Pi bond - wikipedia Pi bond This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. ( February 2013 ) Electron atomic and molecular orbitals, showing a pi bond at the bottom right In chemistry, pi bonds ( π bonds ) are covalent chemical bonds where two lobes of an orbital on one atom overlap two lobes of an orbital on another atom. Each of these atomic orbitals has zero electron density at a shared nodal plane, passing through the two bonded nuclei. The same plane is also a nodal plane for the molecular orbital of the pi bond. Two p - orbitals forming a π - bond. The Greek letter π in their name refers to p orbitals, since the orbital symmetry of the pi bond is the same as that of the p orbital when seen down the bond axis. One common form of this sort of bonding involves p orbitals themselves, though d orbitals also engage in pi bonding. This latter mode forms part of the basis for metal - metal multiple bonding. Pi bonds are usually weaker than sigma bonds. The C-C double bond, composed of one sigma and one pi bond, has a bond energy less than twice that of a C-C single bond, indicating that the stability added by the pi bond is less than the stability of a sigma bond. From the perspective of quantum mechanics, this bond 's weakness is explained by significantly less overlap between the component p - orbitals due to their parallel orientation. This is contrasted by sigma bonds which form bonding orbitals directly between the nuclei of the bonding atoms, resulting in greater overlap and a strong sigma bond. Pi bonds result from overlap of atomic orbitals that are in contact through two areas of overlap. Pi - bonds are more diffuse bonds than the sigma bonds. Electrons in pi bonds are sometimes referred to as pi electrons. Molecular fragments joined by a pi bond can not rotate about that bond without breaking the pi bond, because rotation involves destroying the parallel orientation of the constituent p orbitals. For homonuclear diatomic molecules, bonding π molecular orbitals have only the one nodal plane passing through the bonded atoms, and no nodal planes between the bonded atoms. The corresponding antibonding, or π * ( `` pi - star '' ) molecular orbital, is defined by the presence of an additional nodal plane between these two bonded atoms. Contents 1 Multiple bonds 2 Special cases 3 See also 4 References Multiple bonds A typical double bond consists of one sigma bond and one pi bond ; for example, the C = C double bond in ethylene. A typical triple bond, for example in acetylene, consists of one sigma bond and two pi bonds in two mutually perpendicular planes containing the bond axis. Two pi bonds are the maximum that can exist between a given pair of atoms. Quadruple bonds are extremely rare and can be formed only between transition metal atoms, and consist of one sigma bond, two pi bonds and one delta bond. 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, 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 ethane ( 1 σ bond ) ethylene ( 1 σ bond + 1 π bond ) acetylene ( 1 σ bond + 2 π bonds ) Special cases A pi bond can exist between two atoms that do not have a net sigma - bonding effect between them. In certain metal complexes, pi interactions between a metal atom and alkyne and alkene pi antibonding orbitals form pi - bonds. In some cases of multiple bonds between two atoms, there is no net sigma - bonding at all, only pi bonds. Examples include diiron hexacarbonyl ( Fe ( CO ) ), dicarbon ( C ), and diborane ( 2 ) ( B H ). 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. See also Aromatic interaction Delta bond Molecular geometry Pi backbonding Pi interaction References Jump up ^ Streitwieser, Andrew ; Heathcock, Clayton H. ; Kosower, Edward M. ( 1992 ). Introduction to organic chemistry. Heathcock, Clayton H., Kosower, Edward M. ( 4th ed. ). New York : Macmillan. p. 250. ISBN 0024181706. OCLC 24501305. Jump up ^ Veillard, A. ( 1970 ). `` Relaxation during internal rotation ethane and hydrogen peroxyde ''. Theoretica chimica acta. 18 ( 1 ) : 21 -- 33. doi : 10.1007 / BF00533694. Jump up ^ Harmony, Marlin D. ( 1990 ). `` The equilibrium carbon -- carbon single ‐ bond length in ethane ''. J. Chem. Phys. 93 : 7522 -- 7523. Bibcode : 1990JChPh... 93.7522 H. doi : 10.1063 / 1.459380. Jump up ^ 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. hide Chemical bonds Intramolecular ( strong ) Covalent By symmetry Sigma ( σ ) Pi ( π ) Delta ( δ ) Phi ( φ ) By multiplicity 1 ( single ) 2 ( double ) 3 ( triple ) 4 ( quadruple ) 5 ( quintuple ) 6 ( sextuple ) Miscellaneous Agostic Bent Coordinate ( dipolar ) Pi backbond Charge - shift Hapticity Conjugation Hyperconjugation Antibonding Resonant Electron deficiency 3c -- 2e 4c -- 2e Hypercoordination 3c -- 4e Aromaticity Möbius super sigma homo bicyclo spiro σ - bishomo spherical Y - Metallic Metal aromaticity Ionic Intermolecular ( weak ) van der Waals forces London dispersion Hydrogen Low - barrier Resonance - assisted Symmetric Dihydrogen bonds C -- H O interaction Noncovalent other Mechanical Halogen Chalcogen Aurophilicity Intercalation Stacking Cation -- pi Anion -- pi Salt bridge Bond cleavage Heterolysis Homolysis Retrieved from `` https://en.wikipedia.org/w/index.php?title=Pi_bond&oldid=830933062 '' Categories : Chemical bonding Hidden categories : Articles needing additional references from February 2013 All articles needing additional references Talk Contents About Wikipedia Bosanski Català Deutsch Ελληνικά Español فارسی Français 한국어 Bahasa Indonesia Italiano עברית Magyar Nederlands 日本 語 Norsk Polski Português Română Русский Simple English Slovenščina Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska Українська Tiếng Việt 中文 20 more Edit links This page was last edited on 17 March 2018, at 19 : 45. 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Question: number of pi bonds in a triple bond
Answer: two