This literature discussion focuses on a paper from the Angelici lab that examines the heat of protonation of [CpʹIr(PR3)(CO)] compounds. The compounds presented in the paper provide good introductory examples for electron counting in organometallic compounds. The single carbonyl ligand in these compounds provide an excellent probe to monitor the electron richness at the metal center which is impacted by the electron donor ability of the ligands. The technique of titration calorimetry presented in this paper is likely new to the students, but it is just a combination of two techniques they likely learned about in general chemistry. This LO was developed as part of the 2016 workshop on Organometallic Chemistry.
A student should be able to
- apply the CBC method for electron counting to the Ir complexes in this paper
- explain the titration calorimetry experiment and describe what it measures
- describe the bonding in metal carbonyl compounds and explain how the electron richness at the metal center impacts the bonding in these compounds
- describe the electron donor ability difference of Cp* as compared to Cp
- explain the differences in electron donor ability of phosphine ligands
- relate the basicity of the metal complex to the electron donor ability of the ligands
A more advanced student may
- question the chemical shift of a M-H in the 1H NMR spectrum
- question the chemical shifts in the 31P NMR spectrum if they only have experience with 1H NMR
- connect the reactions performed in this paper to oxidative addition, in particular if they note the change in the valence of the Ir
- read the nucleophilicity studies in more detail
I use the CBC method of electron counting, so that is how the electron counting is presented. Certainly you can modify this to meet your electron counting needs.
The paper presents both the heat of protonation study and the reaction of the Ir compounds with MeI (referred to as nucleophilicity). The general trends in both studies are the same, but the nucleophilicity studies are a bit more complex to read. There is plenty to learn from this paper without worrying about the nucleophilicity studies, so the focus of the LO is on the heat of protonation studies. Additional questions could certainly be developed to include the nucleophilicity studies.
In order to guide the students, I would provide them with a pdf version of the paper with the following comments.
Page 1322 Experimental Section highlighted in yellow - There are lots of great details in the Experimental Section but most of them would only be useful if you are actually repeating this work. For our purposes you can skip most of these fine details.
The sub-sections highlighted in green provide general details about the experiments that were performed and should be read.
Numbers highlighted in blue may prove useful during our discussion of this paper.
Page 1322 Synthesis of CpIr(CO)(PR3) Complexes highlighted in green.
Beginning on page 1322 and ending on page 1324 all of the nCO values are highlighted in blue.
Page 1325 section Calorimetric Studies of Reaction 1 is highlighted in green.
Page 1326 Results is highlighted in yellow - While all of the results section contains interesting details, the sections highlighted in green will be most useful for our discussion.
Page 1327 Characterization of Products in Reactions 1 and 2 the first paragraph is highlighted in green.
Page 1327 Calorimetric Studies section is highlighted in green.
Page 1328 Discussion is highlighted in yellow - While all of this section is an interesting read, the subsection headings highlighted in green will be of the most use for our discussion.
Page 1328 Basicities of CpIr(CO)(PR3) Complexes 1-11 subsection is highlighted in green.
Page 1328 Basicities of Cp*Ir(CO)(PR3) Complexes 12-16 subsection is highlighted in green.
Page 1329 Effects of Cp* and Cp on Metal Basicity (DHHM) in CpʹIr(CO)(PR3) subsection is highlighted in green.