5.5 Nonlinear Molecules with EXn Formulation: Example #6: EX3 … MO... · 2017-03-06 · 131 5.5...

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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. • 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−

The terminal atom SALCs of EX4 with their symmetry labels and phases are:

• 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 s-orbitals 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:

• NOTE: The two p orbitals form an eu set and can therefore mix with both eu SALCs! • All other SALCs only give non-bonding interactions.