Pi bond

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Two p-orbitals forming a π-bond.





Multiple bonds[edit]


A pi bond is weaker than a sigma bond, but the combination of pi and sigma bond is stronger than either bond by itself. The enhanced strength of a multiple bond versus a single (sigma bond) is indicated in many ways, but most obviously by a contraction in bond lengths. For example in organic chemistry, carbon–carbon bond lengths are about 154 pm in ethane,[1][2] 134 pm in ethylene and 120 pm in acetylene. More bonds make the total bond shorter and stronger.

Comparison of bond-lengths in simple structures

Special cases[edit]

A pi bond can exist between two atoms that do not have a net sigma-bonding effect between them.


In some cases of multiple bonds between two atoms, there is no net sigma-bonding at all, only pi bonds. Examples include diiron hexacarbonyl (Fe2(CO)6), dicarbon (C2), and diborane(2) (B2H2). In these compounds the central bond consists only of pi bonding because of a sigma antibond accompanying the sigma bond itself. These compounds have been used as computational models for analysis of pi bonding itself, revealing that in order to achieve maximum orbital overlap the bond distances are much shorter than expected.[3]

See also[edit]


  1. ^ Veillard, A. (1970). "Relaxation during internal rotation ethane and hydrogen peroxyde". Theoretica chimica acta 18 (1): 21–33. doi:10.1007/BF00533694. 
  2. ^ Harmony, Marlin D. (1990). "The equilibrium carbon–carbon single‐bond length in ethane". J. Chem. Phys. 93: 7522–7523. doi:10.1063/1.459380. 
  3. ^ Jemmis, Eluvathingal D.; Pathak, Biswarup; King, R. Bruce; Schaefer III, Henry F. (2006). "Bond length and bond multiplicity: σ-bond prevents short π-bonds". Chemical Communications: 2164–2166. doi:10.1039/b602116f.