This consists of two parts (and a solution, which is linked below under "Related Resources", but for which you will need a faculty privileges): a primer for students (best if handed out prior to class so that students can read it beforehand, or delivered in pre-lecture format) and a worksheet. The worksheet is designed to be done in small groups with assistance from an instructor. In very large classes, in which the instructor cannot circle amongst the groups, the instructor can work through each example after the groups have a few minutes to work the problem on their own.
The idea is for the students to learn to "count to 18" by figuring out their own rules, given that the compounds are 18 (or 16) electron compounds rather than telling them the rules for various ligands.
Omitted from this worksheet are the more complex cases: compounds with metal-ligand multiple bonds and nitrosyls, since these are often handled later in the organometallics unit by many instructors and since instructors and textbooks differ in their treatment of these ligands.
Students will be able to determine dn electon counts, oxidation states, and overall electron count for most organometallic complexes.
Students will be able to explain the rationale behind the counting schemes and to articulate the pros and cons of each method.
Students will be able to identify and become familiar with a number of common ligands without time being specifically dedicated to a survey of these ligands.
Students. Implements of writing.
I primarily evaluate this by circling the room and watching them grow in ability to assign electron counts and justify their methods in their own words. They get pretty good at it by the end. In addition, I later progress to talking about catalytic cycles, and ask them to do electron counts for each species in several catalytic cycles. I can tell how well they've internalized these skills by their ability to approach these new problems.
As long as groups are chosen by the instructor (such that there is a range of ability in each group), most students are able to work through the problems successfully. Students seem to have the most difficulty with the very beginning, in which they have to do their "first electron count" and oxidation state/d-electron configuration count.
However, most groups need some assistance at some point in the worksheet (some at several places), so it really is best to either constantly "check in" and work through the problem on the screen (best for large classes) or provide helpful hints and let each group work at its own pace (for small classes).
If the students are unable to complete the worksheet in time, it can be completed for homework.