# Lesson10 - The MO Diagram of Carbon Monoxide Begin just as...

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The MO Diagram of Carbon Monoxide Oxygen Carbon 2p x 2p x 2p y 2p y 2p z 2p z 2s 2s C O σ s σ s * σ p σ p * π y * π z * π y π z Begin just as with F 2 . Compare the AO energies for CO vs. F 2 . Can we ignore the C2s / O2p x interaction?

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Consider C2s + O2p x Mixing Oxygen Carbon 2p x 2p x 2p y 2p y 2p z 2p z 2s 2s C O σ s σ s * σ p σ p * π y * π z * π y π z C2s + O2px orbitals are closer in energy than O2s + C2s and O2px + C2px Remove σ p and σ s * But orbitals must be conserved (8 AOs produce 8 and only 8 MOs)
Beyond Pairwise Combinations: Little of This, Little of That σ s σ s/p * σ s/p σ p * 2p x (a) 2s(a) 2p x (b) 2s(b) MO theory predicts that orbitals of identical symmetry will mix. As a practical consequence of this mixing, the resultant molecular orbitals become blended. The extent of this mixing (or blending) depends on the relative energies of the like-symmetry MO's. ( s/p ) = c 1 {2s(a)} + c 2 {2s(b)} + c 3 {2p x (a)} + c 4 {2p x (b)}

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An Alternative Approach to Construct the CO Diagram: Mix the MOs σ s σ p * π y * π z * π z Before s/p x mixing After s/p x mixing σ s σ p * π y * π z * π y π z σ p σ s * negligible s/p x mixing negligible s/p x mixing substantial s/p x mixing σ s/p l σ s/p r
New Diagram After C2s + O2p x Mixing Oxygen Carbon 2p x 2p x 2p y 2p y 2p z 2p z 2s 2s C O σ s σ s/p * σ s/p σ p * π y * π z * π y π z Replace σ p and σ s * with σ s/p and σ s/p * Secondary (i.e., less significant interaction)

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Interpreting the CO MO Diagram Oxygen Carbon 2p x 2p y 2p z 2s 2s Ψ 1 Ψ 2 Ψ 4 Ψ 3 Ψ 5 Ψ 7 Ψ 6 Ψ 8 C O σ s σ σ s/p σ p * π y * π z * π y π z Ψ 1 looks like a 2s centered on O Ψ 2 looks like a σ s/p from sheet #1 Ψ 5 looks like a σ s/p * from sheet 1 Bond order: A sigma bond and two pi bonds. Two non-bonded electron pairs.
The Nucleophilic and Electrophilic Sites in CO Carbon 2p x 2p x 2p y 2p y 2p z 2p z 2s Ψ Ψ 2 Ψ 4 Ψ 3 Ψ 5 Ψ 7 Ψ 6 Ψ 8 C O σ s/p * σ s/p σ p * π y * π z * π y π z Nucleophilic site : use the energy diagram to locate the highest occupied MO = HOMO ( σ s/p * ). Look at the corresponding orbital ( Ψ 5 image). On which atom does the largest share of this orbital reside? Electrophilic site : use the energy diagram to locate the lowest unoccupied MO = LUMO ( π * ). Look at the corresponding orbital ( Ψ 6 Ψ 7 images). On which atom does the largest share of this orbital reside?

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Carbon Monoxide’s HOMO and LUMO Define Its Reactivity carbon monoxide as a nucleophile (HOMO) O C N N N N Fe 2+ CH 3 CH=CH 2 CH 3 CH=CH 2 CH 3 O 2 C O 2 C CH 3 Heme N N N N Fe 2+ CH 3 CH=CH 2 CH 3 CH=CH 2 CH 3 O 2 C O 2 C CH 3 Why carbon monoxide is poisonous.
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## This note was uploaded on 01/25/2012 for the course CHEM 232 taught by Professor Miller during the Spring '08 term at University of Illinois, Urbana Champaign.

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Lesson10 - The MO Diagram of Carbon Monoxide Begin just as...

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