17 Jul 2010

Kinetics of Ligand Substitution Reactions of a Pt(II) Complex

Submitted by Scott Cummings, Kenyon College
My Notes
Course Level: 
This inorganic lab experiment, focusing on the kinetics of ligand-substitution reactions of a square-planar Pt(II) complex, involves collecting UV-vis absorption data and analyzing the results to determine a rate law to support one of three proposed mechanisms.
Learning Goals: 
Students will review kinetic rate laws, integrated rate equations, and preparing solution mixtures of known concentrations (from Introductory Chemistry); Students will analyze experimental kinetics data: fitting first-order and second-order rate laws and determining an experimental rate law; Students will evaluate how an experimentally-derived rate law can be used to support or reject proposed mechanisms.
Equipment needs: 
UV-vis spectrophotometer (experiment provides directions for using common Agilent/HP instrument); 100-1000 microliter autopipetters;
Implementation Notes: 
See INSTRUCTOR NOTES for full list of reagents and equipment, some notes and sample results.
Time Required: 
two 3-hour lab sessions
Evaluation Methods: 
(1) Check students' ability to use correct calculations and techniques to prepare solutions of known concentrations; (2) Review students' fit of first-order and second-order integrated rate equations to a kinetics data set, and discuss with them their choices of which data to use and which to reject; Also check their calculation of the observed rate constant from this fit. (3) Review students' analysis of how changes to the concentration of the excess nucleophilic entering ligand affects the observed rate constant, and how this relationship establishes the best rate law for the reaction. (4) Poll students on what follow-up experiments they might use to test their conclusion or resolve uncertainties. (5) Read lab report (includes this and related experiments involving this coordination compound) to ascertain how students' use data to support a conclusion (proposed mechanism) and address uncertainty and error.
Evaluation Results: 
Not all students who enroll in this advance lab course have completed Introductory Chemistry, so this is the first exposure to experimental kinetics for some. Most of my discussions with students address their confidence in knowing solution concentrations (a reflection of lingering difficulties with lab technique and calculations). Nearly all students can satisfactorily fit first-order and second-order integrated rate equations to data for a single kinetics run, but some struggle to understand the purpose of running trials using varying thiol concentrations and how these results are important for determining the form of the rate law. Inviting students to compile their results with those of previous year’s students and primary research literature raises important issues about reproducibility; some students, though, become disappointed when their results display evidence of poor lab technique. Stronger students recognize, in their lab report, how their results relate to relevant research literature and make connections to other experiments they perform with this coordination compound.
Creative Commons License: 
Creative Commons Licence



I love the data. I think you could also use the data as an example problem in a lecture too.

Sibrina Collins, PhD College of Wooster

This sounds like a great lab with lots of meaty kinetics work, and I'm thinking about using it the next time I teach our inorganic lab course.  Scott, do you have any suggestions on where to source the starting Pt complex from?  It's selling for $500+ (for 25 mg) on Aldrich.  Is there a simple synthesis that could save $$?


Hi Anne!  I haven't tried it yet with students (I will be this semester) but you can make the Pt terpy complex from the Pt COD complex.  Check out Annibale, G.; Brandolisio, M.; Pitteri, B. Polyhedron, 1995, 14, 451.  The Pt COD complex is $92 per gram from Strem and the synthesis looks VERY easy.

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