5.5 Nonlinear Molecules with EXn Formulation: Example #6: EX3 (e.g., BF3)
1. Identify the point group: D3h
2. Specify a coordinate system and stick to it!
3. Draw SALCs using s and/or p orbitals of
terminal atoms, including phases.
4. Assign symmetry labels to the SALCs using
the character table and coordinate system.
Source: Purcell & Kotz, "Inorganic Chemistry", Holt-Saunders, 1977.
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5. Mix the terminal atom SALCs with the central atom’s AO’s using symmetry labels as
a guide to give antibonding and bonding MO’s.
Source: Purcell & Kotz, "Inorganic Chemistry", Holt-Saunders, 1977.
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The symmetries of the bonding and antibonding MOs are: a'1, a''2 and e'
All other combinations give non-bonding MOs with symmetries a'2 or e''.
NOTE: Above, we haven’t yet accounted for the second e′ SALC! What should we
do with them?
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The resulting MO scheme and shapes of the individual MOs:
Source: Miessler & Tarr, Inorganic Chemistry, Prentice-Hall, 1998.
Q. Where are the 1a1ʹ′, 2a1ʹ′, and 1eʹ′ MOs?
From which AO’s do they arise?
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Example #7: EX4 type molecule with D4h symmetry: ICl4
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The terminal atom SALCs of EX4 with their symmetry labels and phases are:
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QUESTION: What is the other acceptable way to draw the eu sets (the ones marked
by boxes)? Can you draw the eg set another way, too?
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Assuming in a reasonable 1st approximation that the set of SALCs formed by the sorbitals on the Cl atoms is too low in energy to interact with the central atom, we can
then construct the symmetry derived (anti)bonding MO's:
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NOTE: The two p orbitals form an eu set and can therefore mix with both eu SALCs!
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All other SALCs only give non-bonding interactions.
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