chem 241 final- 3
What do we know before building the MO picture for CO?
Zeff for O is greater than for C, so we expect the atomic orbitals of O to be lower in energy than those of C.
In CO, which orbitals are treated as σ and which as π?
The 2s and 2pz orbitals are σ-type, and the 2px and 2py orbitals are π-type.
What should you do in more complicated MO cases like CO?
It is more important to interpret the MO diagram rather than try to predict it from scratch.
In CO, where do the π bonds come from?
They result from sideways overlap of the px and py orbitals, as usual.
What special feature is important in the σ-bonding network of CO?
There is significant s-p mixing in the σ-bonding network of CO.
Where does the interpretation of strong s-p mixing in CO come from?
From calculations, specifically noted as density functional theory.
Why are the σ orbitals in CO labelled 1σ, 2σ, 3σ, and 4σ?
Because there is enough s-p mixing that the σ orbitals are better treated as a sequence of mixed σ orbitals rather than as pure 2s- or 2p-derived orbitals.
What is the character of each C–O σ molecular orbital in CO?
Each C–O σ molecular orbital has mixed s and p character.
What types of orbital character are mixed into the σ orbitals of CO?
They can contain bonding, nonbonding, and antibonding contributions.
What is the main takeaway about detailed MO construction in CO?
Do not worry too much about deriving it by hand; calculations sort it out, and your job is to interpret the diagram.
According to the notes, what is 1σ in CO like?
1σ is largely bonding with large contributions from O atomic orbitals.
What is 2σ in CO like?
2σ is more bonding than antibonding.
What is 3σ in CO like?
3σ is more antibonding than bonding.
What is 4σ in CO like?
4σ is extremely antibonding.
Why is the concept of bond order a bit fussy in CO?
Because significant s-p mixing makes the simple bonding-minus-antibonding counting less straightforward.
What is the HOMO of CO according to the notes?
The HOMO is the 3σ orbital.
What kind of character does the CO HOMO have?
It has a lot of p character and more C character than O character.
Why is the HOMO of CO often described like a lone pair on carbon?
Because it is closer in energy and character to the carbon atomic orbitals and is concentrated more on C.
What is the LUMO of CO according to the notes?
The LUMO is the π*(2p) orbital.
What kind of character does the CO LUMO have?
It has more carbon character than oxygen character.
Why is it useful that the CO LUMO has more carbon character?
Because incoming electron density or back-bonding into the LUMO will be directed into an orbital with strong carbon-side contribution.
How should you evaluate whether MO calculations are trustworthy?
Ask whether the latest and appropriate models were used, whether the model matches your actual system, whether solvent and reaction conditions were included, whether the calculation treats ΔG or only ΔH, and whether results were compared to real measured values.
What does HOMO stand for?
Highest Energy Occupied Molecular Orbital.
What does LUMO stand for?
Lowest Energy Unoccupied Molecular Orbital.
When interpreting molecular reactivity, what orbitals are usually most important?
Usually the HOMO and the LUMO.
Where do molecules generally donate electrons from?
From the HOMO.
Why do molecules donate from the HOMO?
Because it is the highest-energy occupied orbital, so those electrons are easiest to give up.
Where do molecules generally accept electrons into?
Into the LUMO.
Why do molecules accept electrons into the LUMO?
Because it is the lowest-energy empty orbital available.
What does a HOMO-to-LUMO excitation represent?
Absorption of a photon that promotes an electron from the HOMO to the LUMO.
What does ΔE correspond to in a HOMO-LUMO excitation?
The energy of the absorbed photon.
Why are the HOMO and LUMO so important?
Their energies, shapes, and locations generally determine much of the chemistry of the molecule.
How does CO bond to Fe according to the notes?
By σ donation from the carbon lone-pair-like HOMO into an empty σ orbital on Fe and π back-bonding from filled Fe dπ orbitals into the π* LUMO on CO.
What Fe orbital accepts σ donation from CO?
An empty σ orbital on Fe, described as a blend such as s, pz, and dz2 character.
What is the source of σ donation from CO to Fe?
The lone pair on carbon in the 3σ HOMO.
What is the acceptor orbital for Fe→CO π back-bonding?
The π* orbital on CO, which is the LUMO.
What is the name for simultaneous σ donation and π back-bonding in CO complexes?
Synergistic bonding.
Why is CO strongly bound to Fe?
Because the σ donation and π back-bonding work together synergistically.
Why is CO toxic according to the notes?
Because synergistic bonding of CO to Fe is stronger than Fe–O2 bonding in blood, so CO blocks O2 from binding to Fe.
What safety note is given about CO?
CO is odourless, colourless, and deadly, so you must be trained and follow every safety procedure.
How is methane commonly treated in introductory bonding models?
Using Lewis structures, VSEPR, and hybridization.
What hybridization is usually assigned to carbon in CH4?
sp3.
What does the MO treatment say about the valence atomic orbitals on carbon in methane?
The valence atomic orbitals are the 2s and 2px, 2py, 2pz orbitals.
What do calculations show about s-p mixing in methane?
Calculations show no s-p mixing.
Why is methane different from CO in this respect?
CO has significant s-p mixing in its σ network, but methane does not according to the notes.
