Bonding models: Discrete molecules

17 Apr 2008

Fivefold Bonding in Cr(I) Dimer

Submitted by Maggie Geselbracht, Reed College
Evaluation Methods: 

Student performance on the discussion questions is assessed.

Description: 

This paper describes the synthesis and characterization of a Cr(I) dimer with a very short Cr-Cr distance.  Computational studies support fivefold bonding between the chromium atoms.  I have used this paper to introduce metal-metal multiple bonds and discuss the molecular orbital interactions of homonuclear diatomics including d-orbitals.  More generally, it is a nice example to stimulate the discussion of what constitutes a bond and the various interpretations of bond order.

Prerequisites: 
Corequisites: 
Implementation Notes: 

Students are asked to read the paper and answer the discussion questions before coming to class. I used this paper in a second-year inorganic course after we had talked about MO theory of diatomics but fairly early in our discussion of transition metal chemistry. There is a Perspectives article in Science that goes along with this paper, but I decided not to give it to the students as I wanted them to work out the MO diagram for themselves.

Time Required: 
50 minutes
28 Mar 2008

Miessler and Tarr: Inorganic Chemistry, 3rd. Ed

Submitted by Nancy Scott Burke Williams, Scripps College, Pitzer College, Claremont McKenna College
Description: 

Miessler and Tarr is an inorganic textbook which is is best suited to an upper-division one-semester inorganic course, though there is more material than can be covered in a single semester, so some choice of topics is necessary.  It is very well suited for a course oriented around structure, bonding, and reaction chemistry of transition metal compounds, but is very limited in its treatment of solids, main-group, descriptive chemistry, and bioinorganic.  Pchem would be helpful but is not necessary.  In particular, the treatment of MO theory is very in-depth.  The quality of end-of chapter problems is generally good.  The book is fairly readable, giving it an advantage over some of the more "reference work" style textbooks, but as a result, is a less useful text to have on your bookshelf five years hence.  Pearson Higher Ed. suggests a retail price of $144.20.  

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Course Level: 
28 Mar 2008

Zinc-Zinc Bonds

Submitted by Maggie Geselbracht, Reed College
Evaluation Methods: 
Student performance on the discussion questions is assessed.
Description: 
This paper in Science reports the synthesis of decamethyldizincocene, a stable compound of Zn(I) with a zinc-zinc bond. The title compound and the starting material, bis(pentamethylcyclopentadienyl)zinc, offer a nice link to metallocene chemistry, electron counting, and different modes of binding of cyclopentadienyl rings as well as more advanced discussions of MO diagrams. More fundamental discussion could focus on the question of what constitutes the evidence for a chemical bond, in this case, the existence of a zinc-zinc bond.
Prerequisites: 
Course Level: 
Subdiscipline: 
Corequisites: 
Implementation Notes: 
Students are asked to read the paper and the accompanying Perspecitves article before class as well as answer the discussion questions. The questions serve as a useful starting point for class discussion. I also took this opportunity to introduce the MO diagram for ferrocene to illustrate the 18 electron rule.
Time Required: 
50 minutes
26 Mar 2008

Housecroft and Sharpe: Inorganic Chemistry, 3ed

Submitted by Lori Watson, Earlham College
Description: 

Housecroft and Sharpe (Inorganic Chemistry, 3ed): This is a comprehensive inorganic textbook designed primarily for students at the Junior/Senior level. P-Chem would not be needed as a prerequisite for this text, but would be helpful. It includes both theoretical and descriptive material along with special topics, enough for a two semester course though it is easily adaptable to a one-semester "advanced inorganic" course by choosing only some topics. It is written in a clear and generally readable style and the full-color graphic contribute to student understanding. Ancillaries include electronic versions of most figures, and a student site with a limited number of multiple choice review questions for each chapter. The 3rd edition updates the end-of -the-chapter problems, though disappointingly does not draw problems from the recent literature. In general, these are good review problems to make sure students understand the basic concepts, but some faculty will want to supplement student assignments with more challenging problems. The list price for the student text is $175 for a paperback, 1098p version.

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4 Jan 2008

Generating LGOs (SALCs)

Submitted by Adam R. Johnson, Harvey Mudd College
Evaluation Methods: 

I have weekly in-class problem exercises that emphasize group work and in-class presentation of the answers.  The in-class participation is worth about 10% of the course grade.

