The final exam is a DIY project. You write the final exam and an answer key to go with it. The instructions are very detailed. Please follow them carefully. Due: Thurs, May 16, Noon as a PDF file.
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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.
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.
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.
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.
The standard model of metal-CO bonding invokes two simultaneous orbital interactions:
- sigma donation by ligand: CO HOMO + metal LUMO (a hybrid of d-s-p valence orbitals)
- pi donation by metal: metal HOMO (d) + CO LUMO (π*)
You can investigate these interactions for several compounds of increasing structural complexity by downloading and examining the Spartan’10 models listed below. The models have all been optimized as neutral singlets using B3LYP/6-31G* except where stated otherwise. Tips for downloading and examining the orbital energies and surfaces appear at the bottom.
- Cr (high-spin multiplicity = 7; ground state of Cr is 3d^5 4s^1; donor orbitals are occupied alpha orbitals, acceptor orbitals are unoccupied beta orbitals)
- Cr(CO)5 (geometry obtained by deleted one CO from opt Cr(CO)6)
To download: click link, Download, and Save File. Open Spartan, navigate to the Downloads folder, and open the file.
To see potential MO interactions between fragments (Cr + CO, (CO)5Cr + CO) use Display: Orbital Energies to examine HOMO (donor) and LUMO (acceptor). Orbital energies are displayed in lower right-hand corner. (Note: this is not useful for Cr high-spin; in this case, use Display: Output to find orbital energies and use Display: Surfaces to generate and display orbital surfaces.)
To see resulting bonding orbitals (CrCO, (CO)6Cr) use Display: Orbital Energies to examine HOMO and HOMO-x orbitals. Look for orbitals that appear to combine fragment orbitals in a bonding fashion.
NMR, we all know, is the workhorse for figuring out what we’ve made. The problem with the typical NMR spectrum, though, is that it shows us signals from everything in the sample: the desired product (hooray!), by-products, leftover starting materials, and a variety of impurities (you’ve all heard the speech about how Reed’s CDCl3 always contains HCl, right?). At one time or another every chemist has been fooled by an “impurity” peak masquerading as something important.
Fortunately, help is at hand. Practically everyone encounters the same impurities so some chemists have sat down and tabulated the chemical shifts of these frequently-encountered impurities in several commonly used NMR solvents:
- NMR Chemical Shifts of Trace Impurities: Common Laboratory Solvents, Organics, and Gases in Deuterated Solvents Relevant to the Organometallic Chemist by G.R. Fulmer et al., Organometallics 2010, 29, 2176–2179, DOI: 10.1021/om100106e
- NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities by H.E. Gottlieb et al., Journal of Organic Chemistry, 1997, 62, 7512-7515, DOI: 10.1021/jo971176v
I will have afternoon conflicts on several Fridays during the semester (going out of town, meeting with job candidates, and so on). When that happens, I will have to shift or cancel my office hours.
Based on the ‘office hour’ poll that I conducted Monday, it seems like alternate hours won’t affect your ability to reach me so the important is to let you know as soon as I can when there is a schedule change.