There are 4 bonds, but each is a sigma bond. Observe that the Carbon atom is in sp3 hybridization and is showing a tetrahedral geometry. This bond is characterized as a sigma bond. HCl H Cl Again, the overlap is directly between the two nuclei. The hydrogen atom has a bonding electron in an s-orbital and the chlorine atom has its unpaired electron in a p-orbital. Note again how the overlap is directly between the nuclei of the two atoms. ![]() The p-orbital of the Cl atom on the left. Cl2 Cl Cl The p-orbital of the Cl atom on the right. Remember also that the shape of a p orbital is “figure-8”. Remember that the valence of chlorine = 7 so that the bond between the atoms is formed by the overlap of p orbitals. The bond in this example is characterized as a sigma bond. Note how the overlap of the two atom’s clouds is directly between the nuclei of the atoms. The atom on the right has been made “clear” to show the cloud overlap. Remember that the s orbital is spherical. A sigma bond will always be a single bond (as opposed to a double or triple bond).ģ Sigma Bond – Example #1 H2 This molecule has the two hydrogen atoms having 1 electron in an “s” orbital. A sigma bond will be formed using electrons in s orbitals, p orbitals, or hybrid orbitals. This rule fails further when considering other shapes - toroidal fullerenes will obey the rule that the number of sigma bonds in a molecule is exactly the number of atoms plus the number of rings, as will nanotubes - which, when drawn flat as if looking through one from the end, will have a face in the middle, corresponding to the far end of the nanotube, which is not a ring, and a face corresponding to the outside.2 What is a Sigma Bond? This is a covalent bond in which the electron cloud overlap is directly between the nuclei of the atoms involved in the bonding. Ordinarily, one extra face is assigned to the space not inside any ring, but when Buckminsterfullerene is drawn flat without any crossings, one of the rings makes up the outer pentagon the inside of that ring is the outside of the graph. This is because the sigma rule is a special case of the Euler characteristic, where each ring is considered a face, each sigma bond is an edge, and each atom is a vertex. This rule fails in the case of molecules which, when drawn flat on paper, have a different number of rings than the molecule actually has - for example, Buckminsterfullerene, C 60, which has 32 rings, 60 atoms, and 90 sigma bonds, one for each pair of bonded atoms however, 60 + 32 - 1 = 91, not 90. In this case there are 16 C−C sigma bonds and 10 C−H bonds. ![]() ![]() The anthracene molecule, C 14H 10, has three rings so that the rule gives the number of sigma bonds as 24 + 3 − 1 = 26. Molecules with rings have additional sigma bonds, such as benzene rings, which have 6 C−C sigma bonds within the ring for 6 carbon atoms. There is no more than 1 sigma bond between any two atoms. This rule is a special-case application of the Euler characteristic of the graph which represents the molecule.Ī molecule with no rings can be represented as a tree with a number of bonds equal to the number of atoms minus one (as in dihydrogen, H 2, with only one sigma bond, or ammonia, NH 3, with 3 sigma bonds). According to the sigma bond rule, the number of sigma bonds in a molecule is equivalent to the number of atoms plus the number of rings minus one. Organic molecules are often cyclic compounds containing one or more rings, such as benzene, and are often made up of many sigma bonds along with pi bonds. These sigma bonds can be supplemented with other bonding interactions, such as π-back donation, as in the case of W(CO) 3( PCy 3) 2(H 2), and even δ-bonds, as in the case of chromium(II) acetate. Transition metal complexes that feature multiple bonds, such as the dihydrogen complex, have sigma bonds between the multiple bonded atoms. For example, propane is described as consisting of ten sigma bonds, one each for the two C−C bonds and one each for the eight C−H bonds. The concept of sigma bonding is extended to describe bonding interactions involving overlap of a single lobe of one orbital with a single lobe of another. Sigma bonds are obtained by head-on overlapping of atomic orbitals.
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