Description: 

    After several days of lecturing on the topic of polyatomic molecular orbital diagrams, students break into small groups of 3-4 and form LGO’s that can be used to interact with a central atom to form a Molecular Orbital (MO) diagram.  This assignment is part of a larger 4-5 week unit on MO theory. 

    The first third of my inorganic course is devoted to polyatomic MO theory as it is the basis of modern understanding of bonding, reactivity and spectroscopy.  Generally, even students at the advanced level have not ever formed an MO diagram for a polyatomic compound (MXn;  e.g.  PF5, CCl4,  CoF63-) whether main group or coordination compound.  After lecturing on symmetry and symmetry operations, and a brief review of Lewis theory, Valence Bond theory, and diatomic MO theory, students are shown that interaction of three or more atoms to form a compound is significantly more difficult unless all of the ligand orbitals are taken together as a group.  The LGO’s are derived using an intuitive, symmetry-based approach that does not require linear algebraic techniques (projection operators).  This allows the students to quickly and easily derive LGO’s and thus MO diagrams for complicated molecules and coordination compounds at a “back-of-the-envelope” level of theory suitable for drawing during a seminar or on a cocktail napkin.
    Doing the problems in this way allows me to cover much more complex material that I would not want to ask on a homework assignment.  I circulate through the room answering questions and providing guidance in real time.  The last 15-20 minutes of class are reserved for each group to describe their solution to the problem.

Learning Goals: 

after doing this exercise, a student should be able to:

i)From a predicted molecular geometry, determine the central atom's hybrid orbitals and use them as generator orbitals

ii) Generate the LGOs by taking linear combinations of the ligand bonding orbitals

iii)  assign proper symmetry labels to the LGOs using a character table

iv)  predict the symmetry of lone pairs (if applicable)

Course Level: 
Equipment needs: 

none

Implementation Notes: 

I spend a good deal of time explaining the technique in class (one to two lectures) and do simple examples in class.  This in-class practice session really cements it home for them. One of these days I will post a five-slides-about LO in order to more fully explain the technique.  Please email me if you want additional details on implementation.

Time Required: 
one 50 minute class period
4 Jan 2008

In Lewis' Own Words

Submitted by Nancy Scott Burke Williams, Scripps College, Pitzer College, Claremont McKenna College
Evaluation Methods: 

See implementation notes. What I evaluate is the sorts of questions and answers the students generate both on their own and in the discussion.

Evaluation Results: 

I've been very impressed by the level of discussion this generates. The students love the paper for its readability, the "outdated" ideas, and the cleverness of the hypotheses put forth. They are very good at putting themselves in Lewis' position, knowing what he did, and the discussion quickly developed (this year) to a surprisingly deep discussion of the nature of scientific truth, the iterative scientific method, and the role of theory and experiment. The students "get" these models, and can see how powerful they are even though they are "wrong". I finished by emailing them Bronowski's incredibly powerful monologue on scientific knowledge: 

 

Description: 

This is G. N. Lewis' classic paper explaining his "octet rule" and the idea of bonds being represented as shared electron pairs.  It's beautifully written, and is a lot of fun for students to read.  Highlights include his description of atoms as being concentric "cubes" of eight electrons at the vertecies, philosophical discussions of the importance of letting experimental observations guide the development of theory, and the sense that students gain that Lewis developed this theory completely in the absence of an orbital or Bohr-type model of the atom.  It wonderfully captures the way in which a brilliant mind wrestled with the problems of developing a bonding theory for the periodic table.  Students understand the paper pretty easily, and are capable of picking out little "gems" on a first read of their own which they bring to a discussion.  

Prerequisites: 
Corequisites: 
Learning Goals: 

Students should be able to address the following questions:

(1) Explain Lewis' original "cube model" and his subsequent "tetrahedron model" of the atom.

(2) Why does Lewis abandon his first for his second model in this paper, even though it contradicts the prevailing theory of the day.

(3) Compare and contrast what Lewis actually proposed with the "Lewis Dot Model" which youlearned in genchem. 

(4) Please discuss the usefulness of  low-level theory which is easy to use vs. high-level theory which is hard to use, and articulate the value (or lack thereof) of doing things at a "simplistic" level.

Implementation Notes: 

I hand out this paper in advance of one class and have the students bring the answers to the questions in the handout as well at three questions of their own, which they give to another student. By the next class, they try to answer those student questions, and we use all of the questions so generated as a basis for a full class discussion.

Time Required: 
5 minutes on one day, plus one additional day for discussion (students read beforehand)

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