σ bond between two atoms: localization of electron density
In chemistry, sigma bonds (σ bonds) are the strongest type of covalent chemical bond.[1] They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is symmetrical with respect to rotation about the bond axis. By this definition, common forms of sigma bonds are s+s, pz+pz, s+pz and dz2+dz2 (where z is defined as the axis of the bond or the internuclear axis).[2]
Quantum theory also indicates that molecular orbitals (MO) of identical symmetry actually mix or hybridize. As a practical consequence of this mixing of diatomic molecules, the wavefunctions s+s and pz+pz molecular orbitals become blended. The extent of this mixing (or hybridization or blending) depends on the relative energies of the MOs of like symmetry.
1sσ* antibonding molecular orbital in H2 with nodal plane
For homodiatomics (homonuclear diatomic molecules), bonding σ orbitals have no nodal planes at which the wavefunction is zero, either between the bonded atoms or passing through the bonded atoms. The corresponding antibonding, or σ* orbital, is defined by the presence of one nodal plane between the two bonded atoms.
Sigma bonds are the strongest type of covalent bonds due to the direct overlap of orbitals, and the electrons in these bonds are sometimes referred to as sigma electrons.[3]
The symbol σ is the Greek letter sigma. When viewed down the bond axis, a σ MO has a circular symmetry, hence resembling a similarly sounding "s" atomic orbital.
Typically, a single bond is a sigma bond while a multiple bond is composed of one sigma bond together with pi or other bonds. A double bond has one sigma plus one pi bond, and a triple bond has one sigma plus two pi bonds.
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Atomic
orbitals
Symmetric (s–s and p–p) sigma bonds between atomic orbitals
A pi bond, for comparison
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σs–hybrid
σs–p
^Moore, John; Stanitski, Conrad L.; Jurs, Peter C. (2009-01-21). Principles of Chemistry: The Molecular Science. ISBN 9780495390794.
^Clayden, Jonathan; Greeves, Nick; Warren, Stuart (March 2012) [2002]. Organic Chemistry (2nd ed.). Oxford: OUP Oxford. pp. 101–136. ISBN 978-0199270293.
^Keeler, James; Wothers, Peter (May 2008). Chemical Structure and Reactivity (1st ed.). Oxford: OUP Oxford. pp. 27–46. ISBN 978-0199289301.
sigma bonds (σ bonds) are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigmabonding...
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...
single bonds have one sigma bond, double bonds consist of one sigmabond and one pi bond, and triple bonds contain one sigmabond and two pi bonds. However...
approach to form a sp2-sp2 sigmabond Two p-orbitals overlap to form a pi-bond in a plane parallel to the sigma plane pi bond (green) in ethylene With 133 pm...
orbitals overlap to form an sp-sp sigmabond. At the same time the pz-orbitals approach and together they form a pz-pz pi-bond. Likewise, the other pair of...
for bond cleavage: homolytic and heterolytic, depending on the nature of the process. The triplet and singlet excitation energies of a sigmabond can...
is produced by a covalent sigmabond between the atom hosting the sigma hole and a neighboring atom. The presence of the bond results in the distortion...
non-interacting H atoms. A double bond has two shared pairs of electrons, one in a sigmabond and one in a pi bond with electron density concentrated...
(153 pm). The triple bond is very strong with a bond strength of 839 kJ/mol. The sigmabond contributes 369 kJ/mol, the first pi bond contributes 268 kJ/mol...
a sigmabond. Dihydrogen complexes are examples. Transition metal silane complexes are often especially stable sigma complexes. A subset of sigma complexes...
N(s+{\sqrt {3}}p\sigma )} , where N is a normalisation constant (here 1/2) and pσ is a p orbital directed along the C-H axis to form a sigmabond. The ratio...
In molecular orbital theory, the bonding is described as σ2π4δ2 with one sigmabond, two pi bonds and one delta bond. The [Re2Cl8]2− ion adopts an eclipsed...
reaction in which a sigmabond moves across a conjugated pi system with a concomitant shift in the pi bonds. The shift in the sigmabond may be antarafacial...
pi bond together with the sigmabond for each pair of carbon atoms, giving a calculated bond order of 1.5 (one and a half bond). Furthermore, bond orders...
Sigma (/ˈsɪɡmə/ SIG-mə; uppercase Σ, lowercase σ, lowercase in word-final position ς; Greek: σίγμα) is the eighteenth letter of the Greek alphabet. In...
the bond order of the sigma bonds to include fourth moments of the exact tight binding bond order reveals contributions from both sigma- and pi- bond integrals...
the interaction of the electrons in a sigma (σ) orbital (e.g. C–H or C–C) with an adjacent unpopulated non-bonding p or antibonding σ* or π* orbitals to...
organometallic complexes. Sigma-bond metathesis is a synthetic method for forming new carbon-carbon sigma bonds. Sigma-bond metathesis is typically used...
permanent dipole in a bond. It is present in a σ (sigma) bond, unlike the electromeric effect which is present in a π (pi) bond. The halogen atoms in...
compound that is an alternative to the sigma and pi bond model. Bent bonds are a special type of chemical bonding in which the ordinary hybridization state...
bond distances are also possible: alkenes and alkynes have bond lengths of respectively 133 and 120 pm due to increased s-character of the sigmabond...
non-bonding electrons > pi bond electrons > sigmabond electrons, the order of ionization preference is non-bonding electrons > pi bond electrons > sigma...
pericyclic rearrangement where the net result is one pi bond being converted into one sigmabond or vice versa. These reactions are usually categorized...
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