spectrochemical series and how that will impact the MO of metal−ligand binding. 4. To understand the effect of the ligand, the metal, and its charge on the d orbital splitting diagrams so that a prediction can be made as to whether a complex is highor low-spin. 5. To understand ligand field stabilization energies and how to calculate them. 6. To predict whether a four-coordinate complex will be square planar or tetrahedral. 7. To understand how ligand field theory can predict physical and chemical properties such as lability, magnetism, and UV absorbance.

diagrams to predict whether compounds are high-spin or low-spin. Student understanding of the basics of ligand field theory is then assessed with a short quiz on key ideas (Supporting Information) before progressing to more complex ideas. On day four, the discussion turns to other geometries. Students once again use the orbital model kits to analyze d orbital overlap with surrounding ligands in a given coordination geometry. Students are then able to predict splitting diagrams for different geometries. Once students have developed a firm foundation in bonding, they are introduced to applications such as lability, magnetism and/or UV−vis absorption (days 5 and 6). This unit culminates with a more comprehensive applied assessment. An example of one of these ligand field application quizzes is also included in the Supporting Information. As students progress through our foundational reactivity chemistry courses, they continue to encounter integrative, literature-based problems that require the application of ligand field theory to problems of current research interest. Examples of subsequent applications include bioinorganic topics, ligand exchange mechanisms, an analysis of hemoglobin function, cytochrome P-450 activity, and a nickel complex-based magnetic switch that may find use as an MRI contrast agent. Subsequent topics that utilize an LFT understanding are clearly outlined in the Supporting Information. These latter activities meet a secondary course goal, which it to introduce students to how the chemistry they are studying is being used in modern industrial or academic research settings. An example of one of these integrative problems is also included in the Supporting Information.

Guided Inquiry Activities

There are three types of activities that appear in the worksheets used to guide student learning. In the guided inquiry activity itself, students are led through the chemical logic and data interpretation to develop an understanding of the major concepts. Subsequent practice problems help to reinforce these new concepts. Third, students are given the opportunity to summarize and compile their new knowledge. In addition to the classroom activities, students are provided with readings and problems to complete outside of class. Students consolidate the information through the use of online homework sets and in-class formative multiple-choice quizzes.33−35 A daily schedule of activities, open source readings, and ordering of topics is provided in the Supporting Information. To guide the students through this material, lecture is kept to a minimum. Students primarily work in groups to develop key ideas for each class period while the instructor moves between groups to coach the students through applications. If it appears that many students are struggling with a similar misconception or missing a key concept, the instructor pauses the group work to present a mini-lecture. While there are many resources available to help instructors develop their skills in guiding students through inquiry-based materials,36 the daily schedule provided with this activity also includes notes on typical topics for the mini-lectures and common sources of student confusion (Supporting Information). These instructor notes provide detailed information on implementation of this activity.

Adaptation

This activity can be adapted for use in either an advanced inorganic chemistry course or a lower-division course. During the introduction to the origins of field splitting, there is an assumption that students have covered basic molecular orbital theory. Although crystal field theory, rather than ligand field theory, is commonly emphasized in the lower division courses, that focus is flipped in the treatment presented here. The emphasis on a molecular orbital approach more closely matches a theme of orbital theory throughout our curriculum. However, there is a short introduction to crystal field theory also included. In addition, students are expected to have an understanding of coordination complexes such as ligand binding, geometries, electron counting, and isomerism. We created this activity for our own students but recognize that other users may make different choices of specific topics, the applications they may choose to emphasize or the need to adjust preparatory material based on student needs. For example, this activity could readily be adapted for an alternative approach using only crystal field theory; the instructor might choose to provide an introduction to crystal field theory and omit the pages that deal with orbital explanations. These materials have also been employed in an advanced inorganic course at a regional comprehensive university. The instructor reported that this was a helpful approach to presenting ligand field theory to these students as well.

