I’ve received a number of research ads from my talent scouts. I expect a few more ads to appear over the next 24 hours so please check back. A full set of instructions for what you need to do between now and Thursday has been posted, but here is the short version:
Hi, the last page of my lecture notes from yesterday can be downloaded here.
Take-home message: the mechanism of oxidation addition often changes to SN2 when there is 1) a good leaving group (X) and 2) that group is attached to a suitable electrophile.
This principle is illustrated by the reaction of Vaska’s complex with CH3I.
‘Talent Scout’ is one of two major assignments for our course (the other will be described later in the semester). Talent Scout consists of several stages with separate due dates (see below). Please read the instructions carefully and let me know if you have questions.
- Tu, April 9, 6 PM – research ad due (email, PDF) (note: if you are taking the qual on the preceding weekend, you may turn in your ad by W, April 10, noon)
- Th, April 11, 6 PM – team members, paper for discussion, preferred date (email)
- 4 days before your in-class discussion – instructions for reading paper (email, PDF)
- immediately after in-class discussion – script for leading in-class discussion (hard copy)
- same day as classmate’s discussion, 6 PM – reflection on research ad (hard copy or email)
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.
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:
- 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.
- 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:
- 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.)
- 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.
- 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:
- email your contributions to me (firstname.lastname@example.org)
- 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)
- 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 anion
- Ticp2 (+2 ion)
- Fe(cp*)2 (“cp-star” is C5Me5)
- cpFe(CO)2Cl (“cpFe(CO)2” is sometimes written as “Fp”, pronounced “Fip”)
- cpMn(CO)3 (“piano stool”)
Here are some links to models (all B3LYP/6-31G*) that we will use in class today (Monday, Feb 25). Download and save the files (as needed). Then use Spartan to open them (they are in the Downloads folder).
Fragment MO Analysis of (CO)4Fe(C2H4)
- (CO)4Fe(C2H4) – equilibrium geometry
- (CO)4Fe fragment – geometry from previous model
- C2H4 fragment – geometry from previous model
- (CO)5Fe – equilibrium geometry
Orientation Effects in (CO)4Fe(C2H4)
- (CO)4Fe(C2H4) “EQ” – (CO)4Fe frozen* and C2H4 optimized with C’s coplanar with equatorial CO
- (CO)4Fe(C2H4) “AX” – (CO)4Fe frozen* and C2H4 optimized with C’s coplanar with axial CO
*(CO)4Fe fragment obtained from equilibrium geometry model of Fe(CO)5
Alkene Substituent Effects in (CO)4Fe(alkene)
- (CO)4Fe(C2Z4) – equilibrium geometries (Z = H, CN, F, CH3)
Other metal-pi complexes
- (CO)4Fe(C2H2) – equilibrium geometry
- [2(COD)Rh-eta2-Cl]2 – equilibrium geometry (COD = 1,5-cyclooctadiene)
- (pi-allyl)2Ni – equilibrium geometry
According to the standard M-CO bonding model, sigma donation from CO to the metal transfers electron density from CO to M, while pi donation from the metal to CO reverses the electron flow. The ultimate result of these opposing bonding mechanisms is far from obvious, but one can appeal to the MO model to make a prediction: greater transfer of electron density occurs when the fragment orbitals are:
- closer in energy (better energy match) and
- achieve greater overlap
The situation is further complicated when several ligands, either all CO, or a mix of CO and others, bond to the metal simultaneously. You can investigate the flow of electron density using Spartan’10 electrostatic potential maps of the (CO)5CrL models listed below. Spartan’10 models of (CO)4CrL fragments in which the CO trans to L has been removed are also provided for comparison (how does adding CO to the fragment affect the distribution of electron density?). All pentacarbonyl geometries have been optimized using B3LYP/6-31G*, while the geometries of the tetracarbonyls have been obtained by removing the trans CO in the pentacarbonyl.
- L = dihydroimidazole of type used in Grubbs olefin metathesis catalyst, (CO)5CrL & (CO)4CrL
- L = pyridine, (CO)5CrL & (CO)4CrL
- L = NCCH3, (CO)5CrL & (CO)4CrL
- L = CNCH3, (CO)5CrL & (CO)4CrL
- L = PMe3, (CO)5CrL & (CO)4CrL
- L = PF3, (CO)5CrL & (CO)4CrL
- L = CO, (CO)5CrL & (CO)4CrL
The pentacarbonyl models also contain calculated IR frequencies (unscaled). These frequencies have traditionally been used to assess the degree of pi donation from metal to ligand (and the bonding properties of the ligand trans to CO). In fact, there is a fair correlation between frequencies and electrostatic potentials.
A short summary of the results can be found here.
To download a model: click link, Download, and Save File. Open Spartan, navigate to the Downloads folder, and open the file.