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Extreme cases: ionic compounds (LiF)
Li transfers e- to F, forming Li+ and F-. This means it occupies a MO centered on the F
A1
A1
orbitals
Molecular orbitals for larger molecules
1. Determine point group of molecule (if linear, use D2h and C2v instead of D∞h or C∞v)
2. Assign x, y, z coordinates (z axis is higher rotation axis; if non-linear y axis in outer atoms point to central atom)
3. Find the characters of the reducible representation for the combination of 2s orbitals on the outer atoms, then for px, py, pz. (as for vibrations, orbitals that change position = 0, orbitals that do not change =1; and orbitals that remain in the same position but change sign = -1)
4. Find the irreducible representations (they correspond to the symmetry of group orbitals, also called Symmetry Adapted Linear Combinations SALC’s of the orbitals).
5. Find AO’s in central atom with the same symmetry
6. Combine AO’s from central atom with those group orbitals of same symmetry and similar E
F-H-F- D∞h, use D2h
1st consider combinations of2s and 2p orbitals on F atoms
2s 2 2 0 0 0 0 2 2Obtain the reducible rep based on equivalent F 2s orbitals.
Use Reduction Procedure to get the irreducible reps.
2s = Ag + B1u
Use the Projection Operator to obtain a SALC for each irreducible rep
Repeat for each group of equivalent atomic orbitals to obtain the full set of eight SALC.
SALC can now be treated similarly to the atomic orbitalsand combined with appropriate AO’s from H
1s(H) is Ag so it matches two SALC. The interaction can be bonding or antibonding.
Both interactions are symmetry allowed, how about energies?
Orbital potential energies (see also Table 5-1 in p. 134 of textbook)
Average energies for all electrons in the same level, e.g., 3p(use to estimate which orbitals may interact)
-13.6 eV
-40.2 eV
-18.65 eV
Good E matchStrong interaction
Poor E matchweak interaction
Bonding e
Non-bonding e
Lewis structureF-H-F-
implies 4 e around H !
MO analysisdefines 3c-2e bond
(2e delocalized over 3 atoms)
Characterize the electrons: bonding, non-bonding, antibonding.
CO2
D∞h, use D2h
(O O) group orbitals the same as for (F F)!!
But C has more AO’s to be considered than H !
CO2
D∞h, use D2h
No match
Carbon orbitals
Ag-Ag interactions of C 2s and the SALC of O 2s
The SALC of the O 2s orbitals for Ag symmetry
-32.38 eV
-19.43 eV
Ag-Ag interactions, now C 2s and the Ag SALC of the C 2pz
-19.43 eV
-10.66 eV
B1u-B1u interactions. Carbon pz with SALC of oxygen 2s
SALC
B1u-B1u interactions. Carbon pz with oxygen pz SALC
SALC of Ag and B1uSALC of Ag and B1u
Ag :2s(C); -15.9 --- SALC of 2s(O);– 32.4 : = 16.5vs
2s(C) ); -19.4 --- SALC of 2p(O); -15.9: = 3.5
Primary Ag interaction
B1u: 2pz(C); -10.7 --- SALC of 2s(O); -32.4: = 21.7 vs
2pz(C); -10.7 --- SALC 2p(O); -15.9: = 5.2
Primary B1u interaction
Symmetry allows many interactions. Energy considerations guide as to which is important.
Strengths of Interactions
Primary Ag interaction
Primary B1u interaction
Bonding
Bonding
Non-bonding
Non-bonding
4 bondsAll occupied MO’s are 3c-2e
LUMO
HOMO
The frontier orbitals of CO2
Molecular orbitals for larger molecules: H2O
1. Determine point group of molecule: C2v
2. Assign x, y, z coordinates (z axis is higher rotation axis; if non-linear y axis in outer atoms point to central atom - not necessary for H since s orbitals are non-directional)
3. Find the characters of the representation for the combination of 2s orbitals on the outer atoms, then for px, py, pz. (as for vibrations, orbitals that change position = 0, orbitals that do not change =1; and orbitals that remain in the same position but change sign = -1)
4. Find the irreducible representations (they correspond to the symmetry of group orbitals,also called Symmetry Adapted Linear Combinations SALC’s of the orbitals).
5. Find AO’s in central atom with the same symmetry
6. Combine AO’s from central atom with those group orbitals of same symmetry and similar E
For H H group orbitals
v’ two orbitals interchanged
E two orbitals unchanged
C2 two orbitals interchanged
2 20 0
v two orbitals unchanged
No match
pz
bonding
slightlybonding
antibonding
px
bonding
antibonding
py
non-bonding
a1 sym
b1 sym
b2 sym
3 10
Find reducible representation for 3H’s
Irreducible representations:
Molecular orbitals for NH3
pz
bonding
Slightlybonding
anti-bonding
bonding
anti-bonding
LUMO
HOMO
Acid-base and donor-acceptor chemistry
Hard and soft acids and bases
Classical concepts
Arrhenius:• acids form hydrogen ions H+ (hydronium, oxonium H3O+) in aqueous solution• bases form hydroxide ions OH- in aqueous solution• acid + base salt + water e.g. HNO3 + KOH KNO3 + H2O
Brønsted-Lowry:• acids tend to lose H+
• bases tend to gain H+
• acid 1 + base 1 base 1 + acid 2 (conjugate pairs) H3O+ + NO2
- H2O + HNO2
NH4+ + NH2
- NH3 + NH3
In any solvent, the reaction always favors the formation of the weaker acids or bases
The Lewis concept is more generaland can be interpreted in terms of MO’s
Rememberthat frontier orbitalsdefine the chemistry
of a molecule
-+
C O
C OM
C O M
CO is a -donor anda -acceptor
Acids and bases (the Lewis concept)
A base is an electron-pair donor An acid is an electron-pair acceptor
Lewis acid-base adducts involving metal ionsare called coordination compounds (or complexes)
acid baseadduct
Frontier orbitals and acid-base reactions
Remember the NH3 molecule
The protonation of NH3
Frontier orbitals and acid-base reactions
(C3v)(Td)
(non-bonding)
(bonding)
New HOMO
New LUMO
In most acid-base reactions HOMO-LUMO combinationslead to new HOMO-LUMO of the product
But remember that there must be useful overlap (same symmetry)and similar energies to form new bonding and antibonding orbitals
What reactions take place if energies are very different?