Daily Learning Activities

On the first day, students use d orbital model kits37 to explore the ability of ligands to overlap with orbitals on the metals to form M−L sigma bonds. This allows student to physically manipulate 3D representations to explore these concepts and then discover the orbitals that are nonbonding or bonding in an octahedral molecule. Previous research has suggested that students who use 3D models or simulations have increased conceptual understanding.38−41 On the second day, students study filling diagrams, metal ion effects, and the spectrochemical series. These ideas build on their understanding of splitting diagrams derived from the use of the orbital model kits on the previous day. Experimental data and theoretical frameworks are provided to allow students to develop a working understanding of different aspects of ligand field theory. On the third day, students apply their understanding of these concepts by using LSFE and splitting

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ASSESSMENT AND STUDENT FEEDBACK Student comprehension of ligand field theory was primarily assessed through two short quizzes. The first quiz covers basic ideas of ligand field theory, while the second quiz requires 1370

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students to use ligand field theory to analyze a full problem including properties of a coordination complex. The ligand field theory basics quiz was nearly identical each semester; each instructor modified examples but used the same template. Each instructor developed the application quiz individually. Representative data is summarized in Table 1; examples of quizzes can be found in the Supporting Information. Overall,

good gain). This result was very similar to their ratings of gains in other course topic areas such as introductory thermodynamics, equilibrium, Lewis acid−base reactivity, and enzyme behavior and regulation. Thus, students in a foundation level course were able to master the basics of ligand field theory even though this topic is normally covered in detail at the junior/ senior level.

Table 1. Mean and Standard Deviation on Ligand Field Theory Quizzes

CONCLUSION While ligand field theory is a key approach to explain the physical properties and lability of transition metal complexes, there are no publicly available resources for presenting this material in alternative approaches. A hands-on guided inquiry activity was developed for students to explore the key ideas in understanding ligand field theory. A list of open access resources to support this implementation approach is also provided. It can be used in both an advanced inorganic course and in a first year level course. Students at any level can benefit from this approach.

Section (n) S ’12 (27) F ’12 (9) S ’13 A (25) S ’13 B (23) F ’13 A (29) F ’13 B (14) S ’14 A (26) S ’14 B (24) S ’14 C (30) S ’14 D (29)

LFT Basics Mean % Score (SD) 88 83 68 94 76 84 90 83 92 91

(15) (10) (18) (10) (18) (18) (20) (19) (12) (10)

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LFT Applied Mean % Score (SD) 76 55 52 57 70 80 59 61 77 77

(8) (11) (14) (13) (7) (11) (22) (14) (15) (14)

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ASSOCIATED CONTENT

S Supporting Information *

The guided inquiry activity for LFT, a daily reading and implementation schedule, sample quizzes and an answer key are provided. This material is available via the Internet at http:// pubs.acs.org.

students demonstrated a very strong proficiency with the basic topics of metal ion effects, spectrochemical series, identifying high/low spin complexes, and completing splitting diagrams. Students did not perform as well in the application of this material to new systems. This is probably to be expected, as this type of question requires a higher level of understanding. However, students’ ability to apply these concepts was still satisfactory. Some of the variation in Table 1 can be attributed to different student samples in small sections (N = 25−30) as well as different difficulty levels of the quizzes administered by the individual instructor in each course. As we have refined the order of the material and the instructor expectations of first year students’ ability to apply this material, the student performance has increased on these applied quizzes. Class quizzes do not provide an external measure of student performance; that information is better assessed via American Chemical Society (ACS) standard exams. Four questions from the ACS Inorganic Chemistry Exam were identified as markers to provide an external measure of student mastery of this topic. The average percent correct (or difficulty index) on these questions for our students was 53%; this compares to an average of 52% reported by the ACS Exams Institute. While there is only a small difference between the results for the two groups and a small number of questions are involved, it is important to remember that the comparison groups are very different. The ACS Inorganic Chemistry exam is typically taken by junior or senior chemistry majors, whereas our students are completing this questions in their second semester of college and represent the usual mixture of students taking a second course in the chemistry sequence. However, this result suggests that our approach is assisting students in developing an acceptable understanding of LFT. We have also employed a SALG (Student Assessment of Learning Gains)42 survey to assess whether the students perceive a gain in their understanding as a result of the course. The average response to a question asking them to rate their gain in understanding of “the use of molecular orbital theory and ligand field theory to understand properties and reactivity” was 3.6 on a 1−5 point scale (n = 136; 3 = moderate gain, 4 =

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant No. 1043566. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The authors would like to thank T. Nicholas Jones for contributions to the assessment and Jeremiah Duncan for feedback about implementation.

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REFERENCES

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