What is the general warning about precise MO energy diagrams?
You need calculations to solve MO energy level diagrams with precision.
After calculations are done, what should you do with MO diagrams?
Interpret the MO energy level diagram after the calculations are verified.
What molecular features do you generally look at when using MO theory?
Shapes, geometries, and symmetry, especially of the HOMO and LUMO.
How do the properties of the LUMO affect chemistry?
They determine how a molecule accepts electrons.
What example is given to show how LUMO controls Lewis acidity?
BF3.
What are the empty orbitals mentioned for BF3?
The B–F σ* orbital and an unhybridized 2pz orbital.
What is the LUMO in BF3 according to the notes?
The 2pz orbital on boron.
Why is BF3 a strong Lewis acid?
Because it can accept electrons into the empty 2pz orbital.
How do the properties of the HOMO affect chemistry?
They determine how a molecule gives up electrons.
What photochemistry idea is connected to HOMO and LUMO?
Absorbing light promotes an electron from the HOMO to the LUMO.
What does the energy of absorbed light equal in simple MO photochemistry language?
ELUMO − EHOMO.
What excited-state labels are shown in the notes for photochemistry?
S1 and T1.
What process is noted between singlet and triplet excited states?
Intersystem crossing.
How is methane described in MO terms using the carbon 2s orbital?
The 2s orbital on carbon bonds with four H 1s orbitals, or portions thereof.
What kind of bonding MO does this create in methane?
One σ bonding molecular orbital smeared out over five atoms.
What is the corresponding antibonding picture in methane?
A corresponding antibonding MO spread over the C and H framework.
How do the carbon p orbitals contribute in methane?
Carbon p orbitals also combine with H 1s orbitals to form bonding molecular orbitals.
What note is made about the C and p orbitals drawn in methane?
They are shown in the plane of the page for visualization.
What broad idea does methane illustrate about MO theory?
That molecular orbitals can extend over the whole molecule rather than being localized as simple single bonds.
What are transition metals in the periodic table?
They are the d-block metals.
What broad bonding resources do transition metals use?
They use s, p, and d valence orbitals.
Which orbitals are commonly involved in σ bonding for transition metals?
s, p, dx2−y2, and dz2.
Which orbitals are commonly involved in dπ bonding for transition metals?
dxy, dxz, and dyz.
Why is the variety of transition-metal chemistry so large?
Because transition metals can use many different valence orbitals and bonding modes.
What is the key periodic-table contrast mentioned between s-block and p-block elements?
s-block elements have low electronegativity and low Zeff and tend to donate electrons, while p-block elements tend to have higher Zeff and can accept electrons to complete a valence shell.
How are s-block elements described in bonding terms?
They are electron donors and can make metal ions such as M2+.
How are p-block elements described in bonding terms?
They can make covalent bonds, like CH4, or accept electrons, like in ionic compounds such as Na+Cl−.
What is meant by atomic configuration on the transition-metal slide?
The elemental form electron configuration of the neutral atom.
What is the key electron-configuration rule for transition-metal complexes or charged atoms?
The nd orbital is always lower in energy than the (n+1)s orbital.
Why is this rule important?
Because when forming transition-metal ions and complexes, electrons are removed from the (n+1)s before the nd set.
What shape is [Co(NH3)6]2+ said to have?
Octahedral, an ML6 compound.
What does L mean in ML6?
A ligand, meaning a molecule or atom bonded to the transition metal.
What is the electron configuration of Co in the gas phase according to the notes?
[Ar]4s2 3d7.
What is the electron configuration of Co2+ in a compound according to the notes?
[Ar]3d7.
Why does Co2+ become [Ar]3d7 instead of keeping 4s electrons?
Because in complexes the 3d orbitals are lower in energy than 4s, so the 4s electrons are removed first.
What example on the slide shows the same rule for chromium and vanadium type ions?
V2+ and Cr3+ often have the same chemistry because both can give d3 configurations.
What Ta example is mentioned as analogous?
Ta2+ can be [Xe]5d3.
What special neutral-atom exceptions are shown in the first-row transition series?
Cr is [Ar]4s1 3d5 and Cu is [Ar]4s1 3d10.
What note is made about most copper complexes?
Most Cu complexes are Cu(I) d10 or Cu(II) d9.
What bonding model explains these transition-metal properties?
Metallic bonding.
What reminder is given about transition metals being elements?
They are metallic.
What example electron configuration is written for metallic Cu?
[Ar]4s1 3d10.
What example electron configuration is written for metallic Ni?
[Ar]4s2 3d8.
How can you quickly remember the key ion-configuration rule for transition metals?
For complexes and cations, nd ends up lower than (n+1)s, so remove s electrons first.
How can you quickly remember the main reactivity meaning of HOMO and LUMO?
HOMO donates, LUMO accepts.
How can you quickly remember why CO binds strongly to Fe?
CO donates from carbon through its HOMO and accepts back-bonding into its π* LUMO, giving strong synergistic bonding.
How can you quickly remember why CO is toxic?
It binds Fe more strongly than O2 does and blocks oxygen binding.
How can you quickly remember the MO message for methane?
Hybridization is useful and intuitive, but the real MO picture uses C 2s and 2p orbitals spread over the whole molecule.