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Midterm

CHEM 312 Study Guide - Midterm Guide: 18-Electron Rule, Pi Backbonding, Pnictogen

by mss2

Department
Chemistry
Course Code
CHEM 312
Professor
C Murphy
Study Guide
Midterm

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Lecture 19
Organometallic chemistry: study of chemical compounds containing bonds between C and M.
Combines aspects of inorganic and organic
Ligand cannot be CN, which is a “coordination compound”
Cp = cyclopentadiene, Cp* = pentamethylcyclopentadiene (-1 charge)
Bridging ligands - mu (μ)
the wavenumber of the CO bond decreases as μ increases due
to increase pi backbonding and decreasing strength of CO bond
Hapticity - eta (η)
Number of atoms that is coordinated to the metal
Cp can have three different ways to coordinate - η1, η3, η5
18 electron rule
10 d e- + 8 s/p e-
More stable than compounds with less or more e-
Electron counting
1. Assign M ox number
Based on L charges and overall complex
charge
M-M bond counts as 1 e- for each M
Bridging L share e- equally between M
2. All valence e- on M are d e-
3. Count pairs of e- donated from L (usually 2, can be more)
4. Number of ve- = d
n
+2L
Isolobal analogy
Main group and organometallic fragment comparisons
7e- 17e- (halogens - I, F, “salt forming”) anions, dimers, inter halogens, hydrogenation
6e- 16e- (chalcogen - O, S, “ore forming”)
5e- 15e- (pnicogen - N, P, “choking”)

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Lecture 20
Organometallic compounds make great catalysts
Metal carbonyls
Advantages of CO ligand
Strong field, pi acceptor, 18 e- rule usually applies
Complexes are thermo stable
Complexes are ox stable (can leave in air)
Spectrally traceable
IR distinct region
As e- density on M increases, pi backbonding increases and pi* e- density
increases, which weakens CO bonding which decreases IR cm
-1
stretch.
This is the opposite for the M-C stretch
13
C NMR
Commercially available
Ligands can be
terminal (CO stretch cm
-1
2100-1900)
bridging (CO stretch cm
-1
1900-1750)
facial (CO stretch cm
-1
1800-1600)
Free CO stretch = 2230
Bonding
CO is weak sigma donor
Also strong pi acceptor (pi backbonding)
These 2 are synergistic
Hydrides
Really big range
Base cat. CO oxidation to make M-CO hydride (Hieber Base Reaction)
Hydrogenases - enzymes that reversibly catalyze the conversion of H+ into H2
1H NMR is way upfield (-22 to -38 ppm)
Olefin ligands
Bonding - pi backbonding like CO
As e- density on M increases, pi
backbonding increases and pi* density increases. C=C bond weakens,
bond length increases, C=C stretch decreases
Ferrocene (Fc)
Fast rotation of Cp from eclipsed → staggered due to low ΔG
Easy synthesis because form salt FeCl
2
+ 2NaCp → FeCp
2
+ 2 NaCl (in THF)
Common electrochem standard for redox. -0.4V vs SCE. redox sensor (1 e-)
Reactions on the ring - if something is added to the ring, the ring might move a
bit, allowing the M to associate with another ligand.
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Lecture 21
Ligand substitution
Trans effect to figure out which
ligand will leave
For 14 or 16 e- compounds, mechanism is associative and RDS is L association
Solvent can participate
Migratory Insertion
Anionic and neutral L on M couple to make a new L
General features
No change in ox state
2 groups must be cisoidal (close enough to jump)
Makes open coordination site ( beta hydride elimination must have open coord. site)
Usually cannot tell if insertion or migration
Migration = anionic L does Nu attack
Insertion = neutral L inserts into M-L
anionic
bond
1,1 and 1,2
B hydride elimination is the reverse of a migratory insertion
If you don't want this to happen, good to have L coordinate to empty
site so B hydride elimination less likely to happen
Oxidative addition
2 new L added to M
Ox state of M increases by 2
Favored for: e- rich, low valent M, Sterically unhindered M, Low X-Y BDE, Easily dissociating L
Can result in cis configuration of new ligands, not a requirement
Reductive elimination
Reverse of ox addition
concerted
M ox state decreases by 2
Lose 2 L
Favored for: e- rich L, e- poor M, Bulky L
Eliminating ligands NEED to be cis to start
Photochemical reactions
Light can pop off a L
techniques/characterization
X ray crystallography
GC or LC mass spec
Elemental analysis (% C, H, N…)
FTIR (CO compounds)
NMR (not with paramagnetic species)
Air sensitive compounds
Schlenk line - bubble in N or Ar and evacuate w vacuum pump
Glove box - can get rid of oxygen
Purification: Soxhlet extraction - solvent dissolves impurities but not compound of interest. Boil
solvent and pure compound left behind. Way to recrystallize compounds while avoiding air
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