March 2013

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We are frequently told that the scientific method offers a dispassionate means for disproving hypotheses. What we are not told is that scientists are passionate, just like every other human on the planet. Once a scientist has invested a few neural connections in a flawed hypothesis, the error proves remarkably hard to root out. Scientists get “stuck” just like everyone else.

I was reminded of this fact when I read our textbook’s discussion of metal-to-ligand backbonding in M-PZ3 complexes. Not only do the authors devote more words to the discredited theory (3d orbital participation) than they do to the right theory (P-Z σ* participation), they also give credit for the theory to a brief 1985 communication (Orpen & Connelly, DOI: 10.1039/C39850001310) when a full paper describing the basis for σ* participation had already appeared in JACS in 1983 (Xiao et al., DOI 10.1021/ja00362a004). One wonders if the authors have fully embraced the σ* story.

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Our next discussion (Friday, March 15) will focus on another paper from the Casey research group: Conversion of an h5-Cyclopentadienyl-Metal Complex to an h1-Cyclopentadienyl-Metal Complex upon Addition of Trimethylphosphine, JACS, 1980, 102, 6154-6156, DOI: 10.1021/ja00539a036.

This paper is quite short (barely 3 pages), but it contains a lot of structural data (x-ray, NMR, IR) that will appeal to organic and inorganic chemists alike. The reason for the data-heavy content is that the authors claim that they are observing a transformation that earlier chemists had proposed, but could never document.

Here is how I would like you to prepare for/participate in the discussion:

  1. I would like you to assume responsibility for more of the technical expertise during the in-class discussion. Mainly this means knowing how to interpret the data that appears in the paper. It also means that I won’t provide a discussion “prompt” to get the ball rolling.
  2. Participate in the online discussion before class. One theme that appeared in many reflections on the previous discussion was, “I wish we had been able to discuss X more than we did.” I want to see if we can generate that extra discussion space using this blog. This will be an experiment, but one worth trying. Here’s what you need to do:
    1. Before Wednesday (March 13) midnight, I expect everyone to make one online post about this paper. To do this, click on “Comments” on this post. Type your entry. Upload it. (Note: I am going to make the first comment, but as you will see, it is somewhat off topic. I would prefer that your comments and responses address issues in the paper.)
    2. Before Thursday (March 14) 9 PM, I expect everyone to read all of the online comments and add an online comment to one of the earlier comments.
    3. Between Thursday 9 PM and Friday 10 AM, read all of the new online comments.

Let’s see what happens.

We will be talking about carbenes and metal-carbene complexes on Monday. Your textbook describes MO models of the metal-carbon double bond in two locations: Ch. 6-1-2 and Ch. 10-1.

We will move on to metal-hydrides Ch. 6-2 and metal-phosphines Ch. 6-3 on Wednesday. Then we will jump to Ch. 7, ‘Organometallic Rxns Part I’ right after Spring Break.

I have also found a short paper that I want you to read and discuss on Friday, but information about this will appear in another post.

The downside of in-class discussions of scientific papers is that the discussion can only last 50 minutes or less. What if we could start the discussion before class? What if we could run it past the end of class? We can do both if we conduct some of the discussion online. Here are some ideas about what might go into a pre/post-class online discussion:

  • A shout-out to something noteworthy/cool in the paper
  • A comment on some particularly helpful/worthless reference
  • A request that we devote some time in-class to a particular topic
  • A question about something

Let’s face it, in-class discussions don’t necessarily suit everyone’s taste. Some of us are great speakers, some of us are great writers, some of us are great thinkers, and some of us are all-of-the-above. An online discussion provides some additional venues for your ideas and participation to shine.

Here’s my idea: I will use this blog to post any online contributions you choose to make. Because our blog is currently viewable by the entire internet,* I will make contributions optional. I will also give you the choice between having your contributions attributed to you (only first names will appear online) or having it posted anonymously. And when the semester is over, I will delete the posts that contain the student contributions.
*My understanding of federal law is that your course work can’t be posted online without your permission unless the online site is password-protected and access is limited to our academic community. I’m working on getting our blog changed over, but in the meantime I’m making the online discussion and attachment of (first) names optional.

If you want to participate, here’s what you do:

  1. email your contributions to me (alan@reed.edu)
  2. I will post contributions as comments as they come in here on the blog along with your first name or no name at all (let me know your preference)
  3. I will not post anything received after 5 PM Sunday

Benzene is aromatic. So is cyclopentadienyl anion (the symbol is “cp” and most chemists I know pronounce this “see-pee”). Yet both molecules happily make strong bonds to transition metals. What’s up with that? We’ll try to figure this out on Friday using the following models (all B3LYP/6-31G*):

cp complexes
benzene complexes