4 Jan 2008

Bercaw vs Bergman

Literature Discussion

Submitted by Adam R. Johnson, Harvey Mudd College
Categories
Description: 

Relative metal-hydrogen, -oxygen, -nitrogen, and -carbon bond strengths for organoruthenium and organoplatinum compounds; equilibrium studies of Cp*(PMe3)2RuX and (DPPE)MePtX systems
Henry E. Bryndza, Lawrence K. Fong, Rocco A. Paciello, Wilson Tam, John E. Bercaw
J. Am. Chem. Soc.; 1987 ; 109(5); 1444-1456.

Monomeric Cyclopentadienylnickel Methoxo and Amido Complexes: Synthesis, Characterization, Reactivity, and Use for Exploring the Relationship between H-X and M-X Bond Energies
Holland, P. L.; Andersen, R. A.; Bergman, R. G.; Huang, J.; Nolan, S. P.
J. Am. Chem. Soc.; 1997; 119(52); 12800-12814.

These 2 papers are a good introduction to bond strength thermodynamics, and show that good ideas can be proven wrong, though it may take time.  Bercaw studied M-H, M-O, M-N and M-C bond strengths for Ru and Pt compounds and found a 1:1 correspondence of M-X vs H-X bond strengths (slope is 1:1 plotting M-X vs H-X).  A decade later, Bergman found that the 1:1 rule, as it had come to be known, didn't hold generally, as shown for Ni complexes.  The main problem is that substitution of M-N with H-O is not as simple as replacing M-N with H-N, and the data best fit a qualitative E-C model.  Bergman used (among other things) Hammet parameters and a wider range of ligands than Bercaw.

 I use this paper in a senior level organometallics discussion course where we study "classic" papers. 

Learning Goals: 

Upon completing this LO students will be able to

1) recognize that trends in transition metal chemistry are harder to extrapolate than in main group chemistry

2) count electrons using the CBC method to describe transition metal complexes

3) understand and describe the trend in M-X bond strengths observed by Bercaw for Ru and Pt

4) Understand and describe the breakdown in the trend Bergman showed for Ni

Related activities: 
Time Required: 
1 or 2 75 minutes class periods
Evaluation
Evaluation Methods: 

Students are expected to present the paper at a high level. I assess on preparation, and ability to lead a discussion.

Evaluation Results: 

A lively discussion ensues.

Creative Commons License: 
Creative Commons Licence

Comments

To all the students who think the Bergman paper is too long: blame me, it's my fault! For the benefit of chemists who are interested in the topic, I wrote a shorter and more accessible review article in a difficult-to-find journal. I have posted the paper on my website at the following address: http://chem.chem.rochester.edu/~plhgrp/plh/CIC_article.PDF .

late breaking news... Bill Jones weighs in on this debate.

http://pubs.acs.org/doi/abs/10.1021/jo9012223

a related activity would be the Robinson/Cotton debate on gallyne (https://www.ionicviper.org/five-slides-about/12-slides-about-african-ame...)

And the story continues:

Theory of Late-Transition-Metal Alkyl and Heteroatom Bonding: Analysis of Pt, Ru, Ir, and Rh Complexes
Devarajan, D.; Gunnoe, T. B.; Ess, T. H.
Inorg. Chem. 2012, 51, 6710-6718.

http://pubs.acs.org/doi/ipdf/10.1021/ic300350k

The VIPEr community supports respectful and voluntary sharing. Click here for a description of our default Creative Commons license.