Example #9: EX5 type molecule in D3h symmetry : PCl5
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From VSEPR:
bond distances axial
(3 × 90° interaction)
>
bond distances equatorial
(2 × 90° interaction + 2 × 120° interaction)
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EX5 is the same point group as EX3 (cf. BF3) - the allowed MO's must be a
combination of the new axial SALCs with the already derived MO's for EX3.
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The axial terminal atom SALCs for EX5 are:
Source: Purcell & Kotz, "Inorganic Chemistry", Holt-Saunders, 1977.
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The additional axial interactions with the central atom are then:
Source: Purcell & Kotz, "Inorganic Chemistry", Holt-Saunders, 1977.
NOTE: From above, it is evident that we must mix 3 SALCs to get 3 MO’s.
Q. How to we do this and what do we expect to get?
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Consequences:
The a2" MO, which was purely "out of plane" π-bonding in EX3, is primarily axially
s-bonding, thus one electron pair serves as a π- and σ- bonding pair simultaneously !
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An MO based explanation for the differences in bond length/strength of axial vs.
equatorial bonds in EX5:
Considering σ-interactions only:
E-Xeq
E-Xax
a1'
1/5
1/5
σ-bonding for all 5 X
e'
2 × 1/3
0
σ-bonding for 3 Xeq
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The table shows that BOeq > BOax
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The complete MO Scheme for EX5 is:
a2"
0
1/2
σ-bonding for 2 Xax
Σ BO
13/15
7/10
Source: Purcell & Kotz, "Inorganic Chemistry", Holt-Saunders, 1977.
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The frontier orbitals of PCl5 calculated at the AM1 level using Spartan:
LUMO+1, e',σ-antibonding, E = -1.480 eV
LUMO, a1', σ-antibonding, E = -4.639 eV
HOMO, a1', non-bonding, E = -12.700 eV
HOMO-1, e", non-bonding, E = -12.767 eV
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Note that Purcell + Kotz give a different HOMO than the SPARTAN program - given
that in 1977 no computers were available they probably made an educated guess - the
AM1 calculation is probably closer to reality.
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