[Bassel]

can a pi bond from from s orbital ? or s orbitals constitute only the sigma bonds ?

[magician4]

- a bond can only occur from (at least) two parent orbitals: a single s-orbital hence can form nothing

- [itex] /Sigma [/itex] , [itex] \Pi [/itex] and so on denote the symmetry of the bond forming orbital.
in case of sigma, this symmetry hast to be of the type "cylindrical symmetry with respect to the atom's interconnecting axis"

this is the only type of bonds known with s-parent-orbitals participation

hence, pi orbitals can not be formed from s-orbitals participation

regards

Ingo

[Vidya]

sigma bond is formed by internuclear axis overlapping and when two atoms are forming bond then first bond formed is sigma bond.We know that in valence shell of atoms we have only s orbital of lowest energy so this is the one who formed sigma bond

[magician4]

(...)and when two atoms are forming bond then first bond formed is sigma bond.(..)

this , with all due respect, simply is not true

think for example of cyclopropane and the interconnecting C-C bonds: those are "banana bonds", and they are not of Σ - symmetry

another example of "sigma-free" bonds does occur in the bridging hydrogens of diborane : those B-H bonds are of the three - center - two - electrons type  (i.e are part of a single multiatom connecting MO)

regards

Ingo

[Vidya]

I am discussing for the concept of hybridization

[magician4]

I am discussing for the concept of hybridization

what then do you think "banana bonds" and multicenter MO's are constructed of instead? magic?

no: even with cyclopropane , QM-calculations show that it's still a type of s , p hybrid (approx. 17% s , hence sometimes described as "sp⁵" )*⁾ overlap responsible for the C-C bond

so, yes, I'm talking hybridization, too, and gave you examples of "first" (in this case: the one and only) bonds (from hybrid orbitals) that are NOT of Σ symmetry, hence contradict your false claim


regards

Ingo

*⁾ L. Klasinc, Z. Maksić, M. Randić: Bent bonds in cycloalkanes. In: J. Chem. Soc. A. 1966, p. 755–757

[clarkstill]

Indeed.  There are also numerous examples of alkene d-block metal complexes where the bonding interaction is entirely of π symmetry.

I think by definition it's impossible for anything spherically-symmetrical to undergo π bonding: the planar node of the π component would necessarily interact with the s-orbital with bonding character on one side of the node, and antibonding on the other, giving no net bonding overall.