A base has an electron-pairin a HOMO of suitable symmetry
to interact with the LUMO of the acid
Frontier orbitals and acid-base reactions
Very different energies like A-B or A-E no adducts form
Similar energies like A-C or A-Dadducts form
The MO basis for hydrogen bonding
F-H-F-
Bonding e
Non-bonding e
MO diagram derived from atomic orbitals(using F…….F group orbitals + H orbitals)
But it is also possible from HF + F-
Non-bonding(no E match)
Non-bonding(no symmetry match)
HOMO-LUMO of HF for interaction
First form HF
The MO basis for hydrogen bonding
F-H-F-
HOMO
LUMOHOMO
Formation of the orbitals
First take bonding and antibonding combinations.
HOMO
Similarly for unsymmetrical B-H-A
Total energy of B-H-A lower than the sum of
the energies of reactants
Poor energy match, little or no H-
bondinge.g. CH4 + H2O
Good energy match,strong H-bonding
e.g. CH3COOH + H2O
Very poor energy matchno adduct formed
H+ transfer reactione.g. HCl + H2O
Hard and soft acids and bases
Hard acids or bases are small and non-polarizableSoft acids and bases are larger and more polarizableHalide ions increase in softness: fluoride < chloride<bromide<iodide
Hard-hard or soft-soft interactions are stronger (with less soluble salts) than hard-soft interactions (which tend to be more soluble).
Most metals are classified as Hard (Class a) acids or acceptors.Exceptions shown below: acceptors metals in red box are always soft (Class b). Other metals are soft in low oxidation states and are indicated by symbol.
Class (b) or soft always Solubilities: AgF > AgCl > AgBr >AgI
But…… LiBr > LiCl > LiI > LiF
Chatt’s explanationClass (b) soft metals have d electrons available for -bonding
Higher oxidation states of elements to the right of transition metals have more class b charactersince there are electrons outside the d shell.
Ex. (Tl(III) > Tl(I), has two 6s electrons outside the 5d making them less available for π-bonding)
For transition metals: high oxidation states and position to the left of periodic table are hardlow oxidation states and position to the right of periodic table are soft
Soft donor molecules or ions that are readily polarizable and have vacant d or π* orbitalsavailable for π-bonding react best with class (b) soft metals
Model: Base donates electron density to metal acceptor. Back donation, from acid to base, may occur from the d electrons of the acid metal into vacant orbitals on the base.
Tendency to complex with hard metal ions
N >> P > As > SbO >> S > Se > Te
F > Cl > Br > I
Tendency to complex with soft metal ions
N << P > As > SbO << S > Se ~ Te
F < Cl < Br < I
The hard-soft distinction is linked to polarizability, the degree to which a moleculeor ion may be easily distorted by interaction with other molecules or ions.
Hard acids or bases are small and non-polarizable
Soft acids and bases are larger and more polarizable
Hard acids are cations with high positive charge (3+ or greater),or cations with d electrons not available for π-bonding
Soft acids are cations with a moderate positive charge (2+ or lower),Or cations with d electrons readily availbale for π-bonding
The larger and more massive an ion, the softer (large number of internal electronsShield the outer ones making the atom or ion more polarizable)
For bases, a large number of electrons or a larger size are related to soft character
Hard acids tend to react better with hard bases and soft acids with soft bases, in order to produce hard-hard or soft-soft combinations
In general, hard-hard combinations are energeticallymore favorable than soft-soft
An acid or a base may be hard or softand at the same time it may be strong or weak
Both characteristics must always be taken into account
e.g. If two bases equally soft compete for the same acid, the one with greater basicity will be preferred
but if they are not equally soft, the preference may be inverted
Fajans’ rules
1. For a given cation, covalent character increases with increasing anion size. F<Cl<Br<I2. For a given anion, covalent character increases with decreasing cation size. K<Na<Li3. The covalent character increases
with increasing charge on either ion.4. Covalent character is greater for cations with non-noble gas electronic configurations.
A greater covalent character resulting from a soft-soft interaction is relatedto lower solubility, color and short interionic distances,
whereas hard-hard interactions result in colorless and highly soluble compounds
Quantitative measurements
2
AI
2
AI
Absolute hardness(Pearson)
Mulliken’s absolute electronegativity(Pearson)
1
Softness
EHOMO = -I
ELUMO = -A
Energy levelsfor halogensand relations between, and HOMO-LUMO energies