4.7: Periodic Trends in π Bonding
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As we noted in Section 2.3, pπ-bonding almost always involves a second-row element.
We encounter π-bonding from the sideways overlap of p-orbitals in the MO diagrams of second-row diatomics (B 2 …O 2 ). It is important to remember that π-bonds are weaker than σ bonds made from the same AOs, and are especially weak if they involve elements beyond the second row.
Example:
Ethylene: Stable molecule, doesn't polymerize without a catalyst.
Silylene: Never isolated, spontaneously polymerizes. Calculations indicate 117 kJ/mol stability in the gas phase relative to singly-bonded (triplet) H 2 Si-SiH 2 .
The large Ne core of Si atoms inhibits sideways overlap of 3p orbitals → weak π-bond.
Other examples: P 4 vs. N 2
P cannot make π-bonds with itself, so it forms a tetrahedral molecule with substantial ring strain. This allotrope of P undergoes spontaneous combustion in air. Solid white phosphorus very slowly converts to red phosphorus, a more stable allotrope that contains sheets of pyramidal P atoms, each with bonds to three neighboring atoms and one lone pair.
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White phosphorus (P 4 ) is a soft, waxy solid that ignites spontaneously in air, burning with a bright flame and generating copious white P 4 O 10 smoke. The sample shown here is photographed under water to prevent the oxidation reaction |
N can make π-bonds, so N 2 has a very strong triple bond and is a relatively inert diatomic gas.
(CH 3 ) 2 SiO vs. (CH 3 ) 2 CO
“RTV” silicone polymer (4 single bonds to Si) vs. acetone (C=O double bond). Silicones are soft, flexible polymers that can be heated to high temperatures (>300 °C) without decomposing. Acetone is a flammable molecular liquid that boils at 56 °C.
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Silicone polymers (R 2 SiO) n are used in non-stick cookware like these muffin cups, in Silly Putty, soft robotics, and many other applications. |
Exceptions:
2nd row elements can form reasonably strong π-bonds with the smallest of the 3rd row elements, P, S, and Cl. Thus we find S=N bonds in sulfur-nitrogen compounds such as S 2 N 2 and S 3 N 3 - , P=O bonds in phosphoric acid and P 4 O 10 (shown above), and a delocalized π-molecular orbital in SO 2 (as in ozone).