15 Jul 2010

Electron Counting

In-Class Activity

Submitted by Adam R. Johnson, Harvey Mudd College
Categories
Prerequisites: 
Corequisites: 
Course Level: 
Description: 

I use these two handouts early in my inorganic course to outline how to count electrons and assign ligand types in a metal complex.  I introduce it early so that I can use the terms "X" and "L" in class.  I come back to it and hit it again when I do my unit on organometallics. The "ligands" handout is my interpretation of the MLH Green paper from 1995 (Green, M. L. H., J. Organometal. Chem., 1995, 500, 127-148.).  I really only use X and L in my class (and in my research), though I do make use of X2, L2, etc.  I ignore the bar notation.

Learning Goals: 

Using this worksheet, a student should be able to count electrons using either the ionic or covalent method for simple transition metal complexes and organometallics.

Equipment needs: 

none

Implementation Notes: 

I hand this out in class and then proceed to do a few examples in class.  The examples on the "18 Electron Guideline: A primer" linked from this LO are good examples to do in class.

Time Required: 
one half of a lecture period
Evaluation
Evaluation Methods: 

I routinely ask students to count electrons and assign ligands as X or L on quizzes, exams, and when they come to my office seeking help. 

Evaluation Results: 

Once students figure out the difference between X and L, most can count electrons just fine.  Occassionally, students will count the ligands ionically and the metal covalently (or vice versa), which leads to all sorts of problems.  I specifically alert them to NOT do this when I introduce the topic, but some of them have to make a few mistakes before they get it.  I would say that 90% of my students can count electrons and assign X and L by the end of my course.

Creative Commons License: 
Creative Commons Licence

Comments

I had Ged Parkin out for a visit last year and he really sold me on the CBC method developed by Malcolm Green. I adopted it an used it in my course last year and really had a lot of fun with it. While it is not quite second nature to me yet, I did find myself really questioning the way people think about 'oxidation states' and such at a recent Gordon conference. Ged sold me on thinking about Fe(IV)...are you really going to have an atom that is +4 bound to negatively charged ligands?

 Ged's website on the CBC method is excellent as is his contribution to Comprehensive Organometallic Chemistry Vol III.

 At the GRC I mentioned to Ged how this really should be applied to main group and dare I say it, organic, molecules. Imagine, a unified idea that could be taught starting in gen chem. While supportive, Ged said he wasn't the one to do it. I don't think it is a one person effort, but I do think it is one worth exploring...

do you have a link to Ged's website? 

Darn it, I thought I put it in there. Silly me.

 

http://www.columbia.edu/cu/chemistry/groups/parkin/cbc.htm

thanks!

Hi Adam,

I spend quite a lot of time on electron counting and I teach the ionic method - I think it is confusing to students to learn two methods. I mention that there is another method (because I also refer them to some websites and texts for practice) but I emphasise a single method. I think this helps with reducing those errors you mention. I do point out that oxidation state is a formalism and exactly what Ged says - it is artificial to say a high oxidation metal is right next to anionic ligands - but as long as they know it is just a book-keeping tool I think oxidation state is useful. 

To get from simple "how many electrons" questions to higher order thinking skills, I ask questions about reactivity (eg nickelocene vs ferrocene), number of carbonyl ligands on metal carbonyls, predicting whether a reaction with CO occurs etc.

How are your students with the list of ligands? I keep reducing the number on my list because my students freak out, also they seem to be unable to recognise simple molecules (water! ammonia!) when they are presented as ligands, and want to tell me that they have charge. I am not sure how to get around this!

Cheers,

Madeleine

 

Dr Madeleine Schultz

Chemistry

Queensland University of Technology

Brisbane QLD 4001 Australia

madeleine.schultz@qut.edu.au

Madeleine.... I thought I recognized your name.  We shared a lab in 1997-1998 (I was in the Raymond group). 

The list of ligands is long, but I like to show it to show them the depth of inorganic chemistry.  In the end, we mostly use amines, ammonia, phosphines, CO, water, etc.  I also like teaching both methods, even though it confuses some students.  Some really think better about it as the ionic method, and some better think about it as the covalent method.  I like you idea about getting to the higher order thinking, and now that I think of it, I do something similar in my course.  Specifially, we talk about Mn(CO)5 or V(CO)6 and whether its likely to gain/lose an electron.

Dear Madeline, Chip and Adam,

I also used the CBC method of Electron Counting for the first time in my teaching this year and loved it.  I was inspired by Chip Nataro (Lafayette College) and Matt Whited (Carleton College) who are both advocates of this method of electron counting and I used Parkin and Green's newest J. Chem. Educ. Paper (dx.doi.org/10.1021/ed400504f | J. Chem. Educ. 2014, 91, 807−816).  I used the paper both for the development of my own notes and gave the paper to my students as reference material/reading.  Like Adam I taught both methods  (CBC and Ionic) and I like that the students see the contrast.  When I found ligands that seemed too challenging I drew a ligand structure for them (showing pairs of electrons or single electrons to help them identify X and L ligands:  Y. Jean's book "Molecular Orbitals of Transition Metal Complexes" Oxford University Press, 2005 also gives a nice brief overview of determining X and L ligands in the Introduction).  Finally, I used Matt Whited's LO "In Class Activity: Electron Counting for Organometallic Complexes" and I used examples 1, 2, 3, 4 (I gave them the structure of the ligand in 4 from Matt's Answer Key) and 7.  These are hard problems and we did them as an in-class challenge problem set with teams of 2-3 working on each problem. I found some of the problems (particularly the last two) too hard to give to my students.

One last note.  I liked examining complexes with M-M bonds (I did both single and quadruple) that Parkin and Green note for a Cl-Hg-Hg-Cl example give an unusual answer when considering a molecule with an element-element bond.  I communicated directly with Ged Parkin who was willing and gracious with his time to confirm and clarify my thinking on this.   So for example in [Cl4-Re-Re-Cl4]2- with a quadruple bond in the Ionic Method, we think of each Re as Re(III), d4 with four Cl- ligands as 2 e- donors each. Since each Re shares 4 e- with the other Re, each Re ends up with a 16 electron count.  However in CBC...we treat the quadruple bond as an X4 ligand, so each Re is a [MX8]- species which has the equivalent neutral of [MLX7]. This suggests Re is Re(VII) d0 (yikes!!) but rather than think of it as d0 think of it as v0 = the number of electrons in metal non-bonding orbitals. Also as all have you have pointed out...we have to be careful about putting too much into the meaning of oxidation state.  Back to the problem:  In the Ionic method we have a d4 electron count to make the metal-metal quadruple bond (which works well) but in the CBC Method the metals are considered as having no electrons in non-bonding metal type orbitals which works well.  What doesn't work in the CBC method is you can't really call the oxidation state of the metal Re(VII) because the overall charge doesn't balance.  Here you need a broader view where VN=7 is not the same as the oxidation state of the metal.  The positive of using an example like this is to show students what works well about each method and what doesn't work well for each method.

Finally, I want to thank Chip and Matt for the inspiration to use this method of electron counting.  I found it thought provoking and useful.

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