Bonding Theories in Chemistry
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Transcript of Bonding Theories in Chemistry
3O3
1. Is this molecule stable ?2. Does this molecule have a charge ?3. Is this molecule linear or bent ?4. Is the bond strength higher, the same
or lower than in O2 ?
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2 2H2O2
1. Is this molecule linear or bent ?2. How many different kinds of electrons are in
this molecule ?3. What is the oxidation number of O in this
molecule ?
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CO
1. Is this molecule stable ?2. Is it polar or non-polar ?3. Is this molecule more or less reactive than CO2 ?
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Allred-Rochow EN
Example: Flourine (r = 72 pm)Carbon (r = 77 pm)Calculate the AR electronegativity
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Shielding of 2p electrons:Flourine:S = 6 * 0.35 + 2 * 0.85 = 3.8 => Z* = 9 – 3.8 = 5.2
EN = 3590 * 5.2/(72 2) + 0.744 = 4.35
Carbon:S = 3 * 0.35 + 2 * 0.85 = 2.75 => Z* = 6 – 2.75 = 3.25
EN = 3590 * 3.25 / (77 2) ) + 0.744 = 2.71
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The periodic table is built up so that elements with the same number of VE are in one column
= no. of VE
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Electron configurationdo NOT use core electrons !
Practise: write the configuration for:1. Fe2+
2. Pb3. W4. Ti4+
Write only the Valence
Electrons !
http://www.slideshare.net/Hoegler6/09-lecture14
Examples:(1) Atoms and ions
Try yourself:
1. Al and Al3+
2. F and F-
3. K and K+
4. H and H-
Indicates that O has 2 valences
(can make 2 bonds)
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Lewis MoleculesWrite the atom with the LOWEST EN in the middle !
Example(2) In molecules write all VE for each atom
Try yourself:
1. AlH3
2. LiH3. SiF4
4. C2H6
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Formal Charges
In some cases, VE cannot be arranged without creating charges
Each atom in a molecule has a “formal charge”: Count the electrons that belong to this atom and compare to the VE in the element
N has only 4 electrons, 1 is missing
O has 7 electrons, 1 more
than in oxygen
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Formal charges and Oxidation numbers
Oxidation number:assign all electrons in bonds to
the atom with higher EN !-> N has no el. -> ox no. +5
Formal Charge:split all bonding el. Between the atoms and count the remaining
-> N has 4 el. -> formal charge is +1
Find the same for: CO2 HCHO H3C-OH HCOOH
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Transition metal compoundsExample:
FeCl3 = ionic compound (“salt”)But in water: Fe(3+) (H2O)6 + 3 Cl(-)
Lewis Formulas do not reflect the bonding in coordination compounds:
Fe(3+) has 5 valence electrons, but forms 6 bonds !
Oxidation numbers:can go from -1 to +7, normally +2 or +3Find the numbers for: KMnO4, MnO2, K4Fe(CN)6, Fe(CO)5
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Coordination Compounds
For TM ions the VE are counted ALL as d-electrons !
EMPTY metal orbitals are neededto be filled with ligand-electrons !
We can form a d2 sp3 hybrid – called “inner shell” complex
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch12/valence.php 38
Here we cannot explain 6 ligands around the Ni(2+).In this case we have to use the “outer” 4d orbitals to form a hybrid:
Use a sp3d2 “outer shell” complex
Explain the bonding in a [Fe(CN)6] 4- complex
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No orbital mixing here – because the energy difference between N and O is high -> small s- and pz-interaction
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sp-mixing – example B2 molecule which is a diradical:
https://en.wikipedia.org/wiki/Molecular_orbital_diagram 42
Formation of LGO* ligand group orbitals *
H2O molecule has c2v symmetry
The 2 H-s-orbitals can be combined to form 2 LGO’s: One symmetric, another anti-symmetric
A1 symmetry
B2 symmetric
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Get LGO’s from group theory
http://plato.mercyhurst.edu/chemistry/kjircitano/inorgstudysheets/inorgstudyexamii.htm
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Construct LGO’sGraphical approach
(1)Arrange all ligand orbitals around the central atom
(2)First MO-combination: all are in the same phase
(3)Draw one node plane symmetrically = next energy level
(4)Draw two node planes symmetrically
LGO no. 1
LGO no. 2
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Coordination compounds
Find 6 symmetry adapted ligand combinations (SALC) To fit with the metal s- p- and d-orbitals
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ML6 complex – Co(NH3)6 (2+)
Co(2+)NH3 ligand binds by the lone pair of ammonia:
Insert the
bonding ?
Insert the electrons -which are
bonding, non-bonding, anti-
bonding ?
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