Download Rules for forming Delocalized molecular orbitals

Rules for forming Delocalized molecular orbitals
Symmetry Allowed Linear Combinations of
Atomic Orbitals to form Molecular Orbitals
(SA-LCAO-MO)
This model is used to address questions of the energy or momentum of electrons within a molecule.
The model is based upon the atomic orbitals (which are a description of the momentum of the electron)
combining linearly, i.e. constructive and destructive interference.
Rules:
1 - The number of orbitals is conserved.
When two atomic orbitals are combined, two molecular orbitals will result; the in-phase
combination (constructive interference) gives rise to an energetically stabilized orbital
(bonding) and the out-of-phase combination (destructive interference) gives rise to
an energetically destabilized orbital (anti-bonding).
2 - The buildup principle, the Pauli exclusion principle and Hund’s rule apply to the construction of
electronic configurations using molecular orbitals.
3 - Only orbitals of the same symmetry can interact.
Symmetry in the diatomic molecule is defined as symmetry and momentum with respect to the
internuclear axis. This can be readily discerned by observing the sign change as the orbitals are
rotated about the internuclear axis. No momentum change corresponds with sigma (F)
symmetry (below left); momentum change of one corresponds with pi (B) symmetry;
momentum change of two 2 (two d orbitals face-to-face) is delta (*) symmetry. (Note: This is
observed in transition metal dimers.)
Sigma (F) Symmetry
Pi (B) Symmetry
4 - Orbitals of comparable energy interact.
5 - Filled orbital subshells on both atoms lead to no net bonding between the atoms.
The filled subshells will mix and generate both bonding (in-phase) and antibonding
(out-of-phase) orbitals; adding the electrons leads to both being filled with leads to no net
stabilization of the atoms, i.e., no bonding.
7 - The greater the degree of overlap between the atomic orbitals, the greater the separation or
energy difference between the bonding and antibonding orbitals.
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|....
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anti-bonding
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| Energy
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........................
......... | ...........................
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bonding
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Large Overlap
anti-bonding
...........................
bonding
Small Overlap
Degree of overlap is a measure of the amount the two atomic orbitals share the same
volume of space. Greater overlap means more of the each orbital interacts thereby
making the in-phase combination more stable and destabilizing the out-of-phase
combination.
8 - The orbital of the atom with greater electronegativity lies lower in energy and makes a greater
contribution of its orbital to the bonding molecular orbital; the atom of lesser electronegativity
lies higher in energy and makes a greater orbital contribution to the antibonding molecular
orbital.
Example: H-F . Bonding and antibonding interactions between hydrogen (electronegativity 2.20) 1s
orbital and fluorine (electronegativity 3.98) hybrid sp"
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| Energy
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H 1s
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(2.20)
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F* (mostly H)
F sp"
(3.98)
F (mostly F)